PRACTICE OF.

We feel that it would be unsuitable to occupy the pages of a work like this with an epitome of the Practice of Physic, composed either on such a scientific plan as would adapt it to members of the medical profession, or in such a more popular style as would be requisite were it addressed to general readers. But there are some topics connected with the study and exercise of the medical profession, from a discussion of which general readers may derive, we conceive, both interesting and useful information. In particular, it seems extremely desirable that the public should entertain more accurate notions than generally prevail, as to the nature and extent of the difficulties medical men have to encounter in the exercise of their profession—difficulties that are in part, no doubt, common to medicine with all other practical sciences; but which must also be attributed in a great measure to the peculiar character of the subject itself. By being better informed with regard to the nature and sources of these difficulties, the public would be better able to judge of the qualifications they ought to look for in those to whom they intrust their health; more reasonable in the expectations they form of the benefits to be derived from medical assistance; and less easily estranged than they are too apt to be, from a just reliance on persons of judgment and skill, by the boastful pretensions of ignorant and possibly uneducated empirics.

When the physician is called to a patient, there are two objects to which his attention is successively directed; first, to ascertain with what disease his patient is affected; and, second, to determine the method of treatment best suited for its cure or alleviation. In the following article we purpose to consider what are the means that the practitioner employs for effecting the first of these objects—for determining in practice what the disease is with which he has to contend; and, in particular, to take a general view of the difficulties he has to encounter in endeavouring to arrive at a correct judgment on this subject.

A knowledge of the disease with which his patient is affected, the medical practitioner can hope to attain only by an accurate observation of the morbid phenomena or symptoms that manifest themselves, combined with or followed up by a careful consideration of the pathological conditions from which his own experience and that of others has shown that the particular group of symptoms under his observation, or a group closely resembling it, may possibly originate. We had occasion in the article MEDICINE, in adverting to the attempts which have recently been made to undervalue the study of symptoms as compared with that of proximate causes, to point out the necessary connections and dependencies of these two subjects of medical inquiry; and to show that as, on the one hand, a more accurate observation of symptoms leads to greater precision in establishing the proximate causes of diseases, so, on the other hand, a more correct acquaintance with proximate causes leads to the more accurate noting and the more due appreciation of symptoms. But in considering the relations of symptoms and proximate causes, it is necessary to keep in remembrance, that there is a wide diversity in the degree of directness with which different symptoms indicate the morbid condition upon which they ultimately depend. Thus, if the lining membrane of the air-passages be inflamed, or, in other words, if a person be affected with catarrh, or bronchial inflammation, amongst the principal symptoms that will arise will be fever, cough, and expectoration. Now the fever is immediately an affection of the nervous and circulatory systems, liable to occur in a great variety of diseases besides bronchial inflammation. Its occurrence, therefore, warns us of derangement in some part of the economy, but does not lead us to the respiratory organs as the primary seat of this derangement. The cough depends on a preternatural action of certain of the respiratory muscles, and is liable to be excited by a number of different causes capable of calling these muscles into play. It serves in some degree, therefore, to guide us to the primary seat of disease. The altered secretion, again, of which the expectoration consists, is the immediate result of that state of inflammation of the lining membrane of the air-passages, which constitutes the proximate cause of the disease, and serves very materially to guide us to a knowledge both of its locality and of its nature.

In attempting to ascertain, from a consideration of the symptoms that present themselves, what the disease is with which his patient is affected, the conduct of the practitioner will vary according to the more or less inquisitive character of his mind. The empirical practitioner will content him- self with endeavouring to determine what name the particular group of morbid phenomena or symptoms under his observation bears in systems of nosology; whilst the rational practitioner will aim at forming a judgment, from the consideration of these symptoms, as to the nature of the disease, or, in other words, as to the proximate cause on which it depends, and its immediate seat. Thus, if a patient presents the symptoms of yellowness of the skin and of the conjunctiva of the eye, the alvine evacuations being whitish or clay-coloured, and the urinary of a yellowish red or saffron hue, the empirical practitioner will be satisfied with knowing that he has to do with a case of jaundice, as distinguished from all other nosological diseases; whilst the rational practitioner or dogmatist will push his inquiries with the view of determining, if possible, whether the jaundice, in this particular case, is referrible to some morbid alteration of the structure of the liver itself, to the presence of gall-stones in the gall-ducts, or to the existence of spasm in these canals (morbid conditions or pathological diseases which may all separately or conjointly produce the symptoms that have been enumerated), or, in short, with the view of ascertaining what particular form of jaundice he has to subject to treatment. Both these practitioners must depend upon their observation of the symptoms that present themselves, for attaining the knowledge which they respectively desire. But the rational practitioner, whose object it is to arrive at a knowledge of the particular form of jaundice under consideration, must investigate these symptoms more accurately and minutely than is requisite for him who is content with the general knowledge of the fact, that the disease with which his patient is affected is that which has been termed jaundice. In the investigation of the rationalist, it is obvious, the differences between the objects to be distinguished, viz. the different forms of jaundice, are less apparent, the resemblances of these objects are closer, and their analogies are more striking, than those that exist between the objects of the empiric's comparison, viz. jaundice and other nosological diseases; and, consequently, a nicer and more elaborate analysis is required for their discrimination.

The process of analysis which the scientific medical practitioner has to carry on in his own mind, in endeavouring, from a consideration of the combinations of morbid phenomena or symptoms that present themselves to his observation, to arrive at a knowledge of the nosological disease with which his patient is affected, and of the morbid condition of the economy on which, in the particular instance, this nosological disease may depend, consists of several successive steps. 1st, He has to inquire what is the function, or what are the functions, a morbid affection or derangement of which is indicated by the symptoms observable. 2d, What is the particular process, or what are the particular processes, in the deranged function or functions, that are more immediately indicated as being disturbed. 3d, If several functions be deranged, he has to consider whether these derangements ought to be regarded as separate and independent occurrences, or as being dependent on one another; and if the latter, what is the particular relation subsisting between them in respect of antecedence and consequence. 4th, By these considerations respecting the deranged functions, the practitioner is gradually led to determine in his own mind what is the organ or part of an organ that is primarily and principally affected. And, 5th, having formed his judgment as to what is the organ, part of an organ, or texture affected, he has to consider the manner in which it is affected, or, in other words, what is the nature of its morbid condition; whether the symptoms can be supposed to depend on simple dynamical derangement, or whether they imply the existence of organic alteration of structure (see article Pathology); and also, what is the particular morbid change in property or in structure that must be supposed to exist. Under this last head is obviously comprehended the determination of the pathological nature of the disease with which the practitioner has to contend,—the consideration upon which the practice he is to pursue must necessarily be founded.

In each of the steps of this analytic process there is, in respect of a large number of diseases, ample room for mistake, not only in consequence of ignorance on the part of the practitioner, but in consequence also of the defective state of medical knowledge depending on the inherent difficulties of the subjects to which it relates. In whatever stage of the analytic process such mistakes may originate, their ultimate effect must be to lead the practitioner into one or other of two errors; 1st, that of supposing the existence of morbid conditions which do not actually exist; or, 2d, that of overlooking the presence of morbid conditions which do exist.

We shall endeavour, in the following article, to point out, Sources of and to illustrate by particular examples, some of the circumstances under which the practitioner is liable to commit such mistakes; or, in other words, in which difficulties and fallacies in diagnosis are liable to arise. Our illustrations shall be drawn chiefly from the writings of some of the most eminent clinical physicians of the present day, in order to obviate any suspicion that the difficulties to which we refer are attributable to the want, on the part of the observer, of an acquaintance with those precepts which the present state of medical science furnishes for the discrimination of diseases from one another, or of skill in applying these precepts to particular examples. In the course of these illustrations it will appear, that not only may the several sources of difficulty and fallacy to be noticed exist separately, but that two or more of them may, in some instances, occur in combination, rendering the determination of the pathological condition or conditions existing in the economy a matter of proportionally greater difficulty.

The first circumstance to which we shall advert is the occasional existence of organic or structural pathological lesions conditions in some of the textures, systems, or organs of without the body, without this being indicated by any obvious or characteristic symptom, so that the physician remains entirely ignorant of their existence till it is revealed by examination after death.

Every part of the body, it may be presumed, is destined for the accomplishment of some particular end; and, reciprocally, each phenomenon which the economy exhibits is referrible to the operation of some particular part of the organized frame. It seems natural, therefore, to infer, that a deviation from the healthy structure of any part of the body must produce a determinate and perceptible change in the performance of the corresponding function; and, on the other hand, that where there occurs a deviation from the healthy performance of any of the functions of the economy, this must depend on some change having occurred in the structure of a corresponding part of the body—in short, that where there is altered function, there must be corresponding alteration of structure; and where there is altered structure, there must be corresponding alteration of function. As general principles, these propositions are unquestionably true; but at the same time they are liable to very numerous and important exceptions. It has elsewhere been shown (Pathology, General) that very obvious deviations from the healthy exercise of the function of a part may manifest themselves during life, without its being possible to detect in that part, after death, the minutest deviation from its natural structure. And it is not less fully established, that in a large number of cases there have been found, on the dissection of bodies after death, deviations of parts from their healthy structure, of which no indicatory symptoms, direct or indirect, had presented themselves during life. "There is no anatomist," as was remarked by Dr Thomas Simson, formerly the distinguished professor of medicine at St Andrews, "who has had opportunity of opening any number of persons carried off without a manifest disease, but who must have had convincing proofs that there are great numbers of preternatural appearances in the body, which, during life, give no sensible disturbances." (Edin. Med. Ess. and Observ. vol. v. p. 616.) "It has been observed," remarks another intelligent author (Dr Crampton, Trans. of the Assoc. Phys. in Ireland, vol. i. p. 1), "by those physicians who are most in the habit of opening bodies, that they sometimes find morbid appearances after death, which they had not expected, even though they had closely attended to the antecedent symptoms."

The circumstances upon which it may depend that a part of the body may be altered in its structure, and yet no deviation from its healthy functions, or from those of the economy generally, be observable, or at least be observed to result from this alteration, are very various. First, The exercise of the function of the particular organ may not be conspicuous to observation. Secondly, We may be ignorant of the purpose which the part serves in the economy, and consequently unable to detect any deviation from the proper exercise of its functions. It seems reasonable to expect that symptoms will be the more readily perceived, and the more characteristic when perceived, in proportion as we are well acquainted with the healthy functions of the organs on a morbid condition of which they depend, and can easily discern the exercise of these functions. And as the functions of those organs, and their several parts, are in general most apparent and best known, which either perform some of the vital motions (that is, of those motions without which life cannot be carried on for more than a very minute space of time), or which furnish some sensible secretions or excretions, so the symptoms resulting from diseased states of such organs must be more easy of detection, and more characteristic, than those symptoms that are connected with organs not similarly circumstanced.

Thus, if the organs of respiration are out of order, or if the action of the heart, the spring of the circulation, is disturbed, the symptoms arising from such disorders are more readily perceived, and more easily referred to the organs concerned in their production, than are the symptoms that arise from the diseases of other parts the action of which is not so immediately necessary to life. If the secreting or excreting organs, as the liver or kidneys, fail in their proper offices, the symptoms thence arising are much more evident than those which depend on the derangement of organs that do not secrete any fluid from the blood; and among the organs that secrete a fluid, there will be greater facility in recognising the symptoms connected with those organs the fluid secreted by which is discharged out of the body, than with those organs whose fluid is retained in the body for the performance of some particular purpose; greater facility in recognising the symptoms produced by pathological conditions of the liver or of the kidneys, for example, than in recognising those produced by pathological conditions of the pancreas, though that, too, is a secreting organ, and separates from the blood a large quantity of fluid.

Dr Monro, in his enumeration of the circumstances which concur in rendering the history of diseases imperfect, has very properly included "the imperfect knowledge we possess of the functions of some of the organs of the body. Thus," adds he, "we do not know whether or not the spleen be diseased, unless it has attained so great a bulk as to be felt by an external examination." (Morbid Anatomy of the Gullet, &c. 1811, Introduction, p. xxx.)

"Nullum est viscus in corpore," says Grottanelli, in a work expressly treating of diseases of the spleen (Splenis Morbi, p. 78, Florentiae, 1821, 8vo), "quod diutius subdole segretet, pathologicumque statum, absque manifesto detrimiento servet, quam lien."

Thirdly, The function of the organ may not be essential for the general wellbeing of the economy, as is the case not being with the ovary; or it may be of trivial importance in itself, and exercise little influence on the action of other organs, to life as is the case with the gall-bladder. "Before there be any direct evidence of the existence of enlargement of the ovary, it has generally," says Dr Hamilton, "made a certain progress; and although there may be some data, from the appearances in dead bodies, for conjecturing the phenomena of the early stages of the disease, yet, practically speaking, a considerable advance must always have taken place before either the patient or the practitioner can be aware of its existence." (Pract. Observat. on Subjects relating to Midwifery, part i. p. 71.) "The gall-bladder," says M. André, "may be filled with calculi, and even with pus; its parietes may be the seat of different alterations; its cavity may have diminished in a considerable degree, or even disappeared; lastly, it may cease to receive bile, in consequence of the obliteration of the cystic duct, without there resulting during life, from any of these lesions, the production of any morbid phenomenon which can lead to a suspicion of their existence." (Clinique Médicale, tome iv. p. 492.)

Fourthly, The change which the organ has undergone, though apparently considerable in amount, may not be of a nature to prevent its performing its destined purposes in the economy. This seems in particular to be the case when the structural change occurs in a gradual manner, the parts apparently adapting themselves to their altered circumstances. "When a compressing cause," observes M. Cruveilhier, "acts slowly on the brain, this organ accustomed itself to the compression, or rather the compressed portion becomes atrophied; and if this compressed or atrophied portion is not indispensable to the free exercise either of the sensory and locomotory functions, or of the intellectual faculties, the patient and the physician are not made aware, by any morbid feeling or phenomenon, of the existence of the cerebral lesion." (Anatomie Pathologique, folio, livr. viii. pl. i. ii. and iii.) "The very great frequency," says Dr Sims, "of collections of serous fluid found in the ventricles or membranes of the brain, in cases where no cerebral symptoms were known to have existed, is a subject of great importance to pathologists and practical physicians, especially when viewed in reference to the discrimination and curative treatment of apoplexy and other diseases of the nervous system. There is also a great variety of other morbid appearances found on dissection of the brain, in cases where no symptoms, or no symptoms adequate to explain the phenomena (that is, indicative of organic lesion of this organ), were noticed during life." "Cerebral diseases," says M. Rostan (Med. Clin. ii. 354), "may go on without there existing any symptoms to characterize them. At all times the occurrence of latent diseases has been acknowledged; there exist fewer of them at present than formerly; our means of investigation have been multiplied to such a degree, that many diseases which would formerly have been overlooked, are in our days easily recognised. Nevertheless there are still affections of organs which we are surprised to meet with after death, and which have not produced during life any sign that could

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1 See Introduction to A Treatise on Biliary Concretions, by Thomas Cox, M. D. 8vo. Lond. 1757. 2 With the view of attempting to illustrate this subject, Dr Sims has subjoined a table, containing fifty cases of persons who died of various diseases not cerebral, and who manifested no symptoms referrible to the brain, though, on dissection, effusion of fluid and other morbid appearances were found in the brain or membranes. Med. Chir. Trans. xix. 274-8. have revealed their existence. This point of pathology is truly very obscure, and almost inexplicable. It is very certain that these things happen when the affection has advanced slowly, and has destroyed the organ slowly; it is very certain also that a lesion of very limited extent, that has come on rapidly, gives rise to well-marked phenomena: but this does not alter the fact that, in the first case, an organ which is destroyed continues its functions; and that, in the second, an organ almost unchanged ceases from its functions. How can the functions still go on in the first case? how do they stop in the second? It is therefore very difficult, not to say impossible, to recognise diseases when they are truly latent; and that there are such, is proved by experience.

The secreting glands seem to be capable of undergoing a very great degree of organic change without their secretion being put a stop to, or even sensibly disturbed, so that unless some other symptoms should happen to indicate the affection that exists, it will not be manifested by any deficient exercise of the proper function of the organ. M. Chardel, in his valuable Monograph on Scirrhouss Degenerations of the Stomach (Paris, 1808, p. 139), observes, "that the examination of a great number of dead bodies has convinced him, that frequently even the most considerable diseases of the liver occasion no impediment to the secretion of bile. Very large tumours may form in this organ, appearing to occupy the greatest part of its parenchyma, and still the secretion continue." And he particularly refers to a case of hydatid of monstrous size that had formed for itself a cavity in the liver, without the biliary secretion being put a stop to.

The urinary organs furnish us with numerous illustrations of the existence of organic affections, very considerable in their apparent degree, of which in many instances no indicationary symptoms manifest themselves during life, or at least during a considerable period of their existence. There is perhaps no organ of the body in which a greater extent of structural alteration may take place, without affecting, in a very marked degree, the exercise of its function, than the kidney. In some cases the urine has continued to be secreted when, from the progress of a particular form of disease, little more than the mere shells of the two kidneys has seemed to remain. (See Chardel, as above.) In other cases, calculi in considerable number, and of considerable dimensions, exist in the kidney without materially deranging their function, or otherwise betraying their own existence. "So long," says M. Chopart (Malad. des Voies Urinaires, i. 179), "as calculi remain concealed in the kidneys, they give no sensible signs of their presence. They may exist there and increase, without producing uneasiness, and without causing any symptoms that can even excite a suspicion of their existence. We have quoted examples of persons whose kidneys contained calculi which had not occasioned any pain to them. Baglivi found large calculi in the kidneys of two persons who, during life, had never complained in these viscera." To the same effect a more recent writer, Dr Marcat, in mentioning the symptoms with which calculi lodged in the kidney is generally accompanied, adds, "Yet cases of this kind occur in which these symptoms are scarcely perceptible; it will appear hardly credible, for instance, that the patient from whom the kidney represented in plate i. was taken, died of hydrothorax, at Guy's Hospital, without any symptom having occurred which could lead me to suppose that there was any disease in the urinary organs." (Essay on Calculous Disorders, 2d edit. p. II.) In the case alluded to, the pelvis of the kidney was much enlarged and distended by a number of calculi closely pressed against each other; and other calculi were contained in the enlarged infundibula.

In the bladder, also, there may, under particular circumstances, exist disease to a considerable extent, without any indication of its existence being furnished by the attendant symptoms. "In several cases," says Sir Everard Home (Diseases of the Prostate Gland, ii. 239), "in which I have examined the body after death, calculi have been found enclosed in cysts formed between the fasciculi of the muscular coat of the bladder, so as to be entirely excluded from the general cavity, and therefore had not produced any of the common symptoms of stone. I have seen in the same bladder two, three, and even four such cysts, each containing a calculus of the size of a walnut." And, even independently of the existence of such cysts, calculi may, as Mr Wilson has remarked, "exist in the bladder for years, without producing always those distressing symptoms which render life a burden, and which so frequently occur in this disease." (Lectures on the Urinary Organs, &c. p. 238, 1821.)

Fifthly, Some other organ or part may supply the place of that which, by a change of structure, has, in whole or in part, been rendered incapable of exercising its function. This is especially the case with those glandular organs which are double; for when one of these is morbidly affected, the other not unfrequently performs its function in such a manner as to supply the place of both. "Suppression of urine," says Mr Howship (Complaints that affect the Secretion and Excretion of the Urine, p. 2), "may be complete or incomplete. The first case is very rare; the second comparatively frequent. An affection that either deranges or destroys the functions of one kidney, may leave the other to the more vigorous performance of its duty; a circumstance most happily calculated to guard the constitution from the ill effects of a disease the complete establishment of which is almost invariably fatal."

"What we have said," remarks M. Bouillaud, "of the obscurity of the signs of diseases of the kidneys, is applicable to those of the lesions of the ureters. If these two canals were obliterated simultaneously, fatal consequences would ensue; but this does not happen when one of these organs only is obliterated. From a case which I shall relate, it appears that, in some instances of this kind, the kidney which has its ureter obliterated, has a tendency to become atrophied, whilst the other becomes hypertrophied, as if it may be able of itself to fulfil the function which it formerly divided with that of the opposite side. I do not affirm, however, that this is in all instances the case." (Journ. Compl. xxxi. 15.)

May we, in this point of view, consider the brain or any portion of it as a double organ; that is to say, may we believe that the corresponding portions of its two hemispheres co-operate in the exercise of a common function, and that this function will continue to be performed without any sensible or considerable impairment, though the part concerned in its exercise is injured or diseased on one side, provided the corresponding part of the opposite hemisphere remains sound? M. Bouillaud (Traité de l'Encephalie, p. 263, 1825), in speaking of the lesions of the intellectual functions which occur in encephalitis, or inflammation of the brain, decidedly adopts this idea, "When one of the hemispheres only is diseased, either in whole or in part, the phenomena purely intellectual continue entire. I have adduced a great number of observations in support of this fact. The slightest reflection, indeed, might have enabled us to anticipate this result. It is obvious that the brain, the organ of the intellectual faculties, being composed of two halves, which are symmetrical and perfectly similar, each of them is equivalent, in what regards the mechanism of intelligence, to the two combined." And M. Lallemand had previously expressed his belief (Rech. Anat. Pathol. sur l'Encephale, 1820, i. 435), "that when the sound hemisphere of the brain is not compressed by the one which is diseased, it will continue to perform its functions in respect of intelligence, as in respect of voluntary motion and sensibility; that the patient will continue to think with a half of his brain, as he continues to see with one eye, and to hear with one ear."

Sometimes one organ may supply, in a mechanical manner, a deficiency or imperfection in its neighbour; and in such a case the existence of the deficiency or imperfection may not be indicated by any apparent disturbance of the action of the organ in which it exists. This happens by no means unfrequently, for example, when an aperture in the stomach, produced by chronic ulceration, is filled up by the adhesion of the liver, the pancreas, or the spleen, to the margins of the ulcer. "I have been often surprised," remarks Dr Pemberton (On the Abdominal Viscera, 3d edit. London, 1814, p. 130), "to find very extensive mischief in the structure of the stomach, without the constitution being sensibly affected by it; that is, provided the mischief was so situated as not to interrupt the passage of the food. I have seen a large scirrhus in the stomach, near the pylorus, with an open cancer in one part of it, which had made its way through the stomach and through the left lobe of the liver, and an adhesion had taken place between the sides of the abscess and the peritoneum; so that, had not the patient been taken off by a disease in the aorta, I have no doubt but that this abscess would have made its way out through the integuments of the abdomen. Still, however, though this must have been a disease of very long standing, the body was but little emaciated, and the patient had never shown any one symptom by which such a disease of the stomach could possibly have been suspected. This single case," adds Dr Pemberton, "is sufficient to point out the infinite difficulty there is to determine the nature of internal diseases."

Sixth and lastly, Medical men must confess that, in some instances, they are unable to assign any plausible reason why the diseased organ does not reveal itself by any outward indication of deranged exercise of function. Thus M. Andral has related (Malad. de l'Encephale, p. 402, 403) some cases in which morbid softening of the brain, variable in its extent and in its seat, produced neither pain of the head nor disorder of the intellect, nor alteration of motion, nor disturbance of the sensations; nothing, in short, occurred during life that could even excite a suspicion of the central part of the nervous system being in any way affected. "As in these cases," remarks this accurate observer, "the softening was seated in different parts of the hemispheres of the brain, the absence of symptoms cannot be explained by the particular seat of the alteration. As to the nature of the alteration, it appeared identical in all the cases. In all, certainly, the softening was uncomplicated with any injection, effusion of blood, or other lesion; but its latency could not reasonably be attributed to this circumstance. Shall we explain it by conditions inherent in the individuals themselves? Shall we remark that they were all exhausted by chronic diseases, which must have blunted the sympathies, and diminished the tendency to re-action? But in many other cases we shall find the existence of the same chronic diseases not in any degree preventing the development of the usual symptoms of softening of the brain. Lastly, shall we refer the circumstance to the advanced age of the persons? This reason would not be more available than the others; for, though one of the patients was an octogenarian, another was only forty-five years of age. Let us here acknowledge, then, our complete ignorance; and, in reference to these facts, to which we find analogous ones in respect of each lesion that occurs, let us recall to mind a principle which cannot be too often repeated, viz. that the alterations of texture which an organ has undergone do not necessarily occasion a derangement of its functions. Thus there are organic lesions without symptoms, as there are symptoms without any appreciable lesion that can account for them."

To those who are engaged in the examination of dead bodies with a view to trace the connection between the symptoms of diseases and the morbid alterations of structure that produce or accompany them, it must obviously be of essential importance to distinguish, in particular cases, those alterations of structure that had existed during life, without giving any outward indication of their existence, from those on which the symptoms that manifested themselves really depended. The observation of some cases in which the coronary arteries of the heart (the vessels by which the substance of this organ is supplied with blood) were found after death to be ossified, in persons who during life had exhibited the symptoms of the painful affection of the chest termed angina pectoris, led some practitioners to conclude that they had detected, in that alteration of structure, the proximate cause of this disease. More extensive observation, however, has shown that ossification of the coronary arteries often exists without the symptoms of angina pectoris, or even without any symptoms whatsoever, manifesting themselves; as it has also shown that the symptoms of angina pectoris may occur in persons whose coronary arteries are sound. In the same way some recent pathologists have endeavoured to establish that asthma depends, in all instances, upon organic disease of the heart; though it seems to be well established, by the researches of accurate observers, that, though asthmatic symptoms and structural affection of the heart are frequently conjoined, every form of organic alteration may exist in the heart without inducing the symptoms of an asthmatic paroxysm; and that in many persons liable to such paroxysms, there exists no alteration in the structure of the heart. "One who would investigate diseases among the dead," as was remarked by an author formerly referred to (Dr Thomas Simson, in the Edin. Med. Ess. and Observ. vol. v. p. 617), "should have a large history before him of what the living can bear without disturbance. If this had been always the qualifications of our dissectors, in what different light should our histories of diseased bodies appear. And, in consequence of this, physicians should not have been so far misled in advancing the causes of diseases, and of the death of their patients, as to have placed every irregularity found as the original of the symptoms they had observed in life. A cautious theorist can never be satisfied in his inquiries till he has found out the disorder whence he can deduce all the symptoms that had appeared to him, by easy and natural consequences. A disorder that he observes upon dissection that has no apparent connection with the apparent disorder that happened in the actions of his patient, must be passed by him as foreign to his inquiry, or is only to be left on record to see if others may afterwards find the connection; but till then, such a disorder can never justly be advanced as the cause of any particular disease."

After the preceding view of the circumstances under which an organic or structural morbid affection of some part or other of the body may escape detection, in consequence of its not giving rise to any perceptible alterations in the exercise of the functions of the economy generally, and more particularly of the functions of the part in which it is situated, we come next to consider those sources of difficulty and fallacy in diagnosis, from which it arises, that although morbid phenomena or symptoms do manifest themselves, it is under circumstances calculated to create an uncertainty as to the nature and seat of the pathological condition on which they depend, or even to excite a belief in the existence of morbid conditions very different from those that actually exist.

In the first place, then, groups or trains of symptoms arising from different pathological conditions, may be identical, or at least so similar as to render their discrimination exceedingly difficult, if not impossible. In some cases, this similarity or identity in the groups of symptoms produced by different pathological conditions, is constant; so that the well-informed practitioner is on his guard against the risk of mistake. But, in other instances, a pathological condition usually recognisable by a group or train of symptoms peculiar to itself, assumes appearances foreign to its own habitual or ordinary character, and such as usually accompany some other form of morbid condition, so that the practitioner is taken, as it were, unawares, and exceedingly liable to adopt an erroneous opinion as to the nature of the disease which he is proceeding to treat.

From the illustrations to be quoted, it will be seen that when different pathological conditions or proximate causes give rise to the same group or train of symptoms in different persons, they may have their seat in the same organ, or in different organs. In either case it may be that both the morbid conditions are of an organic character, or that the one is organic and the other dynamical.

We shall, in the first place, adduce some examples of organic pathological conditions, differing in nature, but corresponding in seat, which give rise to groups or trains of symptoms so similar, as to render it difficult, if not impossible, to say, during life, with what morbid condition the patient is actually affected. M. Andral, after describing the different morbid alterations of the mucous membrane of the stomach, and of the subjacent textures, that may arise from chronic inflammation (including cancer, according to the view which he takes of the nature of this disease), puts the question, whether the infinitely varied lesions which he has described are each of them marked by a peculiar train of symptoms? "We do not fear," says he, "to answer in the negative. With the exception of some symptoms that are the mechanical result of the obliteration of the cardia or of the pylorus (the two apertures of the stomach) by a tumour, the same phenomena most generally attend during life these so different alterations of form and of structure." (Malad. de l'Abdomen, p. 429.)

In speaking of the symptoms of diseases of the liver, the same author says, "they are few in number, often too obscure or too little marked to enable us from a consideration of them always to affirm the existence of an affection of the liver. In other circumstances there is no room for doubt as to the existence of an affection of this organ; but it is very difficult, or even impossible, to determine its nature. For, on the one hand, analogous symptoms are often produced by affections of the liver which present the greatest differences in respect of their anatomical characters; and, on the other hand, we cannot say that in any of these affections we find any particular symptom constantly present." The same principle is applicable to the diseases of the urinary organs. Dr Marcat, after enumerating the symptoms generally produced by calculi in the kidneys, or by their passage into the bladder, adds (ib. cit. p. 13), "yet the occurrence of these symptoms cannot be considered as an absolute demonstration of the existence of this disease, unless calculi have been actually discharged; since it is well ascertained that symptoms very nearly similar are also sometimes occasioned by mere inflammation of the kidneys, without any concretion being present."

Before pointing out examples of an organic and a dynamical pathological condition of the same organ producing similar or identical groups or trains of symptoms, it may not be amiss to suggest of how much importance it is that the physician should be able to ascertain whether the proximate cause on which the symptoms in a particular case of disease depend, is organic or dynamical, since, according to the conclusion he may form on this point, the general plan and objects of treatment must be very materially influenced. It is in the class of dynamical diseases chiefly that the efforts of the medical art are likely to be of avail. Of organic diseases there is a very large portion in which, though we may for a time retard their progress, or alleviate the sufferings to which they give rise, we can entertain very little or no rational expectation of effecting a cure. The distinction between an organic and a dynamical disease of the same organ is one, however, which, in practice, it is very far from being at all times easy to make. The existence of an organic disease can, indeed, frequently be ascertained by direct observation, or by what may be called its physical indications or signs, as when it is seated in external parts, or in parts which the practitioner is able to examine by sight or by touch, directly or with the aid of instruments. In this mode of investigation a correct knowledge of the geography of the body, if we may so speak, is obviously essentially necessary. The practitioner must know the ordinary position of each part, its extent and its relations to the other parts in its neighbourhood. He must know in what situations to expect a prominence, and in what a depression; where to expect to find a soft and yielding part, and where a part that is hard and resisting. But even where he is able by direct observation to ascertain the existence of an organic disease, he often derives from this no knowledge of its particular nature, no information, for example, as to whether a tumour is simply inflammatory or scrofulous, whether it is of a mild or of a malignant nature; nor consequently does he obtain any insight into its probable progress: and in a great many of the organic alterations which occur, from the deep and remote situation of the parts which they occupy, as in diseases of the brain, for example, he is not apprized by direct observation, even of their existence. For all that remains to be learned respecting organic diseases, their existence, their nature, and their seat, after the means of investigation deducible from direct observation, or from what we have called their physical indications or signs, have been exhausted, the practitioner must, as for the whole of his knowledge of dynamical diseases, be entirely dependent on the detection of deviations from the regular exercise or performance of the several functions of the body, or of what may be termed the functional indications or signs of diseases. Hence it is obvious of how great importance an accurate knowledge, both of the healthy phenomena of the economy (as embraced in the science of physiology), and of the various modifications which these phenomena are liable to undergo in the state of disease (the branch of general pathology termed Symptomatology, or Semeiotics), must be to the practitioner of physic.

We may now point out some examples of an organic and a dynamical disease of the same organ, producing groups or trains of symptoms apparently identical. In diseases of the brain, the same train of symptoms may arise from an obvious organic alteration, or without any such alteration, and, as it is usually expressed, from mere functional disturbance. "A person previously in perfect health," observes Dr Abercrombie (On Diseases of the Brain, 3d edition, p. 199, 200), "falls down suddenly, deprived of sense and motion, and dies after lying for some time in a state of coma. We find, on examination, a large coagulum of blood compressing the surface of the brain, or filling its ventricles, and the phenomena of the disease appear to be distinctly accounted for. Another person is cut off with the same symptoms, but nothing is met with except serous effusion in no great quantity in the ventricles, or only on the surface of the brain. A third is seized in the same manner, and dies after lying for a considerable time in a state of coma, from which nothing can rouse him for an instant; and, on the most careful examination, we cannot detect in his brain the smallest deviation from the healthy structure." Here, then, are three cases which, as presenting the same external phenomena, all fall under the same nosological disease, apoplexy; but of which the first may be regarded as depending on an organic, and the last, at least, on a dynamical affection; whilst with regard to the second; it may not be easy to determine whether the morbid appearances discovered after death should be regarded as the primary disease, or only as one of its effects.

The stomach may be mentioned as another organ the structural and dynamical affections of which give rise to closely resembling groups or trains of symptoms. "To trace exactly the phenomena of a disease," says M. Charde (lib. cit. p. 159), "is not always sufficient for conveying a proper knowledge of it; it is still necessary to point out any other affections the resemblance of which with the object of our researches might possibly occasion mistake. This observation is of especial importance in the case of incurable diseases, since, to confound these with other diseases that are susceptible of cure, is to deprive the unfortunate persons committed to our charge of the assistance which they had a right to expect, and to deceive their confidence. No affection," this author proceeds to observe, "simulates so completely scirrhouss degenerations of the stomach as spasmodic vomiting, or that arising from an immediate lesion of the principle of life in the nerves of the stomach, from a special sensibility of that viscus."

In other instances in which different pathological conditions or proximate causes give rise to the same group or train of symptoms in different individuals, the different lesions have their seats in different organs or parts of the body. These organs or parts may be related to one another simply in the way of contiguity or superposition; or there may exist some functional dependence or sympathetic connection between them.

The mistakes liable to occur in the judgments of medi- proximity, cal men as to the existence or non-existence of aneurism, furnish an illustration of the similarity that may exist in the morbid phenomena induced by different pathological conditions of organs or parts in near proximity. "The occurrence of a pulsating tumour in the vicinity of an artery," says Mr Hodgson (Diseases of Arteries and Veins, p. 88), "will induce every cautious surgeon to suspect the existence of an aneurism. But there are many diseases which, from their situation, or from other circumstances, may assume the symptoms of aneurisms; and in some instances aneurisms assume the appearances of other diseases." "The circumstances that characterize the existence of an aneurism are much more complicated when the disease is situated in the thorax or abdomen, than when it exists in the extremities. Aneurisms in the thorax or abdomen frequently prove fatal before a knowledge of their existence is confirmed by that evidence which the appearance of the tumour externally generally affords; and the viscera with which they are surrounded are subject to a variety of diseases, many of the symptoms of which are common also to aneurisms of the aorta and of its primary ramifications."

If we turn our attention to the diseases of particular organs, we shall find ample illustrations of the source of fallacy in diagnosis to which we have just alluded. Practitioners are well aware that a difficulty of breathing or of swallowing may arise not only from affections of the organs by which these functions are respectively exercised, but from morbid conditions of one or other of the neighbouring organs or parts, as of the thyroid or bronchial glands, the vertebrae, and the larger blood-vessels; and also, that whilst difficult respiration may be occasioned by morbid conditions of the pharynx and oesophagus (the organs by which deglutition is effected), difficulty of deglutition may arise from morbid conditions of the glottis, of the larynx and trachea, and even of the lungs, that is to say, of the organs of respiration. "Aneu-

1 See generalization of the same fact by Dr Cheyne, in the article LARYNGITIS of the Cyclopaedia of Practical Medicine; and corroborative cases in the Lond. Med. Gaz. i. 322, and the Edin. Med. Surg. Journ. xiii. 293, the latter by Professor Graham. See case of aneurism of the aorta, producing symptoms of pneumonia, related by Rostan, Médecine Clin. i. 64; and on the difficulty of diagnosis in internal aneurisms, particularly abdominal, see case by Dr T. E. Beatty, in Dubl. Hosp. Rep. v. 166. in physical indications, we may take an example from M. Chardel, who, in pointing out certain circumstances under which scirrhus of the stomach may exist without any tumour being perceptible in the epigastric region, suggests (ib. cit. p. 185) that, on the other hand, tumours really existing in some other organ, such as "a prominent liver, an abscess in the posterior cavity of the epiploa, or a swelling of the pancreas, may simulate an induration of the stomach. Tubercular mesenteric glands, when they are in the neighbourhood of the stomach, may easily be taken for a scirrhus of that viscus." And he adds, that "it is possible that the hardness which the muscles of the abdomen, particularly the recti, exhibit in persons in whom they are strong and much exercised, may equally lead into error." Of resemblance in functional indications, we may quote an instance mentioned by Mr Hodgson, who observes that "when an aneurism arises from the aorta at the root of the coeliac artery, it is attended with frequent sickness, more especially after food is taken into the stomach, and by its pressure produces symptoms like those of stricture of the pylorus." (P. 93.)

The reciprocal influence which the different parts of the body exercise upon one another, often throws great difficulty in the way of the practitioner in attempting, in particular instances, to ascertain the seat of disease from a consideration of the symptoms that attend it, seeing that the prominent symptoms may point at organs very different from those which are actually the seat of disease. This influence is sometimes exerted between parts that co-operate in the performance of the same general function, as when vomiting arises in consequence of obstruction of some portion of the intestinal tube; but in other instances it is exerted between parts concerned in the performance of functions essentially distinct, as when headache or palpitation of the heart occur in various derangements of the stomach and bowels. In consequence of this reciprocal dependency of different parts, not merely may a symptom immediately referrible to some particular organ depend on an affection of an organ quite distinct from this, but the same symptom may arise from affections of several different and remote organs. Thus, for example, the symptom of cough, the immediate cause of which is known to be the convulsive or rapid contraction of certain respiratory muscles, may occur as a symptom of various affections of the air-passages or lungs; but it may also form a very prominent symptom in diseases having their seat in parts remote from the respiratory organs; and a late French author, M. de Larroque, has published a work professedly treating of cough as a symptom, more or less severe, of certain abdominal diseases. The circumstances just adverted to render it impossible, it may be remarked, to characterize and recognize diseases by one particular prominent symptom. It is only by marking and analysing the concomitant groups of symptoms that we can arrive at a knowledge of the organ which is the primary seat of disease. When a patient complains of headache, for example, the practitioner must judge, from a consideration of the other symptoms by which this uneasy feeling is accompanied, whether it depends on the existence of some pathological condition in the head, the immediate seat of the symptom, or in the organs of digestion, the derangements of which we know to be capable of inducing it sympathetically. In the same way, when the outstanding or prominent symptom is vomiting, it is only by a careful consideration of the concomitant circumstances that we can hope to determine whether it is in the stomach itself, or in some other organ more or less immediately connected with it, that the original seat of mischief is to be found.

"The general signs by which pericarditis may be recognised," says Mr Adams, "are exceedingly equivocal, and much requires to be done by the physician to clear the obscurity. I have been frequently called upon to examine the bodies of persons supposed to have died of disease of the lungs or liver, and I have found pericarditis the true and single source of death, although during the previous illness the heart was never suspected to be the seat of a fatal affection. I have myself been witness to the treatment of a case of chronic inflammation of the pericardium in a girl aged fourteen, which terminated fatally, and during the whole progress of the disease, phthisis, or rather what was considered hepatic phthisis, was the disease under which this child was supposed to be gradually declining." (Dubl. Hosp. Reports, iv. 360.)

Morbid conditions of the alimentary canal are peculiarly liable to produce disturbance in the functions of some other organs, without betraying their own existence. "Such," says M. Piorry, in an essay on the influences which the stomach exercises on other organs in health and in disease (Journ. Complem. du Dict. des Scien. Med. xvi. 199), "such is frequently, in diseases of the stomach, the intensity to which the sympathetic lesion of the related organs amounts, that it seems to destroy the primary affection, or at least to cause it to be overlooked. Whilst the patient labouring under such disease experiences only slight uneasiness in the epigastrium, he is tormented by the severest and most intolerable cerebral, muscular, thoracic, and other pains." "The exact appreciation of the nature of the symptoms observed," remarks the same author, "is so much the more difficult on this account, that the organ whose sufferance is symptomatic of the lesion of another part becomes itself a new centre of irradiation, which induces, in its turn, secondary irritations or pains. And as there is nothing to prevent these from being, in their turns, new points of departure of physiological influences, there results excessive difficulty in establishing a just diagnosis, in cases so embarrassing." Every one is aware of the influence which the presence of worms in the intestines is generally believed to exercise in producing derangement in the functions of remote organs. "There are few diseases," says M. Andral (Fievres, p. 273-5), "particularly in children, of which worms have not been supposed to be capable of being the cause. Klein has said, 'Nullum tam peregrinum est symptoma, tanquam caecus, quod vermes excitare non possit.' We read in authors numerous examples of palsies, comatose attacks, asthmas, palpitations of the heart, obstinate coughs, epilepsies, and hysterical convulsions, depending on that cause. Even tetanus has been considered as the result of the presence of worms in the intestinal canal. In the time of Alexander Trallianus, it was said that, when worms passed from the small intestine into the stomach, they gave rise to excruciating cardialgia, to syncope, and that sometimes even they occasioned sudden death. It must be confessed, that observations of this kind become the less frequent as pathological anatomy is more generally and more carefully cultivated. But, in hesitating as to the reality of these extraordinary facts, let us be careful not to deny altogether their possibility. A hemiplegia caused by worms would not appear to us a less credible phenomenon than a palsy succeeding to the introduction of particles of lead into the alimentary passages. Now nothing, unfortunately, is more real and more common than this last species of palsy. The symptoms caused by worms may, besides, be expected to vary much according to their nature, their number, their greater or less mobility and activity of existence, their more or less immediate contact with the mucous membrane, their situation in particular parts of the alimentary canal, the energy of the patient's sympathies, his nervous susceptibility, &c. Amongst physicians, said De Haen, some have made worms play too important a part in the production of diseases; and others have not paid sufficient attention to them." To the class of cases in which different pathological conditions, having their seats in different organs or parts of the body, give rise to the same group or train of symptoms, may be referred those in which lesions of the central, or cerebro-spinal and ganglionic, portions of the nervous system occasion the development of symptoms in remote organs of the body, similar to those that are produced, or that may be supposed to be produced, in these organs, by morbid lesions of which they are themselves the immediate seats. In many cases, for instance, of local pains or neuralgias, as of neuralgia of the mamma, of the intercostal muscles, and of the arm, there is reason to believe that the pain is not attributable to an affection of the nerves of the parts to which it is referred, but (as Drs Brown, Darwall, Teale, and other authors, have pointed out) is caused by the presence of a primary diseased state in the spinal cord itself, or in its coverings. Although our limits do not allow us to enter fully into the illustration of the doctrine of spinal and ganglionic irritation, as it has been called, we cannot refrain from quoting a very graphic account of the diversity of symptoms manifested in this class of cases, which is given in a paper on diseases simulating acute inflammatory attacks of various important organs, by Mr Torbet, surgeon in Paisley, contained in the forty-fourth volume of the Edinburgh Med. Surg. Jour. p. 375. "Of the diseases in question," says that author, "females frequently before, sometimes after, puberty, are the most usual victims. The individuals are of what is called irritable constitution, a peculiarity moreover distinctly hereditary, and connected more or less with strumous diathesis. The morbid train of neuralgic symptoms is most usually set in operation either by a serious inflammatory or febrile disease, by excessive fatigue, or by injury, as from a heavy lift or severe fall. The severity and variety of diseases to which the individuals appear to be liable, might almost characterize the affection. You are astonished to hear of their violent illnesses, the powerful remedies that are applied, and the rapidity with which they shake off these attacks. In severity their diseases resemble those of no other persons. They are paralytic, blind, speechless, without food, and without sleep, for weeks or months, and yet they do not die. Is the head the organ affected? Then they have headache, of which no words can convey an idea, ending perhaps in insensibility; or there is a heavy weight pressing down their eyelids, a tight bandage compresses, or hammers beat within their heads, bells ring in their ears, vertigo and sickness overpower them, noise and light disturb them. Is the chest affected? Then a sepulchral cough, each fit of which seems to threaten existence, over which medical art seems to possess no control, harasses from morning to night, and night to morning. There is excruciating pain in the chest, especially during the cough, which is perfectly devoid of expectoration. There is a feeling of swelling and choking at the under part of the neck, tremendous palpitation of the heart; the pulse is quick and agitated, but varying greatly with the severity of the cough; and the heart's pulsations seem to occupy the whole left side of the chest. Is the digestive apparatus the seat of disease? Then ensue vomiting without apparent cause; incessant retching for hours, occurring perhaps every day for weeks, without anything being vomited; no appetite; unquenchable thirst; occasional severe pain in the belly, or muscular cramps; and costiveness alternating with diarrhoea, the stools being unaccountably copious. Are the organs of voluntary motion attacked? Then there are cramps, prickling or numbness, sometimes severe spasms, like tetanus or hydrophobia than any thing else, ending possibly in catalepsy; or paralysis of one or more limbs ensues. Such is an epitome of the symptoms of the more violent cases; but almost endless variety must of necessity occur, according as the several nervous centres are affected, and their affections modified by individual circumstances. It is precisely from this variety and anomaly of appearances that practical men are apt to be misled. The most marked symptoms are apparently so distant from, and unconnected with, nervous irritation, and are moreover so violent and similar in their character to pure inflammatory diseases, that there is great risk of our chief curative efforts being directed to the appendages and coincidents, instead of attacking the disease itself."

Numerous facts upon record show that a pathological condition existing in the trunk of a nerve may produce symptoms that appear referrible to the organ in which its branches are distributed. "Every part to which a nervous filament can be traced," as is remarked by Sir Benjamin Brodie (Lectures illustrative of certain Local Nervous Affections, p. 2), "may be said to have its corresponding point in the brain or spinal marrow; and an impression made either at its origin, or anywhere in the course of the trunk of a nerve, will produce effects which are rendered manifest where the nerve terminates, at that extremity of it which is most distant from the brain." The experiments of different physiologists had fully established, that when the pneumogastric nerve, or the recurrent branch of this nerve, which goes to the larynx, is divided on each side of the neck, paralysis of the muscles that open the glottis is produced, and, as a consequence of this, the state of stridulous breathing. The knowledge of this fact has been very ingeniously applied by the late Dr Hugh Ley to the explanation of certain affections of respiration occurring as consequences of disease. Thus he conceives that the crowing or croup-like inspiration which is not unfrequently met with in infants, depends primarily, not on a morbid state of the parts constituting the glottis itself, but upon a morbid condition of the pneumogastric and recurrent nerves; and that in very many instances it is connected with a diseased state of the lymphatic glands of the neck, producing pressure upon these nerves. He has endeavoured likewise to show, that the fits of dyspnoea and stridulous breathing, which patients affected with tumours, aneurisms, or other organic diseases at the root of the neck, sometimes experience, as well as the asphyxia by which such persons are not unfrequently cut off, are often attributable, not to any mere physical or organic compression upon the trachea or larger bronchi, but to the diseased masses being so situated as to cause compression, stretching, or more or less complete destruction, of the pneumogastric nerve, or its recurrent laryngeal branch. (On Laryngismus Stridulus.)

But whilst morbid conditions of the central parts of the nervous system frequently give rise to groups or trains of remote symptoms that seem referrible to distant organs, on the other hand it sometimes happens, as M. Lallemand has remarked, that diseases having their seat elsewhere than in the brain produce phenomena analogous to those occasioned by certain cerebral affections. (Preface, p. 20.) M. Andral relates (Malad. de Poitr. vol. ii. p. 444) a case in which the symptoms that manifested themselves during life were those of inflammation of the membranes of the brain, whilst the morbid appearances found after death were those of acute inflammation of the serous membrane of the heart; and he refers to a case related by M. Rosan, in which there occurred cerebral symptoms, for the explanation of which no other lesion was found than that of inflammation of the investing membrane of the heart. "These two cases," M. Andral remarks (p. 447), "are well calculated to show, that in consequence of individual susceptibilities, there is no organ the lesion of which may not induce the most varied nervous symptoms, so as to produce sympathetically the different morbid states (i.e., neurological diseases) of which we place the seat in the nervous centres and their dependencies. It has been correctly laid down, that the alimentary canal, when inflamed, reacts more frequently than any other organ, upon the brain, and gives rise to the symptoms of inflammation of the membranes or substance of the brain. But it is not less true, as M. Boisseau has already very well shown, that inflammations of other organs may equally produce, by their sympathetic influence on the encephalon, the different groups of symptoms which constitute the astax fever of M. Pinel." Dr Latham, too, after remarking that the perplexities with which the diagnosis of the diseases of the pericardium is beset, are more considerable than any belonging to the inflammatory class of diseases in other organs of the body, has mentioned some cases in which the symptoms that manifested themselves during life were those which usually attend inflammation of the brain, whilst, upon examination after death, the brain and its coverings were found in a perfectly healthy and natural state, and the pericardium alone, or conjointly with the substance of the heart, discovered unequivocal marks of acute and recent inflammation. "These cases," Dr Latham continues, "furnish specimens of the enormous difficulties which medical men have sometimes to contend with in the diagnostic part of their art, and they ought to mitigate the censure of those who would judge harshly of our mistakes. Here it is remarkable, first, that a vital organ should sustain an acute inflammation without any symptom whatever immediately referrible to itself; and that organ the heart; and, secondly, that it should impart expressly to another organ, not the mere symptoms of common derangement, but the authentic symptoms of inflammation, and that organ the brain. Diagnosis is necessarily perplexed and baffled where no symptoms whatever are found in the organ really diseased, and none in those with which it is functionally allied, but in some other organ with which it has no known connection, save that they are both parts of the same body. And, further, diagnosis becomes absolutely hopeless when the organ, thus suffering sympathetically, puts forth, not the sign of a mere indefinite ailment, but of a real disease, changing and disorganizing its structure; as when the brain, being perfectly healthy, manifests the signs of acute inflammation, while the heart, being acutely inflamed, gives no evidence of its disease whatever."

Of the cases in which the medical practitioner is misled by the symptoms that present themselves, as to the nature of the malady under which his patient is labouring, this arises, in some, from a local affection producing the symptoms of a constitutional disease, without betraying any marks of its own existence; whilst in other instances, on the contrary, it depends upon a constitutional disease, simulating, or producing the symptoms of a local affection.

A learned journalist, in quoting a case related in Hufeland's Journal by Dr Roesser of Wurtemberg, in which inflammation of the heart had assumed the symptoms of chorea, or St Vitus's dance, has justly remarked, that "the medical practitioner is not unfrequently puzzled by nervous affections taking on the character of serious organic diseases; he should be on his guard against the reverse, dangerous maladies concealed under the mask of nervous disorders." (Johnson's Journal, July 1829.)

"In local diseases," says Sir Everard Home, in his work on Strictures (ii. 13), "it is often difficult, and sometimes impossible, to ascertain whether the constitutional affections that are met with are in reality symptomatic of the local irritation, or belong to some new disease which has been superinduced; and for want of some criterion to guide our judgment in the investigation of diseases, we too often mistake the occasional symptoms for the disease itself, which, from the obscurity of its situation, or its apparent insignificance, is totally neglected." "The constitutional symptoms which belong to a stricture in the urethra," he adds, "have been more frequently mistaken for an original disease, than those of any other local complaint; and it is only within these few years that this error has been detected." And to the same effect Dr Thomson observes, in his Lectures on Inflammation (p. 105), that "of fevers resembling intermittent, and arising from local diseases, we have examples in the febrile affections which not unfrequently accompany strictures in the urethra. I have repeatedly known," he adds, "these symptoms mistaken for, and treated as, a true intermittent fever."

It is well known that the "new medical doctrine," maintained by M. Broussais, proceeds on the assumption that certain diseases usually regarded as constitutional or essential, that is, as not proceeding from nor depending on any local affection of a particular organ or texture, always result from certain local internal inflammatory affections, though in some instances there may not occur any well-marked symptoms indicative of the existence of such affections. "All the essential fevers of authors," says M. Broussais, "are referrible to gastro-enteritis (inflammation of the stomach, and adjacent part of the intestines), simple or complicated. Medical men have all overlooked this inflammation when unaccompanied by local pain; and even when pains have occurred, they have always been regarded as accidental." (Propositions de Médecine, cxxxix.) It would be out of place to enter here on an exposition of the errors involved in this fundamental principle of the so-called physiological doctrine of medicine; but whatever may be thought of the doctrine which represents all fevers as originating in and referrible to some local inflammatory affection, it must be allowed, that local inflammations occasionally assume the symptoms and course of particular forms of fever, so as to mislead the practitioner in his judgment as to the nature of the disease which he has to treat. "Before entering," says Professor Van Coetsom of Ghent (Recherches Cliniques, &c. p. 91), "on an elaborate discussion as to the diagnostic value of the local symptoms which manifest themselves during the course of acute arachnoiditis (inflammation of the serous membrane of the brain), we have thought it right to fix the attention of the medical public upon a singular modification of this inflammatory affection, of which we have hitherto in vain sought for any notices in the works of the different authors who have described this affection, if we except only Pinel and Morgagni, who relate with astonishment some scattered observations, which they have, as it were, left to their successors to make use of, when, having become more numerous, they might serve as the foundation for a medical doctrine applicable to analogous cases. We mean to say that this terrible inflammation may sometimes simulate the paroxysms of quotidian or tertian intermittent fever. This result, founded directly on observation, is of the highest importance as regards the treatment; for if, unfortunately, the physician deceives himself respecting the nature of the pretended febrile paroxysms, which are observed at the commencement or during the course of the disease, he loses successively all the patients committed to his care." "Resting on the observations of our predecessors, and on those which we have ourselves collected, we think we may affirm, without fear of being taxed with rashness, that there are acute inflammations of the serous system which sometimes affect the periodical form of intermittent fevers; and that, of these, acute arachnoiditis presents, during its course, more frequently than the inflammations of other serous membranes, the periodicity in question." (P. 121.)

It would appear that under peculiar states of the general constitution, the groups or trains of symptoms characteristic of diseases appertaining to particular organs may cases may manifest themselves, though there do not exist in these organs any morbid conditions, pathological states, or proximate causes, such as those upon which the occurrence of these diseases usually depends. Thus, for example, in certain states of the general constitution, all the symptoms which indicate an apoplectic attack may occur, whilst, from circumstances developing themselves in the progress of the affection, the practitioner has the assurance that no vascular turgescence of the brain, nor serous nor sanguineous effusion, such as genuine apoplexy depends upon, has actually taken place. As illustrative of those states of the general constitution in which such fallacious or simulating diseases occur, we may refer to that induced by the morbid agent which produces ague or intermittent fever, to the gouty habit or diathesis, and to the hysterical.

We shall give a few illustrations of the influence of each of these forms of constitutional disease in inducing phenomena analogous to those of particular local affections; and, first, of ague. Sydenham, in the account he has given of the epidemic diseases which prevailed from the year 1675 to 1680, in speaking of the intermittent fevers of 1678, makes the following observation (Epistola Responsoria, lma, p. 22-3, edit. sec. Londini, 1685; also Works, London, 1742, p. 284, § 34): "Here it is proper to be noticed, that during the first years of this constitution, a very remarkable symptom sometimes succeeded these intermittents; for their paroxysms did not begin with chilliness and shivering, afterwards succeeded by a fever, but the patient was seized with precisely the same symptoms as if he had been affected with true apoplexy, though, however closely it resembled this affection, the ailment was nothing more than the fever itself attacking the head, as was shown by the colour of the urine and other symptoms. Although in a case of this nature all kinds of evacuations seem to be indicated for producing revulsion of the fluids from the brain, as is wont to be done in primary apoplexy, yet these means must be altogether abstained from, as being wholly opposed to the original cause of this symptom, viz. the intermittent fever, and endangering life, as I have myself known. But, on the contrary, we must wait till the paroxysm disappears spontaneously, at which time the bark, if it could not be administered at an earlier period, must be given as soon as possible, and sedulously repeated in the intervals between the paroxysms, till the patient be perfectly recovered."

Dr Morton, in his treatise De Morbis Acutis; Universalibus, published in 1692, has dedicated a chapter to the consideration of the proteiform character of intermittent fever. He observes that intermittent fevers are wont not unfrequently to assume the appearance of several other diseases, some of them of a very acute character. "These diseases yield to the power of bark, the great specific for the cure of ague; and if bark be not administered, they often prove fatal. No one hitherto," he continues, "so far as I know, has treated of this subject, though its investigation seems to me of the greatest importance for the promotion of practical medicine. Among the innumerable symptoms that usually accompany intermittent fevers, there is no one which does not sometimes rise to such a degree of intensity as to bring the life of the patient into immediate danger, whilst at the same time the type of the fever, in its various stages of chill, heat, and sweating, is completely obscured, so that we are not able to detect the existence of the fever by its usual characters. Sometimes it presents itself to the physician under the mask of a fatal chill; sometimes of an unceasing vomiting, of a gripping diarrhoea, of cholera morbus, of colic, of periodical hemicrania, of apoplexy, syncope, rheumatism, of universal spasm, pleurisy, peripneumony, pleurodynie, or of some other disease; and though the remedies be administered that are suitable to the particular disease the form of which it assumes, no benefit is obtained from their use, and it is only by the administration of bark that the symptoms are relieved." (Exercitat. i. chap. ix.)

The class of cases to which Sydenham and Morton refer in the passages which we have quoted, have generally been designated as instances of larvated or masked intermittent fever. But they have not at all times been distinguished with sufficient accuracy from other instances of intermittent fever, in which the symptoms of some severe local malady have manifested themselves in complication with, and not to the exclusion of, the ordinary febrile symptoms of ague; a form of disease to which, from its dangerous character, and frequent rapidly fatal termination, the name of pernicious intermittent fever has been given by Torti and other writers on this subject.

That the gouty or arthritic constitution frequently gives rise to phenomena which simulate very accurately the symptoms of local diseases, although the usual proximate causes of these diseases do not exist, the following passages from Musgrave, a very distinguished writer upon gout, may satisfy our readers. "The diagnosis of anomalous arthritic affections," says this author (De Arthritide Anomala, Exoniam, 1707, § iii.), "is surrounded by a great many difficulties, and involved in very deep obscurity." Indeed those anomalous affections which occur without being preceded by any regular paroxysm of gout, imitate so closely the legitimate and well-marked diseases of the parts which they affect, that it may be considered wholly impossible to discriminate between them. If, for example, angina or quinsy occurs in a person previously healthy, and who has never suffered a single paroxysm of gout, nor shown any tendency to it, and no point of difference can be detected between the symptoms of this case and the ordinary ones of genuine angina, who would dream that this affection was dependent on gout? yet that a gouty angina may occur under such circumstances is established by medical experience beyond all possibility of doubt.

"With regard to the inward part or trunk of the body," observes Musgrave (lib. cit. § viii.), "anomalous gout most frequently seizes upon the stomach and intestines. Hence arise anorexia, dyspepsia, vomiting; hence colic, dysentery, diarrhoea, and hence sometimes arthritic abscess. Frequently it attacks the head, exhibiting at one time the appearance of vertigo, at another of headache, and at another of arthritic apoplexy. Not unfrequently it makes its attack upon the region of the chest and the organs of respiration, presenting itself under the form of asthma, or of cough, or of homopoeis, or of arthritic phthisis. Sometimes it even attacks the fauces, assuming the characters of angina; sometimes the gums, receiving the name of gout in the teeth. Sometimes, in seizing upon the kidneys, it produces the symptoms of calculus, dysuria, and strangury. In fine, it lays hold of the viscera, the membranes, the muscles, the glands, the tendons, the ligaments, perhaps even the bones; so that throughout the whole trunk there can scarcely be found a part which is totally exempt from it."

"It is well ascertained," adds this author in another place, "that the affections thus produced by this wandering poison (derio miassmate), simulate most accurately all the diseases to which the parts in which they occur are subject; assume their characters, exhibit even their pathognomonic symptoms, and can in no ways be distinguished from the indigenous diseases by the concurrent diagnostics, but, on the contrary, are in fact frequently confounded with them."

We have mentioned hysteria as another of those states of the general constitution that are liable to give rise to groups or trains of symptoms closely resembling those which are in wont to arise from morbid conditions of particular organs. Sydenham, in speaking of hysteria, observes (Works, uti cit. p. 370), that "this disease is not more remarkable for its frequency, than for the numerous forms under which it appears, there being scarcely any diseases with which mankind is afflicted, of which it does not assume the semblance. For, of whatever part of the body it may take possession, it immediately produces symptoms corresponding to that part, so that unless the physician be a person of judgment and penetration, and of considerable experience in practice, he will be deceived, and suppose these symptoms to arise from some essential disease of this or that particular part, and not from the hysterical affection on which it really depends.

Among the diseases which Sydenham had observed to be counterfeited by hysteria," says an able writer in Rees' Cyclopaedia (art. Hysteria), "are apoplexy, which terminates in hemiplegia, and is chiefly seen in lying-in women, after difficult parturition attended with great hemorrhage: the clavus hystericus, or acute pain in one small spot in the head, which is often accompanied by vomiting and palpitation of the heart; this is most frequent in delicate and chlorotic girls: pain in the stomach, cholice, with pavoraceous vomiting, resembling the iliac passion, terminating in jaundice, and always accompanied by great despondency; chiefly attacking women of lax fibre, who have suffered severe labours from large children: pains in the kidneys and bladder, like fits of the stone, with suppression of urine: occurring in women much debilitated by previous hysteric fits, and in a bad state of health, who are also liable to long-continued vomiting and diarrhoea without pain, discharging green bile. He likewise remarks, that the disease is liable to attack the external parts and muscular flesh, sometimes causing pain, sometimes swelling, in the throat, shoulders, hands, thighs, and legs, more especially in the last, which swelling, however, is to be distinguished from oedema, by being greatest in a morning, by leaving no pit after pressure with the finger, and by commonly affecting only one leg, though its external appearance much resembles that of oedema. But the most severe of those pains is that which affects the back. Sydenham adds, that these pains have this circumstance in common, that they all leave the parts extremely sore and tender, as if they had been severely beaten, so that they cannot bear the touch for some time; and that these pains and other symptoms are all preceded by a death-like coldness of the external parts." (Diss. Epistolaris, edit. sec. Londini, 1685, p. 124, seqq. Works, ut supra cit. p. 370.)

Dr Ferriar, in his essay on the Conversion of Diseases (Med. Hist. and Reflections, ii. 17), in speaking of hysteria, remarks that, "whoever would present us with a good book on the fallacy of symptoms, which is greatly wanted, must be completely master of this unaccountable disease." "I have met," says he, "with several cases of hysterical haemoptoe, in which the quantity of blood evacuated was very considerable; six or eight ounces were sometimes spit up daily, for a fortnight or three weeks successively. Most of the usual symptoms attended which denote danger in this complaint when it arises from other causes; but the equal, moderate state of the pulse, and the appearance of some degree of the globus hystericus, seemed to determine the nature of the complaint; a conversion, accordingly, soon took place to the ordinary hysteric paroxysm, and no bad consequence followed the hemorrhage from the lungs." "In one case," says the same author, "the bowels were attacked, and the symptoms of enteritis (inflammation of the bowels) were so precisely imitated, as to give much alarm for the patient's safety. I suspected the real nature of the disease from observing that the pulse was soft and full, that the evacuations were natural, and that her spirits were agitated, even to involuntary emotions, by slight causes. This case terminated successfully on the accession of clear hysterical symptoms." "Several years ago, I attended an elderly lady, for a complaint which seemed to vibrate between apoplexy and palsy. After lying for several weeks in a state which afforded little hope of amendment, she saw affected with involuntary sobbing and weeping; the complaints in her head and limbs were converted into hysterical convulsions, and she recovered completely. It is very common to meet with syncope, or palpitations of the heart and great vessels, accompanied with a soporific depression, or extreme dejection of strength and spirits, and converted, after deep sighing or discharge of tears, into the hysterical paroxysm. In these cases the pulse is sometimes full and regular during the most alarming appearance of sinking; and sometimes variable to such a degree as to exclude all conjecture, excepting that of an hysteric origin." (Pp. 29, 30, 32.) It would be easy to swell to a large amount our quotations illustrative of the great variety of diseases which hysteria occasionally simulates. But we must content ourselves with referring to the account which Dr Marshall Hall has given in his work on Diagnosis (p. 214), of hysteric croup and hysteric pleurodynia; to some cases of hysteria imitating inflammatory action, paralysis, &c., which are related by Dr Bright in his very valuable Clinical Reports (ii. 452); and to a selection of cases presenting aggravated and irregular forms of hysteria, &c. by Mr Laycock, published in the 134th and subsequent numbers of the Edinburgh Medical and Surgical Journal.

In some cases it is not a single form of disease only of which an hysterical state of the constitution induces the phenomena, but several different forms of disease may be simulated in succession. "When the hysteric disposition is set in motion," says Dr Ferriar (p. 31), "it is not uncommon to find many of the different viscera attacked by it in turns, and the disease peculiar to each counterfeited with much exactness. I have seen symptoms of paralysis, jaundice, palpitation, and nephritis, succeed each other rapidly in the same patient, while some of the characteristic marks of hysteria have been discernible, and where the unity of the disease was proved, by the disappearance of all menacing affections on the approach of regular fits."

It is well known to medical men that there are certain diseases which possess what is called a specific character, of distinction, which depend on the introduction of a peculiar poison into the system, animal or mineral, such as those of syphilis, and of lead, or on a peculiar state of constitution, sometimes hereditary and sometimes acquired, such as cancer. These diseases, in their general features, resemble affections of a simple or non-specific character. Hence it is not frequently a matter of great difficulty, when affections present themselves resembling in their symptoms those which are known to be produced by specific diseases, to determine whether or not there exists in the constitution such a modifying influence as should communicate to these affections a specific character, or whether they be the results of diseased action in a constitution not peculiarly modified. Thus, it is well known how much controversy has arisen among medical men as to the possibility of distinguishing the various symptoms of syphilis from analogous affections occurring independently of the operation of the syphilitic virus. "There is hardly any disorder," says Mr Hunter, "that has more diseases resembling it in all its different forms than the venereal disease." (On the Ven. Dis. p. 380.) In the same way, it is, in many instances, impossible to judge, by the symptoms which present themselves, whether an affection, as of the breast, of the lymphatic glands, of the tongue, &c., ought or ought not to be considered as cancerous; and it is only the melancholy distinction of incurability that ultimately clears up the diagnosis. Again, when lead is introduced into the economy, as it may be in a variety of ways, it is liable to give rise to a certain combination of symptoms which, from the predominance of abdominal pains, and the retraction of the belly, bilious vomitings, and obstinate constipation, with which it is accompanied, is usually designated lead colic. But the symptoms of lead colic, as M. Andral has observed, sometimes manifest themselves in per- sons who have not been subjected to the influence of any saturnine preparation. (Malad. de l'Abdomen, p. 507.) To the same effect, Dr Graves, in the conclusion of a clinical lecture, published in the Lond. Med. Gaz. for 26th February 1831 (vi. 675-6), has suggested, "that perhaps the following general law will be found applicable to the effects which poisons produce on the constitution, viz. that all poisons give rise to a train of symptoms, constituting a certain disease, of which we may always find a counterpart among those diseases which arise in the system from other causes. Thus, symptoms exactly resembling those of colica pictorum (or lead colic) may occur where no lead has acted on the system; and the remarkable epidemic paralysis of the extremities observed in Paris in 1828 bears a striking resemblance to the paralysis caused by lead. If time permitted me to enter upon this subject," adds Dr Graves, "I think I could succeed in bringing forward examples of morbid states of the system, in which symptoms similar to those arising from mercury, arsenic, narcotic poisons, &c. are produced totally independent of the ingestion of these substances."

The resemblance in the groups or trains of symptoms produced by different pathological conditions, which is liable to embarrass the practitioner, or lead him into error in forming his diagnosis, exists in some instances only during a particular period of the course of the disease, as at its commencement or termination. Thus, for example, a great many diseases, in their incipient stages, exhibit the general characters of fever, without any symptom manifesting itself by which the physician can be enabled to foretell whether, in its progress, the disease will prove to be of the nature of simple fever, or one of the various forms of febrile eruption, or whether an inflammatory affection is about to develop itself in one or other of the internal organs of the body. Unless there happen to be some adventitious circumstances, by which in such cases the judgment can be assisted, as, for example, the presumption afforded by the knowledge of exposure to a specific contagion, the practitioner must wait for a more advanced stage of the malady before he can pronounce with confidence upon its nature.

M. Pinel, in noticing some remarkable varieties which mania presents at its first commencement, remarks, that "the extreme intensity of the symptoms may sometimes lead to error, and the name of mania may be given to a malignant or ataxic fever; for, besides that the same causes may equally produce one or other of these diseases, they may also exhibit common characters, violent agitations, furious delirium, alternations of stupor and nervous excitement; sometimes a marked acceleration of the pulse, dryness of the tongue, a pale and exhausted countenance, slight and interrupted sleep, and an entire rejection of all nourishment. The distinction is then difficult to make at the commencement, and it is only by watching the disease for several days that the practitioner can arrive at a positive conclusion." (Sur l'Aliénation Mentale, Paris, 1809, p. 59-60.)

As there is in many instances a close resemblance in the symptoms by which different diseases commence, so also it frequently happens that diseases of different natures, at an advanced stage of their course, give rise to some determinate group of symptoms which mask or conceal those of the primary disease, and themselves assume the prominent character. Thus various organic diseases situated in different organs or parts are liable to induce, at an advanced period of their progress, the state of dropsy; and when the dropsical effusion has once attained a considerable extent, it frequently masks the symptoms of the original disease so much as to render it difficult, if not impossible, to ascertain its nature, from the consideration of the existing symptoms. It would appear, however, from the results of recent clinical observation, that if the practitioner is able to ascertain in what situation the dropsical effusion commenced, and to trace its progressive extension to other parts of the body, these circumstances of themselves often afford sufficient data for determining in what organ the morbid condition is situated on which the production of the dropsy depends.

So much for the circumstances under which the practitioner may be embarrassed or misled in his judgments respecting the seat and nature of diseases, from resemblances in the group or train of symptoms produced by different pathological conditions. The next source of difficulty and fallacy in the diagnosis of diseases which we propose to illustrate, arises from the fact that the group or train of symptoms produced by a particular pathological condition, or concourse of pathological conditions, in one individual, may be very different from the group or train of symptoms produced by an apparently identical lesion in another individual. This production of two or more different groups or trains of symptoms in different individuals, by a pathological condition or proximate cause that seems to be identical, may occur, as will appear from the illustrations which we shall adduce, whether the pathological state or lesion be organic or simply dynamical.

The diversity in the group of symptoms attending a particular morbid condition, which misleads a practitioner in his diagnosis, may extend to the whole or greater part of the symptoms, or be confined to one symptom, or to a few only. In the latter case the deviation from the ordinary characters of the disease may consist either in the presence of a symptom or symptoms that do not usually attend the morbid condition which exists, or in the absence of symptoms by which it is usually accompanied. The disease drops, as it were, some of its usual characters, or assumes some that appertain to other diseases. "A disease," says M. Rostan (Méd. Clin. ii. 355), "without being entirely latent, may want many of its characteristic symptoms; and this circumstance must occasion great difficulties in diagnosis. This is of frequent occurrence in cerebral diseases. Thus, when softening of the brain is unattended by precursory symptoms, its diagnosis is of extreme difficulty. 'Cerebral haemorrhage,' says the same author, 'may exhibit precursory symptoms, and cerebral softening want them. How, in such a case, can these two affections be distinguished?' Pain being a symptom to which the attention of the practitioner is always forcibly directed by his patient, much reliance is usually placed on its presence or absence in the diagnosis of diseases, and particularly in discriminating between those which are and those which are not inflammatory. It is far, however, from being an infallible sign. 'The signs of the different alterations of which the kidney may be the seat are,' observes M. Bouillaud (Journ. Compl. xxxi. 13), 'very obscure. All that semiologists have said on this subject is far from being exactly conformable to observation.' We may easily conceive why the diagnosis of affections of the kidney presents so many difficulties, if we reflect, 1st, that the depth at which that organ is situated withdraws it almost entirely from our means of exploration; 2dly, that the disturbance of its functions produces phenomena which may equally be occasioned by different diseases of the bladder and ureters; 3dly, that pain is not a constant accompaniment of renal affections. If we trusted to most pathologists, the most acute pains should be almost inseparable from inflammation of the kidneys. I do not pretend to deny that this may sometimes be the case, but I can affirm that I have met with incontestable traces of nephritis in persons who had not complained of any pain in the region of the kidneys. This absence of pain in the diseases of these organs is, besides, the more easily conceivable, that they are not sensible in the state of health. It may be added, that it is not very uncommon to observe violent pains in the region of the kidneys in persons who are not affected with any disease of these organs." No circumstance certainly tends more to perplex the medical observer, in attempting to determine, from the consideration of the symptoms that manifest themselves in individual cases, what the disease is which presents itself to his observation, than the great diversity which a disease may exhibit in its phenomena, according to a variety of circumstances not at all times easily appreciable; such as atmospheric influence, individual constitution, or, perhaps, mode of treatment. Those diseases that arise from the action of a contagious principle afford us the best opportunity of noticing the great diversities of appearance which the same disease may exhibit in particular epidemics, and as modified by diversity of constitution, mode of treatment, &c. If we compare, for example, the severest and the mildest case of typhus fever, or of smallpox, or scarlet fever, we shall have difficulty in believing that they are instances of the same diseases. Yet, in examining a considerable number of cases of any one of these diseases, we shall be satisfied, that between these extreme points there is a series of intermediate links which exhibit the most gradual transition from the most mild to the most severe.

In the diseases to which we have just alluded, we are unable to detect after death, when it occurs, any structural morbid conditions of the different textures, systems, and organs of the body, as being constantly and necessarily present, so as to justify us in regarding them as the proximate cause of the symptoms that manifest themselves; and this circumstance must no doubt add considerably to our difficulty in determining what are the essential and characteristic symptoms by which we should be able to recognise these diseases under the great diversity of appearances which they present in particular cases. But even in diseases in which we meet with structural alterations on examination after death, and these too having proceeded, in some cases, to a very considerable extent, we often find that there had been great diversity in the groups or trains of symptoms by which, in different individuals, the same structural change had been attended.

The various affections to which the contents of the cavity of the cranium are subject, furnish numerous illustrations of the fact that the same structural morbid condition may be attended in different cases by different groups or trains of symptoms. M. Andral, in his Resumé (Malad. de l'Encephale, p. 522) of the cases which he has published illustrative of softening of the hemispheres of the brain, observes, "In showing us the softening of the brain under all its forms, both anatomical and symptomatical, the preceding cases have shown us also how far the diagnosis of this affection is from being in all instances easy." "In each of the cases, it is not only that the symptoms are not the same, but that the same symptoms are combined, or succeed each other in the most different ways. Hence there result for one particular anatomical lesion, several forms of disease, which, in a nosological table, might be placed at a great distance from one another. In some cases the nosological disease is what is called an attack of apoplexy, with or without loss of consciousness; in others, it is a so-called ataxic or malignant fever; and in others, again, it is a gradual diminution either of motion only, or of intellect and motion at the same time."

To the same purpose Dr Abercrombie remarks, that "when we endeavour to trace the leading symptoms connected with the different forms of organic diseases of the brain (comprehending under that name permanent changes of the cerebral substance itself, and new formations within the head), we do not find any uniformity, by which particular symptoms can be distinctly referred to the various forms of the morbid affections." He afterwards alludes to the extreme difficulty of fixing upon any general principles, or of referring the particular character of the symptoms to anything in the seat or nature of the disease. "Tumours, for example, will be found, in one class of cases, unaccompanied by any remarkable symptoms; while, in other cases, tumours in the same situation, and of no larger size, are accompanied by blindness, convulsions, or paralysis. It does not appear that these diversities depend either upon the size of the tumours, or, as far as we know at present, upon their particular structure. But these points remain to be investigated; particularly what diversity of symptoms is connected with the nature of the tumours, and especially with their characters, as being tumours distinct from the cerebral mass, or as being indurations of the substance of the brain itself." (On Diseases of the Brain, third edit. p. 316, 322-3.)

In the viscera of the chest, as well as in those of the thoracic morbidity, examples are to be met with of the same structural diseases, morbid condition giving rise, in different persons, to different combinations of symptoms in concourse and succession. Thus, Dr Latham, in illustrating his observation, that one of the common difficulties attending the diagnosis of pericarditis is referrible to the variability of its own symptoms, remarks (Lond. Med. Gazette, iii. 211): "Disease belongs most unequivocally to the heart when pain is immediately referred to it, and when its action is disturbed. And when there is fever withal, it bespeaks the essence of the disease to be inflammatory, and fixes it, almost to a certainty, in the pericardium. But why in the pericardium more than in other parts of the heart? Because it is matter of experience, that, above all others, the pericardium is liable to acute inflammation in an incalculable proportion. But, strange to say, the pericardium may be acutely inflamed, and yet there may be no pain. And the disturbance of the heart's action is so little of one kind in all cases of pericarditis, that no certain mode of disturbance can be relied upon as a diagnostic symptom; and though fever may be present wherever there is pericarditis, there is often, at the same time, inflammation of some other organ, out of which it may arise."

Nor are the viscera of the abdomen more constant in respect of the symptoms by which their several structural pathological conditions are attended, than those of the head and chest. Dr Coe, after describing the ordinary signs of biliary concretions when they are making their way through the ducts, remarks (ibid. supra cit. pp. 145-6, 147-8) that these "do not happen precisely alike in all persons, but will be found to vary in different patients, and even in the same person at different times, as well as to the number of the symptoms as the degrees of them, and the order in which they come on. Which variations," he continues, "will arise from several causes (in like manner as we see happens in other diseases), according to the difference of constitutions and other circumstances, and according to the size and shape, and perhaps other properties, of the calculi, as also of the ducts through which they pass. From which variation of the symptoms there sometimes arises an obscurity and uncertainty in judging of particular cases. So that we may have good reason to suspect that the disorder is occasioned by a stone, though we cannot be clear enough to pronounce that it is so; which at other times we can do with sufficient certainty." "As in the more uncommon and less known diseases, some particular cases may be so strong and clear as to be sufficiently obvious, so in some of the most evident diseases, which are readily known at first view when the train of symptoms is regular and uniform, we sometimes meet with so great irregularity, and such a perplexed combination of circumstances, that we are at a loss to unravel them, and find some difficulty in determining the particular nature of the case."

From the illustrations that have been given of the two last-mentioned sources of difficulty and fallacy in diagnosis, it must be obvious that, in practice, they frequently occur combined with one another. A concourse of symptoms which we shall designate as A, may arise from two different lesions y and z; whilst at the same time the lesion y, besides giving rise in one individual or class of individuals to the train of symptoms A, may in another individual give rise to a different train B. When, therefore, A presents itself to our notice, we may suppose it to depend on y in cases in which it actually depends on z. And when B presents itself to our notice, we may fail to detect its dependence on y, from its not occurring to us that this lesion can exist without being attended by the symptoms A. "If inflammation of the contents of the cranium always announced itself by the same symptoms," says M. Charpentier, "or if the symptoms by which it usually announces itself did not sometimes depend solely upon simple irritation reflected upon the brain by some organ or organic texture in a state of inflammation, its diagnosis would never be very difficult. This, however, is not the case, for inflammation of these parts occasionally differs so much from its ordinary physiognomy, both in respect of its course and of its symptoms, that the opening of the dead body can alone reveal its existence; whilst at other times simple sympathetic irritation of the brain deceives so much by the perfect resemblance of its symptoms to those which meningo-cephalitis (inflammation of the membranes and substance of the brain) presents, that the sight of the un-inflamed condition of these parts can alone undeceive us as to the nature of the disease. Among the number of affections capable of inducing cerebral symptoms which by their concurrence simulate meningo-cephalitis, we must particularly reckon the presence of worms in the intestines, dentition, pneumonia at its height, but, above all, inflammations of the stomach and small intestines." (De la Nature, &c. de la Maladie dite Hydrocéphale Aiguë, Paris, 1829, p. 195.) M. Rostan, in showing that pain, vomiting, and swelling form the principal characters of cancer of the stomach, remarks (Médicine Clinique, ii. 477), "These symptoms, however, may exist, and the disease (that is, the pathological condition of cancer of the stomach) not exist, and vice versa, that is to say, the symptoms may be absent, and yet the disease exist. And, in the first place, pain appertains to all diseases of the stomach, and cannot be regarded as a special sign of that in question; the same may be said of vomiting; and, lastly, swelling may appertain to a multitude of different lesions. Engorgement of the liver, of the spleen, or of the pancreas, the accumulation of fecal matters in the arch of the colon, aneurismal dilatation of the aorta, and other accidental tumours, may so much the more readily pass for cancer of the stomach, that most of them occasion disturbance of the digestive functions. In the second place, the pain may not exist, as happens in cases of indolent cancer, which, as is well known, are not uncommon. The vomiting, also, may not occur, 1st, when the pylorus is free; 2d, when it is extensively ulcerated; 3d, when the whole of the stomach is involved in the disorganization. The swelling may not be sensible, which, so far as regards diagnosis, is the same as if it did not exist, 1st, when the false ribs and sternum descend very low, and cover it; 2d, when the free border of the liver lies over it; 3d, when the subject is pretty stout; 4th, when the abdominal muscles contract," &c. "Notwithstanding these difficulties," adds M. Rostan, "it is in general possible to arrive at a precise diagnosis, because it rarely happens that they occur in sufficient number entirely to mask the disease."

But although a train of symptoms apparently identical may arise from different morbid lesions, and different trains of symptoms may arise from what appears to be an identical lesion, it is possible that in neither case does there actually exist the identity supposed. For, notwithstanding the apparent resemblance of the symptoms arising from different lesions, it seems reasonable to suspect that there may exist points of difference between these symptoms, if we were capable of detecting them. And, accordingly, the progress of improvement in diagnosis has been in numerous instances brought to light symptoms or marks by which we are enabled to distinguish between different morbid lesions of the economy, that had previously been supposed to produce precisely the same trains of symptoms.

Of many nosological diseases, such as asthma, ileus, and jaundice, it may be said that they afford examples of the same group or train of symptoms, arising from different pathological conditions. Asthma, for instance, understanding by that term "difficulty of breathing returning at intervals, with a sense of oppression in the chest and sibilous respiration, cough difficult at the commencement of the paroxysm, or absent, towards the termination free, with mucous expectoration, frequently copious," may proceed from diseases of the respiratory organs, or of the heart and large vessels, or of the organs belonging to the nervous system. But though on a first view it may appear true in respect of each of the nosological diseases that has been mentioned, and of many others that might be enumerated, that different lesions give rise to the same group or train of symptoms, on more accurate examination it will generally be found possible to detect, in the cases which depend on one pathological condition, symptoms that are absent in the cases which depend upon a different pathological condition; or, though the symptoms of the two sets of cases are the same in their general nature, one or more of them may be found to be differently modified. "We must at all times," observes M. Dalmas, "conform to that rule of logic which teaches that one same effect always results from one same cause. If, in cases which have exhibited the same symptoms during life, we meet, after death, with different lesions, we may be sure that the true seat of the disease has escaped our researches. It is proper to remark here, however, that there are some vague words employed in symptomatology to express general results, for which it would be unjust to pretend that, in conformity with the rule alluded to, we ought always to find a single and identical lesion. The words dyspepsia and palsy are so circumstanced. If there be twenty species of dyspepsia, we need not be astonished if we find as many different lesions capable of producing it." It is upon such diversities in the minor features of particular sets of cases, depending on differences of pathological condition, that nosologists found, or should found, their varieties of diseases; and it is by the accurate marking of these diversities that they should seek to improve their systems of definitions.

In like manner, it seems probable, that when different groups or trains of symptoms arise from what appears to be the same morbid lesion, there is in reality some unperceived specific diversity in the lesion. It may be identical in respect of its nature in the different cases, but different in its seat or in its relations to other parts. For, supposing that in two or more cases the same organ is affected with the same form of disease, it is obvious that there may be considerable difference in the functional disturbance, and consequently in the symptoms that arise, according to the particular part of the organ in which the pathological condition may be seated. In cases of cancer of the stomach, for example, the symptoms vary greatly according as one or other aperture, or the body of the stomach, are affected separately, or in conjunction with one another.

In illustration of the remark, that where the same lesion appears to give rise to different symptoms, there may actually be some difference in the relation of the lesion to other parts, though not adverted to by a superficial observer, we may quote an observation made by M. Cruveilhier in reference to cancerous tumours in the brain. (Anatomie Pa- "A point extremely important in the history of these tumours," says he, "is the difference of the effects produced by causes apparently identical. Thus, in one individual, hemiplegia is occasioned by a spheroidal tumour of two inches in diameter, while in another a tumour of four inches may exist upon the brain without any paralysis being induced, though everything is in other respects correspondent as regards the seat and nature of the disease. I think," adds this intelligent author, "that one of the circumstances to which this diversity of result is attributable is the difference of the direction in which the compressing power acts."

M. Lallemand has with much justice observed, in the preface to his work on Diseases of the Brain (p. xvi.), that "The circumstance which has most influence on the course of symptoms, is the more or less rapid, the more or less slow course of the disease. This influence is such as to cause that very different pathological alterations manifest themselves by phenomena sufficiently similar to deceive the most attentive observer, simply from their corresponding in their general course or progress. Thus, for example, there is more resemblance between the symptoms of an acute inflammation of the brain, and of a softening of the cerebral substance, and those of an apoplexy, or sudden compression of the brain, whatever be its cause, than between the symptoms of an acute inflammation and those of a chronic inflammation of this organ. In the same way, an encysted abscess has less resemblance, in respect of the external phenomena which it produces, to a recent abscess, than to a scrofulous, fibrous, bony, or hydatid tumour, or a foreign body lodged in the brain. The same," he adds, "may be said of the affections of the arachnoid membrane."

"Reason and common sense," observes Dr. Latham, "seem to declare, that there cannot be the same disease of the same organ, and yet the most prominent symptoms be absolutely different. But morbid anatomy is not content merely to contemplate a disease in its general character, and have done with it: it takes into account also the various morbid products and changes of structure accompanying it, or resulting from it. In pericarditis it takes cognizance of the lymph deposited upon the surface, and of the fluid effused into the cavity of the pericardium; the different quantities of each in different cases, and the different proportions which they bear to each other. It notices, also, that the lymph sometimes contracts a partial and sometimes a complete adhesion, and sometimes none at all; that the fluid is sometimes freely effused into the pericardium, and envelopes the whole heart; and is sometimes limited by adhesions to a part of it. These conditions must influence the action of the heart in various ways; and morbid anatomy, knowing these things, suggests to clinical observation, with respect to pericarditis, that as its products are different, so too must its symptoms be in different cases, and even in the same case at different periods." (Lond. Med. Gaz. iii. 212.) These observations are well calculated to excite the most minute attention, on the part of medical observers, to the less obvious differences in individual symptoms or groups of symptoms produced by different lesions, and to the less obvious differences in particular lesions which seem to give rise to different symptoms or groups of symptoms.

The last source of difficulty and fallacy which we have to notice as occurring to the medical practitioner, in attempting to judge, from the symptoms that manifest themselves in any particular case, as to the seat and nature of the disease with which his patient is affected, arises from the circumstance, that two or more parts may be the seats of morbid conditions in the same individual at the same time, whether these morbid conditions shall correspond or differ in their nature, and whether they shall have come on simultaneously or in succession. Such a complication of pathological conditions will possibly produce a complication of symptoms which may render it very difficult for the practitioner to ascertain how many parts are disordered, where producing the principal seat of disorder is, what part was primarily affected, and how far the disorders in the others depend upon that of the part first affected. Thus, if we suppose an individual to be labouring at the same time under the symptoms unravelled of disorder of the stomach and of disorder of the heart, the practitioner may experience considerable difficulty in determining whether the symptoms of deranged digestion and those of disordered circulation are referrible to two distinct primary morbid conditions, or whether one or other of these groups of symptoms is not produced sympathetically; whether, whilst one of the groups arises directly from a primary morbid condition of the corresponding organ, as those of indigestion from disease of the stomach, the other group of symptoms, those of disordered circulation, may not depend on sympathy of the heart with the stomach, rather than on a primary morbid condition of the heart itself.

In the case just supposed, the co-existence of two distinct pathological conditions in the economy is obviously accidental, there being no necessary bond of connection between them. But in other instances, from the dependency of different organs and functions upon one another, the existence of diseased action in one organ may occasion diseased action (whether of a simply dynamical or of an organic character) in another. Thus, it is now well established, that in a large proportion of cases of disease of the heart, the change of structure or of action which this organ undergoes is occasioned by an interruption to the circulation of the blood through some other organ of the body, as, for example, through the lungs, in consequence of previous disease of these viscera. Hence, in the progress of cases of this kind, the symptoms belonging to the organ in which the impediment to the circulation exists, come to be complicated with those immediately referrible to the altered condition of the heart, occasioned by that impediment.

The reciprocal dependency of the stomach and liver on one another furnishes us with another example of complication of morbid lesions producing complication of symptoms. "It must be admitted," says Dr Cheyne (Cases of Apoplexy, &c. Lond. 1812, p. 171), "that it is often difficult to know to which viscus of the abdomen we ought to refer the uneasy sensations that attend imperfect digestion. When disease of structure is apparently confined to the liver, the patient has been known to suffer greatly from dyspeptic symptoms (that is, symptoms of derangement of stomach)—great anxiety, sickness, flatulence, and prostration. And, on the contrary, when the stomach alone appears to be the seat of disease, there is not unfrequently an imperfect or redundant secretion of bile (indicating derangement in the action of the liver), with listlessness, depraved sensation, tedium vitae, &c. The work of the stomach is never well performed unless when the liver is sound, and the functions of the liver equally require a healthy condition of the stomach. As when either viscus is in a diseased state, the uneasy sensations felt in consequence are referrible to both, the stomach and liver are to be considered, in their morbid states, as firmly linked as in health. But in practice it is of consequence to discover in which organ disease has begun; for, if we are able to restore the organ primarily affected, the organs sympathetically drawn into disease will often return to a healthy state in consequence."

The supervention of a new morbid condition in a person previously labouring under some complaint, whether in the same or in a neighbouring organ, is very liable to be overlooked till it has made considerable progress, in consequence of the premonitory and early symptoms being suffocated, as it were, in those of the original malady. And if in a case in which there exist two or more distinct pathological conditions at the time of the physician's examination, the morbid phenomena or symptoms produced by one of them be, from the nature or the seat of the organ affected, much more apparent than those produced by the other, then it may very readily happen that the practitioner, having satisfied himself of the existence of the more considerable pathological condition, may take it for granted that this is in itself sufficient to explain all the phenomena that present themselves, and thus fail to detect the existence of the other pathological condition, upon which the less marked, though perhaps not the least important, portion of the symptoms actually depends.

But it is not the mere circumstance of complication, and the difficulty of separating the symptoms arising out of different co-existing pathological conditions, into their respective groups, that render cases of this kind puzzling and perplexing to the practitioner. It frequently happens that the symptoms of one or other, or possibly even of both, the co-existing pathological conditions, are modified in such a degree as to throw over them a great degree of obscurity. "In many diseases," says M. Melier (sur le Diagnostique), "instead of a single organ, or of a single texture, there are two, or a still greater number, affected. It rarely happens, for example, that a mucous membrane is alone affected; it is the same with the skin. It is necessary to attend to this coincidence of several diseases, or rather of several organic foci of disease; for in almost all cases the physiognomy of the symptoms is thereby changed. Two diseases existing in this way together, are far from proceeding as if they existed separately. On the contrary, they modify one another. Sometimes they mutually diminish one another's intensity; at other times one of them, in running through its stages, preserves a preponderating intensity, whilst the other, restrained, so to speak, in its development, advances slowly. The first only attracts attention; but when once it has terminated, the second, which, up to that time, had been slight or unperceived, is awakened, and excites fresh symptoms, which the existence of the other seems to have prevented; as if there were in our economy, as Bichat said on this subject, only a certain amount of strength which cannot exist in excess in one place without being diminished in another, and that nature does not indulge in the development of several diseases at the same time."

"It rarely happens," observes M. Lallemand (p. xix. xx.), "that a patient dies of a simple and single affection. In almost all cases, several organs have been involved simultaneously or successively. Then the symptoms of the one or the other disease predominate; the others are very obscure; and, in all cases, these complications influence one another reciprocally, so as to give an unusual aspect to the concourse of symptoms, and to the progress of the diseases. Formerly, in the examination of dead bodies, attention was paid only to the organ which was supposed to be diseased; and when the alterations did not correspond exactly to the symptoms, medical men were contented with regarding the fact as anomalous; or they supposed they had explained it by attributing it to a peculiar idiosyncrasy; or, better still, they made an abstract being of it, existing by itself, and called it an essential disease. The progress of the different parts of medicine, the exactness that is now bestowed upon the observation of symptoms, and the attention that is given to the examination of dead bodies, enables us to appreciate more accurately than was possible till recent times, the influence of each of these complications on the course and concourse of the symptoms."

In some instances the co-existing pathological conditions, each of which furnishes its particular symptoms to the resulting concourse, are seated in the same organ, or in textures immediately adjacent; as in the investing membranes of parenchymatous organs. "Next to those diseases of the brain," observes M. Rostan (Med. Clin. ii. 355-6), "which do not exhibit any symptom, or what are called latent diseases, there are none more obscure than those which are complicated with one another. Thus, when there exist at the same time several different alterations of the brain, as haemorrhage and primary or consecutive softening, it is almost impossible accurately to determine the nature of the affection; but as in general we recognise at least one of the alterations, these cases cannot rigorously be regarded as errors." M. Lallemand, in remarking that the pathological alterations of the brain are not always accompanied with the same symptoms, and in pointing out that "this apparent disagreement depends sometimes on the different seat of the affection, sometimes on the age, on the sex, or on the temperament of the patient; sometimes on important complications; and, lastly, at other times on the acute or chronic course of the disease;" adds, "when we compare together cases of affection of the brain and of the arachnoid membrane, exempt as much as possible from complication, we see that they exhibit symptoms very distinct, a particular character (cachet); whilst those cases that are complicated, as is most frequently the case, present a mixture of the two orders of symptoms, a circumstance which has not been sufficiently attended to, and which has frequently led observers into error." (Preface, p. xvi.) The same observation has been specially applied by Professor Van Coetsem to the complication of one particular form of cerebral disease, viz. softening, with inflammation of the arachnoid or serous membrane of the brain. "One of the causes," says this author (ib. cit. p. 374), "which seems to us to render the diagnosis of cerebral softening so obscure, is the complication of arachnitis (inflammation of the serous membrane of the brain), sometimes of an acute and sometimes of a chronic character, which frequently accompanies or precedes the morbid degeneration of the substance of the encephalon, and in other instances succeeds to it. In fact, it has more than once happened to us, after conceiving that we had recognised the nature and the seat of the disease by its symptoms, to find in the bodies of those who have died, independently of the lesion of the substance of the brain, which we expected, unequivocal traces of inflammation of the serous membrane that surrounds this viscus, such as adhesion between the different laminae of the arachnoid, sero-purulent effusion into the lateral ventricles, &c. In such circumstances the facies mortis appears to us very doubtful, in respect of diagnosis, because the symptoms connected with each of the co-existing affections are far from being characteristic."

In other instances the different pathological conditions which exist simultaneously have their seats in distant organs. When one of these organs is the brain, the symptoms that might be expected to arise from the diseased condition of the other organ are exceedingly liable to be obscured. "The diseases of the brain," says M. Bourdon (Principes de Physiologie Médicale, i. 164), mask the diseases of other organs. Far from requiring opium, cerebral affections serve as such to the whole body." M. Lallemand, in speaking of the influence of cerebral affections on the development of the symptoms of such diseases as may exist simultaneously with them, observes (i. 233), "Whenever the functions of the brain are disturbed, and there exists at the same time an inflammation in some other organ, the pain produced by that inflammation diminishes, and even entirely disappears." "It is not," he adds, "because the pain of the head is greater, that that of the abdomen or of the chest, &c. disappears, for frequently there is no headache in these cases. Nor is it because the inflammation in the brain is more severe, seeing that an effusion of serum or blood may produce the same effect; it is because the pain being the result of a sensation perceived by the brain, whatever deranges its functions will cause this symptom of the inflammation to disappear. But if the inflammation is severe, it is not arrested in its development by the cerebral affection; all the other phenomena continue, because they are not, like the sensibility, dependent on the brain. The disease runs through its course in a manner so much the more dangerous, as it is the more difficult to be recognised, and produces the death which is attributed to the affection of the brain, though this be in reality only accessory. If the body be opened with some attention, we are surprised to find so little relation between the symptoms observed during life and the morbid alterations found after death; and this misapprehension throws discredit on medicine, and on pathological anatomy in particular. It is, therefore, of the greatest importance, whenever we observe any symptom which indicates an alteration in the functions of the brain, not to allow ourselves to be imposed upon by the absence of pain. We must then investigate with so much the greater attention the other phenomena, which, being independent of sensibility and volition, do not vary with the state of the brain." "These proper symptoms are so much the more precious, that, if alterations of the brain mask, so to speak, certain inflammations of the stomach, by causing the disappearance of the pain, it also happens very frequently, by a singular sort of compensation, as it were, that certain cerebral affections produce sympathetically nausea and obstinate vomiting, which might be regarded as symptoms of inflammation of the stomach." "These considerations on the perception of pain apply to all inflammations complicated with cerebral affection."

The influence of delirium in concealing the symptoms of diseases seated in other organs than the brain must be explained upon the same principles. In reference to a case of fever in which, as appeared on examination after death, there occurred perforation of the small intestine, followed by inflammation of the serous membrane of the abdomen, but in which the usual symptoms of these morbid conditions did not manifest themselves, M. Louis takes occasion to observe, that "the most formidable symptoms are usually so completely masked by delirium, that as soon as it declares itself, the diagnosis of diseases that are most easily recognised becomes difficult, and sometimes even impossible." (Recherches Anatom. Pathologiques, p. 187.)

In treating of "ramollissement (softening) of the central parts of the brain, combined with serous effusion," Dr Abercrombie remarks, "an interesting modification of this affection is that which supervenes upon other diseases, chiefly of a chronic nature, especially affections of the lungs; and the remarkable circumstance in these cases is, that the pectoral symptoms generally cease after the commencement of the symptoms in the head." (On the Brain, 3d edit., p. 137.)

M. Andral, in speaking of the varieties of pneumonia, or inflammation of the substance of the lungs, observes (Malad. de Poitrine, i. 369), that some very important modifications of this disease depend on the absence of one or of several of its usual symptoms, whence results latent pneumonia. "It is well established," he continues, "that an intense inflammation of the parenchyma of the lungs may exist without being announced by dyspnoea, cough, or expectoration. It may sometimes happen, even, that at the same time that these symptoms are wanting, auscultation and percussion themselves cease to reveal the true state of the lung; in this case it becomes impossible to establish a diagnosis. We have already remarked that these latent pneumonies are rarely primary, but generally supervene when an inflammation of the lung complicates some other disease." "We have seen," remarks the same author (Fièvres, p. 441), "in recapitulating the lesions discovered by opening the bodies of the dead, that in severe fevers pneumonia was a very frequent complication, and most frequently a fatal one. If we pass in review the symptoms which have announced this inflammatory affection, we shall find that, in most of the cases, the diagnosis has been very obscure. These intercurrent pneumonic affections have frequently not been announced by any apparent oppression of respiration, nor by any characteristic expectoration. The cough itself was very slight, and the disorganisation of the lung might be completed before even any suspicion of the existence of pulmonary inflammation was excited. Hence the indispensable necessity of frequently practising percussion and auscultation in such cases." Lastly, we have seen these two modes of investigation become themselves useless, when the inflammation was settled in the centre, the base, or roots, of the lungs. The absence of all characteristic expectoration in these cases of intercurrent pneumonia may often depend on the circumstance that the patients, having reached the last degree of debility, swallow their expectoration, when they have no longer the strength or instinct to spit it out. But sometimes also it appears that there is really no secretion, and this seems to occur particularly in cases of splenisation, or putaceous softening of the lungs."

We have thus endeavoured to illustrate the various circumstances under which it may happen that, in the examination of a patient, a well-informed medical practitioner may fail to detect the existence of any morbid or pathological condition of the economy, though such a condition actually exists; or, in recognising the existence of some such condition, may form an incorrect judgment respecting its nature or its seat. These sources of error in diagnosis we have attempted to reduce to four general heads; which may be shortly stated in the following terms: 1st, Morbid lesions without symptoms; 2d, identical symptoms with different lesions; 3d, different symptoms with identical lesions; and, 4th, complications of lesions and symptoms.

We have seen, in the first place, that many changes may take place in the structure of the different textures, systems, and organs of the body, and that even to a considerable degree, without producing any symptoms which attract the notice of the individual himself, or of his medical attendant, in the event of some other ailment giving occasion to the latter to inquire into his patient's state of health; or, at all events, without producing any symptoms calculated to direct attention to the organ or part in which the change has occurred. The causes that may be assigned for this occasional latency of morbid lesions we have endeavoured to refer to the following heads: 1st, The lesion being seated in an organ the function of which is not exercised in a conspicuous manner; 2d, its being seated in an organ, of the function performed by which we are ignorant; 3d, its being seated in an organ that is not essential to the wellbeing of the economy, or that is of trivial importance; 4th, the exercise of the function not being prevented by the change that has taken place in the structure of the organ, as happens particularly when the alteration is slow in its progress; and, 5th, some other organ exercising the function or supplying the place of that which is injured. We found it necessary to admit, also, that in some instances no explanation of the latency of the morbid lesion can be given; and we may remark, that in explaining the same particular instance of latency, we may sometimes find it necessary to refer it to two or more of the causes which we have mentioned, operating in conjunction.

We have seen, in the second place, that a particular, and, to all appearance, identical group or train of symptoms, which in one individual is connected with one form of pathological condition, may, in another individual, be con- connected with a pathological condition of an entirely different nature. We have seen, that when different pathological conditions or proximate causes give rise to the same group or train of symptoms in different persons, these lesions may be both organic, or the one lesion may be organic and the other dynamical. In some instances the different lesions giving rise to the same group or train of symptoms have their seats in the same organ; in other instances they are seated in different organs or parts of the body, and these different organs or parts may be related to one another simply in the way of contiguity or superposition, or there may exist functional dependencies or sympathetic connections between them. To this head we have referred the class of cases in which lesions of the central portions of the nervous system give rise to symptoms in particular organs of the body similar to those that are produced, or that may be supposed to be produced, by lesions of which these organs are themselves the immediate seats; and that other class of cases in which a central portion of the nervous system exhibits symptoms of being affected with some morbid condition, while in fact it is in some other organ that the primary source of disturbance is seated. We had occasion to observe, that in some of the cases in which one disease assumes the characters that usually appertain to another, it is a constitutional disease which assumes the appearance of a local one; and that in other instances, again, it is a local disease which assumes the semblance of one that is constitutional. Sometimes the simulating disease retains one constant character throughout its progress, while in other instances it assumes in succession a variety of different characters. We have seen that the characters by which certain specific diseases can be distinguished from analogous diseases that are destitute of a specific character, are often far from being obvious, and that consequently the discrimination of these two classes of diseases is frequently a matter of difficulty and uncertainty. We have seen, likewise, that in some cases the community of symptoms between two diseases may exist only during a particular period of their progress, as at their commencement or termination; and that in other cases the community of symptoms may continue during their whole progress.

In the third place, we have seen that different groups or trains of symptoms may arise, in different individuals, from pathological conditions or proximate causes that appear to be identical. This seems to occur whether the pathological state or lesion be simply dynamical or organic. We have seen that the diversity in the group or train of symptoms may be confined to one of the symptoms, or to a few only, or may extend to the greater part or to the whole of them; and that it may consist in the presence of a symptom or symptoms not usually attendant upon the morbid condition, or in the absence of a symptom or symptoms by which it is usually accompanied. Under this head we have illustrated, or rather suggested, the influence of external and internal circumstances in modifying the characters which the same disease exhibits in different persons.

In relation to these two last-mentioned sources of difficulty and fallacy in diagnosis, we have seen reason to suspect that when groups or trains of symptoms apparently identical arise from different morbid lesions, and different groups or trains of symptoms arise from what appears to be an identical lesion, it is possible that in neither case does there actually exist the identity supposed; but that the apparent identity is attributable only to our inaccurate observation, or to the imperfection of the means of investigation hitherto ascertained.

Fourth and lastly, we have seen that several different pathological conditions, having their seats in the same or contiguous organs or parts, or in organs or parts remote from one another, may exist in the economy simultaneously, whether they shall have come on simultaneously, or shall have followed one another in succession; and, in the latter case, whether they shall stand in the relation of cause and effect, or shall be entirely independent of one another; and that not only may these co-existing pathological conditions, in producing in a more or less marked manner their respective symptoms, give rise to a complication of phenomena which in many instances it is by no means easy to unravel, but they may in various ways modify and obscure the phenomena appertaining to one another.

In illustrating these several sources of difficulty and fallacy in diagnosis separately, it has very clearly appeared, that in a large number of cases they are met with, not singly, but more or fewer in conjunction; a circumstance which cannot fail seriously to increase the embarrassments of the practitioner. Had our space allowed of it, it was our wish, with a view to impress more forcibly on our readers the practical operation of these sources of difficulty in ascertaining and distinguishing diseases, to have traced their influence, separately or variously combined, in rendering difficult and obscure the diagnosis of some one particular disease. For that purpose we had made choice of pulmonary consumption; and it is on this account that, in illustrating the several sources of difficulty individually, we have not taken advantage of the numerous illustrations of these which consumption affords, and which have been so ably pointed out in the writings of Laennec, Andral, Louis, Abercrombie, Craigie, and Clark.

With this recapitulation we must bring to a close our review of the various sources from which those difficulties proceed that the medical practitioner has to encounter, either in detecting the presence of disease in the human economy, or, when its existence is manifestly proclaimed by obvious signs, in discriminating, in individual cases as they present themselves to his observation, the true nature and seat of the affection. To the professional reader, we flatter ourselves, such a review may be not without its use, as tending to bring before him, under somewhat of a systematic arrangement, a number of particular facts, of which, in order to avoid the risk of perpetual errors of diagnosis, it is necessary not only that he should be aware, but that he should be able, in the exercise of his art, readily to recall the recollection, and make the appropriate application. On the general reader, we venture to hope, a knowledge of the nature and extent of these difficulties may tend to impress, first, the importance of the friends and attendants of the sick, as well as the sick themselves, not only facilitating such inquiries as the medical practitioner may himself institute, but spontaneously affording every information from which it can be supposed that he may possibly derive aid in forming a judgment respecting the disease under which his patient labours; secondly, the inexpediency of urging the medical practitioner to the precipitate expression of an opinion respecting the nature of his patient's malady, which, however incorrect it may be, must, when once it has been pronounced, enlist upon its side both his pride and his interests, and thus render it difficult for him to perceive the true posture of affairs, and even more difficult for him, perhaps, to avow it when perceived; and, thirdly, the unreasonableness of withdrawing confidence from a practitioner of whose knowledge and skill they have had manifold personal experience, or sufficient warrant on the testimony of others, because in some particular case or cases the judgments which he has formed may have proved to be incorrect. It has been justly remarked, that "in the practice of physic, there is doubtless a great difference in men as to quickness of apprehension, and also as to the improvement derived from experience; so that one practitioner may discover a difficult case much sooner than another. But there is also a difference in respect of cases, Some are obvious and easy to proper judges at first view; others are so obscure and perplexed that they are not to be hit off at first sight, nor to be discovered by a superficial inquiry, but require time and attention, with a careful discriminating examination of all the circumstances of the patient, before the true state of them can be determined. Some that are not knowable in their beginning, will become evident in their progress or maturity; and it must be owned there are some that will be too hard for all our inquiry and consideration, and cannot be fully discovered but by the anatomical knife after the death of the patient." (Dr Coe, lib. cit. p. 5, 6.)

As a knowledge of the existence of these difficulties seems calculated to bespeak from the public a more favourable consideration for the practitioner who has availed himself of all those means which medical science affords for improving his acquaintance with diseases and his means of discriminating them, so we cannot but think that there is another purpose to which it may, in some degree, also be subservient. We have already expressed our conviction (art. Medicine), that it is vain to trust to any legislative enactments for the suppression of quackery; and that the only mode in which any one can hope to promote this end, is by assisting in opening the eyes of the public to the grossness of the delusions which, in matters relating to medicine, they practise upon themselves, or allow others to practise upon them. But if it be admitted that a knowledge of the nature of the disease under which a patient labours, is an indispensable preliminary to the safe and proper management of his case; and if, in attempting to arrive at this knowledge, so many sources of difficulty and fallacy await even the best-informed medical practitioner,—him who is furnished with all the means of avoiding and overcoming them which the accumulated experience of ages has been able to discover or invent,—the prudent man may hesitate before he admits the belief that these difficulties can be overcome by persons of no scientific instruction, and who professedly reject all aid from the observations or experience of others; he may well pause to consider whether the discrimination of such a person can be with safety relied upon, even in those cases of which Dr Coe speaks as "obvious and easy to proper judges," far less in those obscure and perplexed cases, which require so much skilled observation "before the true state of them can be determined;" and whether the freedom of such a person, from doubt or uncertainty in diagnosis, may not arise from the rash presumptuousness of ignorance, rather than from the reasonable confidence which knowledge is calculated to inspire.

We could have wished, before terminating this article, to have taken a view of the means by which the medical practitioner endeavours to overcome those difficulties, and avoid those fallacies of diagnosis, which we have represented as arising from the uncertain relations that subsist between internal morbid conditions of the economy and their external manifestations or symptoms. The observations we can offer on this subject must necessarily be very brief and imperfect; but we are unwilling to leave on the minds of our readers the impression that medical science has accomplished, and can be expected to accomplish, nothing towards the more accurate determination and marking of these relations, which can aid the practitioner in his diagnostic judgments.

Previously to entering on these observations, however, we would repeat, what we have already hinted, that diligence in seeking out, and precision in examining, the symptoms of diseases, are often of themselves sufficient for the correction of those errors into which a less diligent investigation and less precise examination of these phenomena would be liable to lead the practitioner. "There is probably," as Mr Hunter has remarked in his work on the Ve- tions of the economy; so that not only are the means which medical men possess of discriminating diseases, through the medium of symptoms, becoming gradually more numerous, but they are also becoming more valuable.

But when the symptoms actually present are insufficient to guide the practitioner to a satisfactory judgment respecting the nature of the disease before him, are there any subsidiary aids towards the accomplishment of this object, of which he may avail himself?

In the first place, a careful consideration of the previous history of the case, from the earliest development of morbid phenomena to the period of examination, may often afford important assistance. For even in cases in which there exists a striking coincidence between the symptoms produced by two distinct lesions at a particular stage of the progress of one or other of them, there may be such differences either in the concourse of their symptoms at other periods, or in their general course, as to enable the practitioner who is duly informed respecting these particulars to decide on which of the lesions the case under his immediate observation depends.

In the second place, when, after the most careful consideration of the symptoms as they actually exist, and of the course which the disease has run previous to the time of examination, the nature of the malady is still doubtful, the practitioner may in some instances be assisted in its determination by the knowledge he obtains of the causes by which it has seemed to be induced.

In the third place, it happens in some cases that the first certainty the practitioner acquires respecting the nature of the disease with which he has to deal, is from the effects of the remedies administered for its removal; whether these effects shall be such as to confirm him in the correctness of the opinion upon which his mode of practice has been founded, or to satisfy him of the error of his diagnosis. It must not be forgotten, however, that inferences of this kind must be subject to the numerous fallacies to which the judgments of medical men, respecting the influence of remedies over diseases, are exposed. Thus, it is well known that medical men long imagined that the most certain test of a syphilitic or non-syphilitic taint in particular cases was to be derived from the fact whether or not the malady disappeared when no mercury was employed: an opinion which recent experience has shown to be wholly without foundation. In modern times Italian physicians have established, as a general law in pathology, a principle, which they designate the law of tolerance of medicines, that would, if well established, render a knowledge of the *povantia* and *leovantia* a very powerful assistance in forming our diagnosis, and particularly in judging of the intensity of a disease. The principle to which we allude is, that the human body, in becoming diseased, acquires the faculty of supporting medicines suited to its state, in doses exactly proportional to the intensity of the disease; and that it loses this faculty in proportion as the disease diminishes, and in the ratio of its diminution.

It has generally been understood, that when there existed a doubt as to the inflammatory or non-inflammatory character of a disease, a cautious trial of blood-letting might advantageously be made, with a view to throw light on the nature of the disease, and to determine the proper mode of treatment to be pursued. "The effects of remedies carefully considered," says Dr Marshall Hall, "throw an important ray of light upon the nature and force of the disease, and upon the condition and energies of the system." (i. l.) And in reference to this proposition the author observes, "One of the sources of diagnosis enumerated constitutes a department of knowledge which may be termed new: it is that of the effect of remedies, and especially of blood-letting, as a diagnostic of diseases, and as a criterion of the general powers of the system. In cases in which it is doubtful whether the pain or other local affection be the effect of inflammation or of irritation, the question is immediately determined by placing the patient upright, and bleeding to incipient syncope: in inflammation much blood flows; in irritation, very little. The violence of the disease, the powers of the system, and the due measure of the remedy, are determined at the same time. There is, in my opinion," adds Dr Marshall Hall, "no single fact in physic of equal importance and value in the diagnosis of acute diseases and the use of an important remedy." (P. 2, 3.) How far this application of the Italian doctrine of tolerance of medicines to the practice of blood-letting affords a safe rule of practice, we cannot here inquire; but we may observe, that common opinion assigns more influence in the production of syncope from venesection to individual constitution, than to the nature or intensity of the disease for which it is practised.

Lastly, when a case, respecting the exact nature of which any doubt has existed, proves fatal, it is of great importance that the practitioner should have an opportunity of disconfirming or correcting the judgment he may have formed by the examination, after death, of the several organs and systems of the body; for though the information then acquired can be of no practical use in the individual case, and the seeking it may, with the ignorant, expose the medical man to Molière's sneer, "d'aviser et voir ce qu'il aurait fallu faire pour guérir un homme qui mourut hier," it may on a future occasion exempt him from doubt, or rescue him from error. It cannot be questioned that a much greater degree of precision than formerly prevailed in medical diagnosis has been introduced, and is in the course of being introduced, into this branch, since medical men have become duly aware of the importance of examining dead bodies for the purpose of determining the existence or non-existence, in particular diseases, of organic alterations of structure, and, when these exist, of ascertaining their nature and seat. But in endeavouring to connect the symptoms that manifest themselves during life, with the morbid alterations of structure found on examination after death, much attention is requisite, as has been already hinted, to enable us to discriminate between those structural changes that give rise to diseases, and those that only develop themselves during their progress, or the existence of which may be merely fortuitous. "Multum prodesset ad intelligendam morborum causas cadaverum sectionem," says Van Swieten in his Commentaries on Boerhaave's Lectures (sect. 1041), "neque dubitari; interim tamen magna cautela hic opus est; ne pro morbi causa habeatur illud, quod potius morbi effectus est; multa enim in cadavere inveneriuntur mutata per mortem ipsum, quae non praestitentes ante mortem." To assist us in this discrimination, it is very desirable to have an opportunity of making post-mortem examinations in every stage of the progress of a disease. But in some diseases in which there is no tendency to death, particularly in their early periods, it is only by availing ourselves of deaths accidentally occurring that we are able to ascertain whether the disease depends primarily on organic or structural lesions, and, if so, of what nature these may be.

It would perhaps be impossible to refer to a more happy illustration of the reciprocal light which the attentive examination of the symptoms that present themselves during life, and the careful inspection of the structural lesions met with after death, are calculated to throw upon one another, than is afforded by the connection traced by Dr Bright between a particular affection of the kidney (now well known to medical men under the name of its first accurate describer), and a train of symptoms too often forcing themselves on the attention of the medical practitioner, but which had never before been recognised as depending on a specific pathological condition or proximate cause.

It must, we conceive, be acknowledged upon all hands, that the farther medical inquiries have been pushed, the smaller has the number of diseases become which are not known to be connected with, if not immediately dependent upon, perceptible alterations in the structure of particular portions of the body; or, in other words, that the diseases reputed dynamic have gradually diminished, while those reputed organic or structural have proportionally increased in number. It may be doubted, however, whether medical men, in their anxiety to trace the symptoms of diseases to organic causes, have not occasionally attributed to structural alterations a greater influence in the production of these phenomena than they actually exercise. For, even when structural alterations are found in the organs which, during life, had exhibited symptoms of a deranged condition, it does not necessarily follow that these symptoms and organic alterations were immediately and necessarily connected. In many cases, indeed, the symptoms seem to depend rather on some superadded dynamical derangement, than on the change of texture which has occurred.

In corroboration of this opinion may be mentioned the well-ascertained facts, first, that the same organic lesion which in one individual is accompanied with well-marked symptoms, in another individual may give rise to no indication of its existence, which, indeed, may become known only on examination after death; secondly, that the same combination of symptoms which post mortem examination would lead us to refer in one case to obvious organic alterations, may be found in another case totally unconnected with any perceptible change in the structure of any part of the body; and, thirdly, that in some cases of disease in which organic alterations are found after death, the symptoms have exhibited during life a periodical or intermittent character, which could not be the case if they depended simply on organic alterations of structure, seeing that these are not subject to variation. Thus, in a large proportion of epileptic subjects, organic alterations have been met with in the encephalon; but, in the first place, it sometimes happens that similar organic alterations are found after death in cases in which no symptoms of epilepsy occurred; in the second place, in some cases of epilepsy, no marks of organic disease can be detected; and, in the third place, though the organic alterations found in a number of cases of epilepsy may possibly account for such deviations from the healthy exercise of their functions as are manifested by some epileptics between their paroxysms, we must suppose that some other morbid condition of the encephalon, not permanent in its existence, but only occasional in its occurrence, is requisite for the production of the paroxysms themselves. "We know," says an intelligent author,¹ that morbid alterations in the substance of the brain sometimes produce periodic dis-

¹ Mackenzie on the Eye, p. 765.

² Médecine Éclairée, p. lili. See reasons assigned by Dr Abercrombie (on Diseases of the Brain, 3d edit, p. 217) for believing that the affection which has been called Serous Apoplexy is to be considered as Simple Apoplexy (that is, apoplexy not depending on any perceptible deviation from healthy structure) terminating in effusion.

cases, and that certain additional causes of excitement operating upon an unsound brain, one or other of the functions of that organ is for a time impeded, till the new cause ceases to operate, when the individual immediately returns to his former state of apparent health."

In attempting to distinguish between those diseases which are referrible to an organic pathological condition, and those which are to be considered as dynamical, in the sense that has elsewhere been explained (in the article PATHOLOGY), it is necessary also to keep in mind that a deranged exercise of some, at least, of the functions of the economy, though not originating in any perceptible alteration of structure, may in its continuance give rise to some such alteration. The morbid appearances met with on the examination of dead bodies may, consequently, sometimes stand in the relation of effect to the derangement of function, and the consequent symptoms, of which medical men are disposed to regard them as the cause. "Les troubles des fonctions de la vie," said M. Prost, "et les altérations organiques, sont la double source des maladies, qui s'enchâinent, s'accroissent, et se reproduisent mutuellement."

There is another source of fallacy in the attempts made by medical men to refer the symptoms that present themselves during life to such altered appearances as are found after death, in the different textures and organs of the body. We allude to certain changes that are liable to occur during the last periods of life, or even subsequently to death, and which those who are not duly warned of their occurrence, or not aware of the characters by which they may be distinguished, are liable to regard as products of disease. Much has been done, of late years, to facilitate the distinction between truly morbid, and, as they are termed, pseudo-morbid appearances, particularly between those turgescences of the extreme vessels which do, and those which do not, depend on inflammatory action. But, even from the difficulties that have presented themselves in establishing the grounds of distinction between these two sets of appearances, it may be inferred, that when these distinctions were not sufficiently understood or attended to, the symptoms that had manifested themselves during life must not unfrequently have been supposed to find their explanation in altered appearances of different textures and organs, which were not in reality products of morbid action.

From these observations, it is obvious that medical men require to be at all times equally on their guard against attributing to mere dynamical disturbance morbid phenomena which depend upon organic alteration of structure, and against attributing to structural alterations phenomena that arise simply from dynamical derangement. (L.) A TREATISE ON PHYSICAL GEOGRAPHY should comprehend a description of the general structure of our planet, of the arrangement of the solid and liquid materials of which its surface consists; of the nature of the elastic invisible fluids with which it is surrounded, and an account of the distribution of the organized beings by which it is inhabited. A detailed description of a field so vast, of a range of subjects so multifarious, would embrace every branch of natural history; but the province of physical geography is understood to be limited to general views of the different subjects now indicated. These views, if founded on legitimate deductions from observed phenomena, should enable us to reduce to a few general principles the infinite variety of appearances presented to us by nature; and it is this generalization which in modern times has elevated the study to the dignity of a science. Physical geography, however, in this philosophical sense, is of very recent origin; much of every domain of terrestrial nature yet remains to be explored; the surface of our earth is still but partially known; and although each succeeding age adds prodigiously to the knowledge of mankind, this exhaustless field will, for ages to come, afford materials to exercise the industry and reward the investigations of the philosophical inquirer, ere physical geography approaches to its limits of perfection.

Sect. I.—Position of the Earth in the System of the Universe; its Form, Magnitude, Density.

The sublime conceptions of astronomy, founded on observation and on mathematical inductions, have taught us to consider the innumerable stars visible to the naked eye and indicated by the telescope, as so many suns, probably enlightening and gladdening unseen worlds, which revolve round each, as our earth and its kindred planets revolve round our glorious luminary. The stars nearest to our earth are grouped into constellations, for the convenience of descriptive astronomy; but the more distant appear to the eye as nebulous patches or streaks of diluted light, which the highest powers of optical instruments have barely enabled us to resolve partially into clusters of stars, which analogy would lead us to regard as myriads of other suns; while imagination, ranging through illimitable space, pictures still more remote orbs, whose light has for ages travelled the profound, without as yet reaching the abodes of men. Such lofty conceptions of infinite power and beneficence are calculated to elevate the mind which entertains them, and to afford a compensation to human vanity for the comparative insignificance of our earth in the scale of creation. The solar system, of which our planet forms only a small part, seems to be itself but a fragment of a more extended system, round the centre of which the observations of Maskelyne and Herschel have rendered it probable that the sun and the fixed stars all revolve.

The terraqueous globe, without including comets, is one of twenty-nine bodies wheeling round our sun. Of these, eleven are termed primary planets, and revolve round the sun as a common centre; the remaining eighteen are named moons, satellites, or secondary planets, because they also revolve round some one of the primaries. The earth is accompanied in her orbit by one of these satellites. The orbits of two other planets are included within her path round the sun. The orbit of the earth is elliptical, but differs so little from a circle, that, in estimating its diameter, it may be considered as such; and its semidiameter, or the earth's distance from the sun, as determined from various astronomical data, especially from the transits of Venus over the sun's disc, equals ninety-five millions of miles.

That the earth is globular, is proved by ships steering in a general direction, either westward or eastward, arriving again at the point of departure; and very satisfactorily by the phenomena exhibited by vessels in receding from an island in the ocean. At a short distance the hull of the ship is sunk below the horizon; a little further and the courses or lower sails are lost; and soon only the upper sails are descried on the circular horizon. The rotundity of the earth is such, that when the eye is elevated six feet above the water, an object on its surface may be seen at a distance of three miles; and at all other altitudes of the eye, the distance of the utmost visible point varies as the square root of the altitude.

The earth, however, is not a perfect sphere, but is somewhat flattened at the poles, or forms what is termed an oblate spheroid. Newton attempted to ascertain the ellipticity of the earth by calculation; and, considering the effect of its diurnal velocity on a homogeneous fluid of the mean density of the earth, he determined its equatorial to be to its polar axis as 236 to 230; but he imagined, that if the density were greater at the centre than at the surface, the compression would be greater than in a spheroid of equal density. This error was pointed out by Huygens, who, on the supposition that the density increases regularly with the depth in the earth, estimated the relative axes as 578 to 579. But the determination of both philosophers is at variance with the results of geodetical experiments made about that time, and with the more refined measurements of meridional degrees in different parts of the earth in later times, as well as with the results of observations with the pendulums made by Maskelyne on the mountains of Perthshire, and the very recent investigations on the same subject by Forster, Sabine, Duperrey, Freycinet, Kater, Brisbane, and others.

On comparing the two methods of determining the figure of the earth, we are inclined to place more confidence in the result of measurements of meridional arcs than on the pendulum experiments, because the latter are liable to be disturbed by the geological structure of the regions in which they are made; and this is probably the cause why they accord less with each other than we should have expected from the delicacy of the apparatus, and the expertise of the observers. They give a greater ellipticity to the earth than the comparison of meridional arcs. The latter are not only more accordant with each other, but agree very nearly with the form deduced from the inequalities in the moon's motions, the equations for which were discovered by Laplace. The result of the whole is, that we may safely assume the ellipticity as equal to $\frac{1}{45}$th of the earth's axis; or the polar is to the equatorial diameter as 305 to 306.

The solution of this problem is essential to the determination of the exact length of the radius of the earth, and a most important problem in astronomy, as it is the unit by which we measure the planetary distances. But the determination of meridional arcs affords the solution of the point in question; and, aided by calculations, we obtain the following results in English miles:

| Equatorial radius | = 39637 | |------------------|---------| | Polar radius | = 39498 | | Ellipticity | = 139 | | Mean radius | = 395675| | Mean diameter | = 79135 | | Mean circumference| = 24871 |

The oblate figure of the earth appears to be a consequence of its rotation on its axis. Mathematicians have de- Sect. II.—General Distribution of the Land and Water.

On glancing at a map of the world, we are immediately struck with the great extent of its superficies occupied by the waters, which are united into one great ocean, filling up the depressions which separate the more elevated portions of the solid crust of the globe, and dividing the land into detached masses of very various extent, which we denominate continents, islands, or insulated rocks. These divisions, however, differ only in size; the continent is but a larger island; and the insulated rock is merely the summit of a mountain, the flanks and base of which are buried below the general level of the sea.

We are in the habit of speaking of the four quarters of the world, and sometimes of its four continents; but it is sufficiently obvious that there are only two great distinct divisions or principal masses of the dry land to which the name of continent should be applied; the eastern, including Europe, Asia, and Africa, which, from being the earliest abodes of civilization, may be termed the Old Continent; and the western, or American, which is usually known as the New Continent. By confining the designation of continent to these prime divisions of the land, we shall find no difficulty in classing Australia with islands. The old continent includes three of the great divisions of the earth, Europe, Asia, and Africa; the western continent, though chiefly occupied by the descendants of Europeans, is naturally divided into two great portions, which in future ages may become as distinctly marked by political and moral differences as they are by geographical peculiarities.

The vast extent of Australia has induced some geographers to consider it as a continent; but it may more justly be reckoned the first of islands in magnitude; and it might be grouped with the numerous and extensive islands lying to the south-east of the old continent, as a fifth great division of the globe, under the title of the Oceanic or Polynesian.

The terms employed in describing the various forms of the land, and the subdivisions of the liquid surface of our planet, have been sufficiently noticed under the article Geography, to which the reader is referred.

The slightest glance at a map of the world will show, that there exists a greater accumulation of dry land in the northern than in the southern hemisphere. Recent voyages have indeed proved that this disproportion was over-rated by the geographers of the last age. The late discoveries of the extensive tracts of land to which the names of New Shetland, New Orkney, and Sandwich Land have been given, prove that considerable deductions must be made from what was at one time considered as the domain of the Antarctic Ocean. But the voyages of Cook, Furneaux, Weddell, and others, have shown that these, although extensive groups of islands, do not belong to a great antarctic continent; and even if such existed, the general form of the ancient, as well as of the western continent, towards the north, would still leave a great preponderance of dry land in the northern hemisphere. It is not unworthy of notice, that the proximate sides of the old and new continent seem as if they had mutually influenced the forms of each other; for where any remarkable projection of the one is perceived, we find an indentation nearly opposite in the other. Thus, the principal eastern projection of South America is almost opposite to the vast African bight, extending from Cape Palmas to Cape Negro; the great western projection of Africa is opposed to the basin of the Carribbean Sea and the Gulf of Mexico; and the north-eastern trending of the coasts of Europe is balanced by the opposite direction of the shores of North America.

The general disposition of the land is different in each. In the old continent the principal extension is to the west and east; in the new it is to the north and south. In the former we may draw continuous straight lines of immense extent in different directions without encountering one sea. Thus a line from Cape de Verd in Western Africa will pass through continuous land till it reaches the promontory which separates the Yellow Sea from the Gulf of Leatong, a distance of 7800 geographical miles. One projected from Cape Palmas, on the Gulf of Guinea, to the Asiatic promontory called Tscheoutsko-nos, will extend 8700; and another, from Tangier to the Cape of Good Hope, will be nearly 5800 geographical miles. The longest straight line which we can project on the land of the new continent is from Cumana to Tierra del Fuego, a distance of 4600 geographical miles; and the next, from Icy Cape to Puerto de los Angeles, on the western coast of Mexico, is about 4100. In the old continent the lines we have described have large portions of the surface on either hand; but in the new continent the greater part of the land is on the eastern side of the imaginary lines.

The greatest extension of each considerable division of the two continents lies in the direction of its principal mountain chains. This is strikingly exemplified in America, where the Rocky Mountains and Western Cordillera of Mexico in the northern, and the gigantic Andes in the southern hemisphere, pursue the general direction of the American continent. The greatest extent of Europe is from west to east, and this is the general direction of the chains of the Pyrenees, the Alps, the mountains which form the northern boundary of Turkey in Europe, and the Carpathians. A similar direction is pursued by Mount Taurus, and other ranges of Central Asia, the Himalaya chain, and the mountains of Thibet. In Africa the position of the mountain chains is less known; but the direction of the rivers, and the notices of travellers, would lead to the inference, that, on its eastern side, a spine of mountains, more or less interrupted, extends from Abyssinia towards the southern extremity of Africa. Even the direction of smaller portions of both worlds, as of islands and peninsulas, coincides with their principal mountain chains. This is well illustrated in Norway, in Italy, in the so-called peninsula of India, in Siam, Cambodia, Florida, and California, and on a smaller scale in the islands of Ceylon, Madagascar, Cuba, and Great Britain.

The expanse of the waters is termed the Ocean; but, for the convenience of descriptive geography, it is divided into several, though, strictly speaking, there is but one ocean; and the different seas form but parts of a vast whole, which communicate freely with each other, and can scarcely be considered as marked by definite limits. Most geographers speak of two grand oceans, which have derived their appellations from their position relative to the old continent.

I. The Western Ocean, which includes three principal subdivisions. 1. The Northern Ocean, of which the southern limits are the British coasts, and a line drawn thence to the southern extremity of Greenland. 2. The Atlantic Ocean, which should be limited to the south by the extreme projections of Guinea and Brazil. 3. The Ethiopic Ocean, which lies between the limits of the last, and a line joining the southern extremities of Africa and America. All three are sometimes, though less correctly, denominated the Atlantic.

II. The Eastern Ocean, which is also subdivided into three principal portions. 1. The Indian Ocean, comprehending the seas between Southern Africa, Australia, and the Malayan Islands. 2. The Pacific Ocean, extending from Behring's Straits to Cape Horn, and the southern side of Australia. It is often subdivided into the Northern and Southern Pacific, of which the limits are not well defined.

3. The Austral or Southern Ocean, which in general terms may be described as extending southwards from a line passing round the globe and touching Van Diemen's Land, and the Cape of Good Hope, and Cape Horn. With these main oceans various seas communicate, either by narrow straits, as the Persian Gulf, the Red Sea, the Mediterranean, and the Baltic; or by wider channels, as the Sea of Okhotsk, the Yellow Sea, the Sea of Hudson, and of Baffin; or by numerous openings, as the Sea of China, the Caribbean Sea, and the Mexican Gulf. The Euxine, the Caspian, the Sea of Aral, and of Baikal, and the magnificent fresh-water lakes of North America, have many of the characteristics of lakes; but their gigantic scale, and their important influence on the climate of the adjacent countries, entitle them to the designation of Inland Seas.

The portions of the earth's surface which are covered by water bear a large proportion to the dry land. Dr Halley first suggested the idea of ascertaining the comparative surface of the land and water, by cutting them out of a map, and weighing them separately. It was by this means that he ascertained the relative area of the English counties. The astronomy of Dr Long, published in 1742, contains the application of this principle for ascertaining the relative surface of the land and water on the earth. He describes his having cut out each from the gores of Senex's sixteen-inch globes, and on carefully weighing them he obtained the ratio of the water to the land as 349 to 124, or very nearly as 3 to 1. Dr Long's experiment appears to have been conducted with great nicety; for Professor Rigaud of Cambridge has very lately confirmed it by a laborious and careful repetition upon the great scale. This latter gentleman used the gores printed for Addison's three-feet globes, which contain a superficies of 4071½ square inches. The gores of this magnificent globe are twenty-four in number. The torrid zone was divided at these lines and at the equator, and thus formed into forty-eight divisions; the divisions of the two temperate zones were also forty-eight, and these together make ninety-six divisions, each of which was carefully examined separately, divided into land and water, and accurately weighed in a balance which easily indicated one tenth of a grain.

The southern frigid zone was considered as wholly consisting of water; but the northern frigid zone, from longitude 180° to 270° west, was taken as land, and, from 270° to 360°, was computed to consist, one half of land and one half of water. The whole surface of the earth was supposed to be divided into 1000 parts, and all the estimates of the different regions were expressed in proportions of this standard. Rigaud's paper contains several tables, of which the following are the general results:

| Region | Land | Water | Ratio | |-------------------------|------------|------------|-------| | North polar zone | 18°02'63" | 23°44'37" | 100 to 139 | | North temperate | 126°63'08" | 132°52'47" | 100 | 105 | | North half of torrid | 52°55'82" | 146°81'62" | 100 | 279 | | South temperate | 46°15'92" | 158°21'56" | 100 | 332 | | South polar | 22°54'88" | 236°60'60" | 100 | 1049 | | Whole sphere | 265°92'33" | 734°07'62" | 100 | 276 | | North hemisphere | 197°21'53" | 309°78'46" | 100 | 154 | | South hemisphere | 68°70'80" | 431°29'06" | 100 | 628 | | Whole torrid zone | 98°71'71" | 300°03'18" | 100 | 204 |

This differs surprisingly little from the estimate of Dr Long, considering that each was deduced from the works of different geographers, and at a considerable interval of time. If we express the ratios of each determination, we shall have, in

Dr Long's estimate, the land to the water = 1 : 281 Professor Rigaud's = 1 : 276

And it is also deserving of notice that Rigaud's last esti- mate differs but little from his former experiment on Carey's twenty-one-inch globe; but both differ considerably from the calculations of Malte-Brun. As, however, that author has not furnished us with the elements of his calculation, it ought not to be put in competition with the elaborate investigations of Long and of Rigaud.

The dry superficies of each great division of the globe was ascertained by the same method; and the tables of Rigaud enable us to give an accurate comparative view of their relative area in thousandths of the earth's surface.

| Europe | = 156989 | | Asia | = 887320 | | Africa | = 595764 | | America| = 912573 | | New Holland | = 99063 | | Smaller islands | = 07524 |

Total = 2659233

If we take the whole superficies of the globe = 196,896,658 square miles, as the land is to the water in the proportion nearly of 266 to 734, it follows that the whole land occupies a surface of 52,863,231 square miles, and the ocean an area of 144,473,427 square miles.

Sect. III.—Variations of the Surface.

Besides the inequality produced by the oblate figure of the earth, different portions of its surface are at different distances from its centre. The dry land is elevated into mountains, depressed into valleys, or stretched out into plains, varying in extent and elevation above the sea; and the ocean itself must be considered as effused into a vast basin, seemingly diversified, like the dry land, with mountains, precipices, valleys, and plateaux of an infinite variety of shapes and dimensions. In fact, the whole dry land may be considered as a series of islands, or of plains and mountains, elevated above the general surface of the ambient fluid which occupies the deeper valleys that divide them.

The whole surface of the dry land is elevated more or less above the general level of the ocean; with one remarkable exception, which, however, has only of late years been detected. Barometrical measurements have shown that a vast area of Central Asia, no less than 18,000 square leagues, is considerably below the level of the ocean. This tract includes the Caspian and the Aral, the surfaces of which have recently been shown to be 101-2 feet lower than that of the Euxine. Should any convulsion of nature, such as earthquakes be known to produce in other quarters of the earth, depress the low sandy tract which now separates the sea of Azof and the Caspian, the waters of the Euxine, and also of the Mediterranean and the Atlantic, would inundate an enormous extent of the sandy steppes of Asia, and entirely change the climate and face of that portion of the earth.

Plains of greater or less extent, with inconsiderable inequalities, are found in every region. The most considerable plain in Europe is that which, commencing on the Northern Ocean, between the extremity of Jutland and the mouths of the Scheldt, extends eastwards through Prussia and Poland into Russia, and constitutes in that empire an almost unbroken plain, from the confines of the Frozen Ocean to the shores of the Euxine and the Caspian, until it meets the Uralian chain, which separates Northern Europe from Northern Asia. In this vast expanse, the surface is scarcely broken by any hills, except the inconsiderable ridge of Valdai, between Toropetz, Smolensk, and Moscow, which nowhere rises to 1200 feet above the sea. This magnificent plateau is divided by the Uralian Mountains from a still more extensive plain, which may be denominated the great plain of Northern Asia, the boundaries of which are the Altaian chain and the Icy Ocean. It commences at the eastern side of the Ural, and inclines towards the northeast until it almost touches the Northern Pacific Ocean.

Both plains together form a remarkable feature in the surface of our earth, and being only interrupted by the Ural chain, which never exceeds 4500 feet in height, have been regarded by some geographers as but one plateau; which, thus considered, is the most extensive on our planet, with a medium breadth of 1100 miles, a length of 6000, and a surface of about 6,600,000 square miles. The surface of much of these two great plains is fertile; some of it is covered with heaths or swamps, producing dwarf birches, willows, and a stunted vegetation; a considerable part is shrouded in dark pine forests.

Far different in appearance are the enormous wastes of arid sand in Northern Africa and Central Asia, the vast saunas of the Mississippi and the Missouri, and the pampas and llanos of South America.

A prodigious zone of sand stretches like an ocean, in Deserts. Northern Africa, between the ridges of Mount Atlas and the parallels of the Senegal and the Niger, and from the Atlantic to the narrow valley of the Nile. It may be said to cover a tract across that continent between the eighteenth and thirty-first degrees of north latitude, extending from long. 15° W. to 30° E. This enormous area may be considered as having a length of 2470 miles, a breadth of 900 miles, and consequently a superficies of about 2,200,000 square miles. It is, however, by no means an uniform surface of loose sand. In many parts the dreary waste is broken by low hills of naked sandstone, or by tracts of arid clay; occasionally it is enlivened in a few spots by verdant isles, or oases, of various extent, according as water finds its way to the parched surface. The principalities of Fezzan and Darfur are the most considerable of these islands of the desert; but the most celebrated of these insulated spots is the far less considerable oasis of Siwah, the palms of which still wave over the ruins of the Temple of Ammon, and indicate the most ancient seat of African civilization.

Egypt, Nubia, and Sennaar, the eastern boundaries of this desert, would long ago have been overwhelmed by the moving sands of the western desert, but for the annual inundation of the majestic Nile; and thus the empire of sand would have stretched uninterruptedly into Arabia. As it is, the narrow valley of that river, and the Red Sea, are all that separate the African deserts from those of Asia. Soon after quitting the Nile, the traveller by the route of Suez encounters sand, which is continued into the centre of Arabia, where it forms the Desert of Nejd, extending to the valley of the Euphrates. The sandy zone then inclines northwards, enters Persia, and forms the saline deserts of Adjem, Kerman, and Mekran; it is turned north-east by the valley of the Indus, passes through Caubul and Little Bukhara, till it joins the vast deserts of Cobi and Shamo, which occupy so large a portion of Central Asia between the Altaian and Mustag chains, and reach to the confines of China. The sandy zone thus traced throughout the breadth of the ancient continent, from Western Africa to the 120° of east longitude, has been computed to cover an area of 6,500,000 square miles; but the Asiatic portion of this tract includes many chains of mountains and fertile valleys. It is characterized by the occurrence of arid wastes of sand or clay, sometimes with saline incrustations on the surface, and is remarkably deficient in considerable rivers; except the Nile, the Euphrates, the Indus, and the Oxus, there are no large rivers in a region which embraces almost a fourth part of both Africa and Asia. This portion of Central Asia forms a series of elevated plains six thousand miles in length from west to east. "Some of these plains," says Humboldt, "are covered with herbage; others produce only evergreen saliferous plants, with fleshy and jointed stems; but a great..." number glitter from afar with a saline efflorescence, that crystallizes in the semblance of lichens, and covers the clayey soil with scattered patches like new-fallen snow."

From these elevated plains have issued the barbarous hordes which at different periods have swept like a whirlwind over the regions where the human intellect had achieved its proudest triumphs, and devastated the seats of Asiatic and European civilization. From this officina gentium have successively issued the Huns, the Avars, the Mongols, the Alans, and Uzes—the scourges of humanity; and possibly also the tribes by whom the American continent was originally peopled.

The great valley of the Mississippi is a magnificent plain, covered with primeval forests, or spreading in vast savannahs; but a great portion of it is liable to periodical inundations from the river. At 100 miles from the sea its annual inundation has a breadth of from 80 to 100 miles; and even at the mouth of the Ohio, which is 1000 miles from the sea, the inundating waters have the breadth of thirty miles. This shows the extreme flatness of the soil. The river has been extensively embanked, to rescue a portion of the rich soil from these inundations; but, during the season of the inundations, the greatest part of this extensive plain is still abandoned to the alligator and other reptiles, and to enormous herds of wild animals during the rest of the year.

The banks of the Lower Missouri, for 2500 miles, present a succession of fertile plains beyond the reach of inundations. This district, which may be considered as a continuation of the great valley of the Mississippi, exhibits an agreeable intermixture of natural prairies and forests, abounding in wild animals, which retreat before the rapidly advancing tide of human colonization.

These immense plains are probably destined to become the abodes of a dense population, and perhaps the seat of a mighty empire, subject to few of the natural vicissitudes that impede the march of improvement, if we except the contingent risk of some mighty convulsion, by which the waters of the extensive fresh-water seas of Canada may be permitted to overwhelm this extensive realm with a far more formidable catastrophe than the deluge of Deucalion. Lake Superior has a depth of about 600 feet, and its surface is more than 300 feet above the level of the plains of the Mississippi. Should earthquakes disrupt the intervening barrier, the waters of that and the other great inland seas would sweep these plains with a devastation quite unparalleled in the history of our planet since the deluge of Noah.

The interior of South America presents plains not less extensive, the peculiarities of which have been sketched with a masterly hand by Humboldt. The first may be termed the valley or plain of the Orinoco. It is bounded on the north by the maritime ridge of Caracas, on the south by the forests of Guiana, and extends westward to the mountain chain of New Granada, the summits of which are lost in perennial snow. It is a solitude of more than 20,000 square leagues.

The second is divided from the first by the mountain-arm which the Andes sends eastward to separate the waters of the Orinoco from those of the Maranon, an interrupted branch, stretching from the towering ridges of Popayan, to the humbler granitic Sierra de Tumucurique, on the confines of Guiana. The southern boundary of this second plateau consists of the chains of Santa Cruz de la Sierra, of Chiquito, and Cuyaba, which separate it from the plains of the Paraguay, the Parana, and La Plata. The western boundary is the cordillera of the Andes. This territory includes the llanos, extended solitudes, with a scanty soil, destitute of trees, but covered with a luxuriant herbage, the consequence of the periodical inundations, which convert this portion of South America into inland seas; amidst the turbid waters of which, as has been forcibly described by Humboldt, may be seen herds of wild cattle and of horses, swimming from one gradually-diminishing island to another, in search of food, or escaping from the attacks of the jaguar, to become the prey of the alligator, or perhaps to be stunned by the strokes of the electric gymnusotus. In the immense tract of country included in this second division, are some detached groups of mountains, indicated on the magnificent map of Omedilla; but they are generally of inconceivable elevation, and the face of the country may be considered as one enormous plain, or succession of contiguous plains, little elevated above the general level.

The third system of great South American plains forms the basins of the Paraguay and La Plata. This region includes the vast pampas of Buenos Ayres, extending from the foot of the eastern ridge of the Andes to the "sea-like Plata," and stretching southward into the cheerless deserts of Patagonia, presenting to the eye interminable plains, with a surface often as unvaried as the ocean, inhabited by vast herds of wild cattle and droves of horses; while the tall form of the Rhea Americana, a species of ostrich, frequently gives local peculiarity to the picture.

Besides the extensive plains already described, which are either not much elevated above the sea, or rise by imperceptible degrees, there are other plains, with steep acclivities on every side, which have thence been termed table-lands. Some of these are very extended, and retain a general level of several thousand feet above the sea. Sometimes they are bordered with chains of mountains, much less elevated on the side towards the table-land than in the opposite direction; sometimes they have surfaces much undulated or broken. There are few table-lands in Europe of sufficient importance to be here noticed. The most considerable is that of Central Spain, embracing the two Castilles, which has a general elevation of nearly 2000 feet above the level of the sea. The barometer at Madrid has a mean height of about twenty-seven English inches, which, supposing the instrument on the shores to have a mean of thirty inches, would give to the capital of Spain an elevation of about 1900 English feet. The thermometer in winter often descends below the freezing point; and the chaemerosps and the agave, which flourish luxuriantly in the plains around the table-land, are unknown at that elevation, though under the fortieth parallel of north latitude. The descent from this table-land on all sides is steep; and the transition from the arid and almost treeless plains of Castille, into the milder climate at the foot of the mountain ridges which support it, is marked by the vegetable productions of a warmer latitude.

Our imperfect knowledge of Central Asia prevents any accurate notion of the extent and form of these remote regions; but there can be little doubt, that in this quarter there is the most extensive table-land on the face of our globe. The severity of the climate, the direction of the rivers flowing from it on every side, and the scanty information derived from Asiatic caravans, all indicate that there is a prodigious central elevation, supported on the north by the chain of the Altai, on the west by that of Belug Tag, on the south by the mountains of Thibet, and on the east by the chain of Sialki, which divides the country of the Mandshurs from their Tatar brethren of Mongolia. This enormous tract, however, can scarcely be described as consisting of one table-land; for it is intersected by the lofty mountain ranges of Akaki and Mus Tag, which are capped with perennial snows, and it contains the vast, treeless, sandy deserts of Cobi and Shamoo.

The central plateau of Asia, from the severity of its climate, is inferred by Humboldt to have an elevation of not less than 9000 or 10,000 feet above the sea; and this region, including Thibet and the desert of Shamoo, is believed to have a superficies of 1,500,000 square miles. According to the observations of Olivier, Persia should likewise be considered as a table-land of great elevation. Southern India affords an excellent instance of a table-land, which includes the whole kingdom of Mysore. It stretches from the banks of the Kistna on the north, and is supported by the steep acclivities of the Western, and of the less abrupt Eastern Ghauts. The general elevation of this region is about 3000 feet; but at its southern extremity it abuts against a smaller though much loftier table-land, the district of the Neilgherry Hills, which, till lately, was almost unknown to Europeans. A good account of it has recently been published at Calcutta, by Dr Robert Baikie, from which we learn that the district occupies an area of 600 or 700 square miles, between the eleventh and twelfth degrees of north latitude, and the seventy-sixth and seventy-seventh degrees of east longitude. The general level of its finely-undulated surface is between six and seven thousand feet above the sea. It is connected with the Mysore country by a neck of land about fifteen miles in width; but its northern flank towers above that kingdom by 3500 feet, and it is separated from the subjacent plains of Malabar and Coimbatore by an almost precipitous boundary 7000 feet in perpendicular altitude. The surface is fertile, the climate mild and salubrious; and there the invalid, suffering from the baneful influence of a residence in the sultry plains of India, finds a European climate. The barometer varies from 22:840 to 23:220; whilst the thermometer throughout the year ranges from 73°-51 to 42°, and has a mean of 52°-50. The quantity of rain is = 42-251 inches; heavy rain falls on nineteen days, showers on eighty days, and the clear days are 179 in the year.

The Mexican plateau (No. 1) has also a general elevation of 7000 feet above the sea; it rises as abruptly as the Neilgherry district, especially from Vera Cruz, and in a single day the European may escape from the pestilential atmosphere of the plains, to an elevation at which the black comet is unknown. The sea-coasts of Mexico are hot and unhealthy at certain seasons, and are known to the natives by the name of Tierras Calientes, or burning regions, producing the vegetable riches of the torrid zone. A single day, from Vera Cruz, suffices to reach the height of 4000 feet, where there reigns a perpetual spring; and this region, termed Tierras Templadas, though often enveloped in clouds and fogs, is healthy. On ascending to the height of 7000 feet, the traveller gains the Tierras Frios of the Mexicans, or great table-land, which has generally a mean temperature of 63° Fahrenheit. It produces the cerealia of Europe, and the vine and the olive flourish on its plains. But its surface is varied by hills; and along its margins, stupendous peaks, such as Orizaba, Popocatepetl, and the Sierra Nevada of Mexico, have their summits far above the limits of perpetual snow. There are other plains in Mexico which are more than 1000 feet above the general level of the great plateau. Such is the plain of Toluca, where the climate is severe, and the mean temperature so low that the olive there refuses to ripen its fruit.

Several less extensive table-lands are included between the two lofty parallel chains which constitute the Southern Andes. Thus the plains of Quito and Riobamba are true table-lands; the general level of the first being 8000 feet above the sea, and the city of Riobamba having an elevation of 10,500 feet, so that it is probably the most elevated city in the world.

Mountain chains and ranges of hills are separated by valleys, which have obtained the names of ravines, dells, defiles, or passes, when narrow and difficult of access. These chiefly occur among lofty mountains, and sometimes present mural precipices of wonderful sublimity. Narrow valleys often present a striking coincidence in the position of the salient and receding angles of their opposite sides. This has been well described, as observed in the Pyrenees, by Ramond; and some geologists have considered it as a proof that such valleys had been formed by the sudden disruption of continuous strata.

In describing valleys, geologists likewise distinguish them into different kinds, according to their respective relation to the mountain ranges on which they occur. Principal valleys are such as run parallel to and separate great chains of mountains. Those which have a direction at a considerable angle with the principal chain, separating the branches or arms of the mountain chain, are termed lateral valleys; and the smaller ones, running into these lateral arms, are denominated subordinate valleys. The lateral open into the principal, and the subordinate into the lateral valleys.

Sometimes we find a valley, consisting of a chain or succession of basin-shaped cavities, at different levels, separated from each other by narrow gorges. Such appearances are not uncommon among all mountains; and several examples occur in our Highland glens, as in Glen Roy, Glen Gluoy, and Glen Spean, where the fancied parallel roads mark the successive heights at which the waters formerly stood, until the rocky barriers which pent them up were opened.

Sometimes a valley has a circular form, or is basin-shaped, and on all hands surrounded with mountains, excepting at a narrow gorge, where the waters of the valley make their escape. Two of the most remarkable instances of this are found in Bohemia and in Cashmere. The former consists of a single circular valley, nearly 200 miles in diameter, which appears at one time to have been a vast lake, until the formation of the defile in the Erzgebirge, through which the Elbe escapes into Saxony. Cashmere is a fertile valley of difficult approach, nearly circular, and ninety miles in diameter, embraced by the arms of the Himalaya Mountains. Native tradition represents it as a lake, until one of the Hindu deities burst the rocky boundary, by the formation of the ravine through which the Vindus now pours its waters.

Some valleys contain large rivers and lakes, from which there is no issue or outlet. Such valleys have not yet been drained by the giving way of some point of their boundaries. Central Asia affords many instances of this species of valley, as in the districts around the Koko-noi, the Lokenor, the lakes of Zaizan, Kirgha, and Palkati, and the Sea of Aral. A very remarkable instance occurs amidst the loftiest peaks of the Andes, in the valley which contains the great lake of Titicaca, which the ancient Peruvians regarded as sacred, the residence of the founder of their mo- narchy, and which is sufficiently remarkable, by its extent, and the sublimity of the surrounding scenery.

The most elevated portions of the earth's surface are either connected in extensive mountain chains, or they exist in aggregated groups branching from a common centre, or they form insulated heights, rising more or less abruptly from plains. We have seen that the direction of mountain chains is that of the principal extension of the regions in which they occur; and the same thing holds good in peninsulas and islands, in which we generally find the chains longitudinal. Thus the principal mountains of Europe, Asia, and America form extensive chains in the direction of the greatest extent of each of these divisions of the earth.

The direction of peninsular chains is well seen in Norway, in the Apennines, in the mountains of the western side of Arabia, in the Ghauts of India, and in the mountains of Corea, Kamtschatka, and California. The direction of insular chains, as corresponding with the principal dimension of each island, is also illustrated in Madagascar, Ceylon, Japan, Cuba, St Domingo, the British isles, Sardinia, Sicily, and in many of the smaller islands.

If we endeavour to generalize the observations on the direction of mountain chains, one very remarkable peculiarity seems deducible, namely, that they very generally present their steepest acclivities towards the great basins to which they are contiguous, while they slope more gradually in the opposite direction; and on examining their intimate structure, we find their stratified beds dipping generally from the basins to which their escarpments are presented. Thus the ridges of the Scandinavian peninsula present their boldest escarpments to the basin of the Northern Atlantic; while the opposite ranges of Greenland and Iceland also show their steepest acclivities towards that basin. Round the shores of the Mediterranean the same arrangements are especially observable. The lofty ridge of the Atlas is very bold on the northern side, and declines more gradually towards the Sahara. The chain of Spanish mountains which skirt the Mediterranean, from Gibraltar to the Pyrenees, present their escarpments towards that sea; and the Maritime Alps of France, the mountains of Switzerland and the Tyrol, of Istria, Dalmatia, and Thrace, all present their most precipitous sides to the basin of the Mediterranean. We believe that the considerable chain which in Asia Minor extends from Mount Ida to the country around Scanderoon, has the escarpment directed towards the Levant seas; while the mountains of Armenia present their boldest declivities to the basin of the Euxine, and the Caucasus and mountains of Mazanderan towards that of the Caspian. The escarpment of the great chain of Africa, termed that of Lupata, which seems to be prolonged from the lofty mountains of Abyssinia to the south of the Mozambique Channel, would appear, from the little we have learned of its structure, to face the basin of the Indian Ocean; and we know that the steepest declivities of the Western Ghauts of India are directed towards the same basin. The mighty spine of the American continent, from the shores of the Arctic Frozen Ocean to the extremity of South America, through a course of more than 8600 British miles, presents a series of rugged precipices to the vast basin of the Pacific; and, if we might indulge in one sweeping generalization, it would seem that the chains stretching from the Persian Gulf eastward through Thibet, and thence bending to the north-east through Mongolia and Northern China to Tscheoutskoi-nos on the Frozen Ocean, present their fronts also to the Pacific Ocean.

As such coincidences can scarcely be considered as accidental, they afford a wide field for speculation. Can we suppose that appearances, on such an immense scale, have any relation to the operation of the force which caused the elevation of the land, acting towards a central point, and producing the dip of the elevated strata all around, from a common axis of movement? Such speculations, aided by the position of volcanoes, and other mountains of igneous formation, might lead us to infer the direction of the great lines of subterranean disturbances which have modified the appearance of the crust of the earth.

The inequalities of the earth's surface, however considerable they may appear to the eye that contemplates them, bear but a trifling proportion to a sphere of nearly 8000 miles in diameter. The loftiest peaks are not quite five miles in height above the sea; and we may obtain some idea of their effects on its general form, by considering, that such inequalities on the surface of our earth produce no greater proportional deviation from its spherical shape than a roughness equal to $\frac{1}{7500}$th of an inch would on a ball an inch in diameter; nor so considerable as the inequalities produced upon the form of an orange by the papillose of its rind. The heights of mountains, however, exercise an important influence on the climate and productions of the regions where they occur; and it becomes an important point in physical geography to determine their respective altitudes. This has usually been attempted by two methods; by geodesical measurement, and by marking the difference of the barometrical columns on their summits and at their bases. The first is the most accurate when we can obtain a level base at a moderate distance from the height to be ascertained, and when the measurements of the angles are carefully taken by expert operators, and with good trigonometrical instruments. The use of the barometer is confined to accessible heights; but it is much more easily managed, under such circumstances, and requires little more than attention to the true height of the barometrical column, corrected for the temperature of the instrument and of the ambient air. In variable climates, accuracy would require synchronous observations at the foot and on the summit of the height to be ascertained; but in tropical climates, or even in the south of Europe, the barometer scarcely varies throughout the year at the level of the sea, and therefore such measurements are more easily made. The methods of both kinds of observation are sufficiently detailed in the articles Barometrical Measurements and Trigonometry.

Accessible heights have also been estimated by the fall of the thermometer on ascending. It has been found that, in our latitudes, an ascent of every 300 feet may be considered as equivalent to a fall of 1° of Fahrenheit's thermometer; but this formula does not answer so well either in high or in low latitudes.

It has also been proposed to measure such elevations by marking the temperature at which water boils at the base and on the summit. A convenient instrument for such observations was invented by the Rev. F. Wollaston, but it is liable to accidents in carriage; and, on the whole, geologists trust either to barometrical or to trigonometrical observations for measuring mountain elevations.

The most lofty mountains in general form parts of extensive chains; but there are many instances of great elevations being attained by isolated peaks. Instances of the former are familiar in the Alps, the Carpathians, the Himalaya, and the Andes; of the latter, in the towering summits of Etna, Teneriffe, and Mouna Roa in the Sandwich Islands. But such detached elevations are either active or extinct volcanoes.

In the following table we have arranged the heights of mountains, according to the regions in which they occur, and marked the method of determination by the letters B. and T., according as the determination has been made by the barometer or by trigonometrical operations, while E. signifies estimated height. ### Table of Heights of Mountains, &c.

#### AFRICA

- Abyssinia: Mountains of Geesh, 15,000 E. - Amid, 13,000 E. - Lamalmon, 11,200 T. - Gondar, 6,450 T. - Taranta, 7,800 T. - Morocco: Atlas, two peaks of, 11,400 T. - Abyla or Ape's Hill, 3,000 E. - Mountains near Fez, 10,000 E. - South Africa: Chain of Lupata, 12,000 E. - Nieweldt, Cape of Good Hope, 10,000 E. - Kom, ditto, 5,000 E. - Khamies, ditto, 4,300 E. - Table Mountains, ditto, 3,532 T. - Devil's Head, ditto, 3,315 T. - Lion's Head, ditto, 2,166 T. - African Islands: Teneriffe, the peak, 12,236 B. - Trigo, Canaries, 7,400 B. - Ruivo, Madeira, 5,162 T. - Diana's Peak, St Helena, 2,692 T. - Gros Morse, Isle de Bourbon, 9,600 B. - Volcano in ditto, 7,680 B. - Bonnet Pointu, ditto, 6,050 B.

#### SOUTH AMERICA

- Peru and Bolivia: Pico de Illimani, first peak, 24,450 T. - Second peak of ditto, 24,200 T. - Sorata, 25,000 T. - Chimborazo, 21,440 T. - Antisana, v., 19,150 B. - Cotopaxi, v., 18,890 B. - Tunguragua, v., 16,579 B. - Pichincha, v., 15,940 B. - Antiquipa, v., 17,800 T. - Corazon, 15,800 T. - Potosi, 16,000 B. - Defile of Assuy, 15,540 B. - Huancavelica, 14,960 T. - Pass of Quindiu, 11,500 R. - Lake of Titicaca, 12,000 R. - City of Rio Bamba, 16,600 B. - City of Cuzco, 9,356 T. - Columbia: Nevado de Merida, 16,420 T. - Nevado de Sta Martha, 15,200 T. - Volcan de Duida, 8,467 T. - Guadarrama, 6,400 T. - City of Bogota de Sta Fe, 8,650 T. - Mountains of Venezuela, 5,000 E. - Guachano, 5,550 T. - Silla de Caracas, 6,432 T. - Tuniriquiri, 6,250 B.

#### NORTH AMERICA

- Mexico: Volcan de Popocateptl, 17,716 T. - Pico de Orizaba, 17,371 T. - Nevado de Mexico, 15,700 T. - Nevado de Toluca, 15,159 T. - Cofre de Perote, 13,514 T. - Cerro de Axusco, 12,032 B. - Pico de Tancitaro, 10,498 B. - Volcan de Colima, 9,186 B. - Volcan de Jorullo, 4,267 B. - City of Mexico, 7,470 B. - City of Toluca, 8,818 B. - City of Xalapa, 4,333 B. - Guanaxuato mine, 6,636 B. - Real del Monte mine, 9,057 B. - Valenciana mine, 7,637 B. - North-West Coast: Mount St Elias, 12,672 T. - Mount Fairweather, 8,970 T. - Crillon, 5,440 T. - East Greenland: Blaserk, 6,000 T. - Werner Mountains, east coast, 6,000 T. - Coast from 70° 33' to 71° 12', 3,000 T. - Church Mountain, 2,967 T. - Double Mount, 3,444 T. - Roscoe Mountains, 3,690 T. - Cape Brewer, 4,000 T. - Raffles's Island, 2,000 T. - Träll Island, 1,390 T. - United States: White Mountains, Massachusetts, 6,230 B. - Kaatskill, New York, 3,454 B. - Killington, Vermont, 3,450 B. - Ridge of Alleghany, 3,000 B. - Appalachian Peak, 2,700 B.

#### ASIA

- South Sea Islands: Ophir, Sumatra, 13,842 T. - Volcano in ditto, 12,465 T. - Egmont, New Zealand, 11,433 T. - Volcano, Isle de Bourbon, 7,689 T. - Parmesan, Banka, 10,050 T. - Jesso, Isle of Jesso, 7,689 T. - Quilpaoert, 6,400 E. - Mountain in Behring's Isle, 6,000 E. - Mouna Kaah, Sandwich Islands, 18,400 E. - Mouna Roa, ditto, 16,029 T. - Himalaya: Dhawala-giri, 26,862 T. - Jamautri, 25,500 T. - Dhaulum, 24,740 T. - Twenty-seven other peaks raised from 15,700 T. - Nitee Ghaut, 16,814 T. - Temple of Kedar Nath, 12,000 T. - Soomoonang, Bootan, 14,500 T. - Ghassa, ditto, 13,030 T. - Chumularee, Thibet, 11,960 T. - Hindustan: Western Ghauts, 3,500 E. - Town of Octacamund, Neilgherries, 7,416 B. - Dodabetta, ditto, 8,760 T. - Morkoorte Peak, ditto, 8,402 T. - Kodanad, 6,815 T. - China: Petcha, Chinese Tartary, 15,000 E. - Sochendo, ditto, 12,300 E. - Mountains of Corea, 4,480 T. - Asiatic Russia: Italtizkoi, Altai, 10,735 T. - Torgonskol, ditto, 10,700 T. - Katumayaiskoi, ditto, 10,650 T. - Awatscha, Kamtschatka, 9,600 T. - Sludina, 7,722 T. - Tangui, Ural, 4,912 T. - Kyria, ditto, 3,015 T. - Western Asia: Olympus, Asiatic, 6,500 T. - Ida, 4,960 T. - Ararat, 9,500 T. - Lebanon, 9,320 T. - Gargara, 4,960 T. - Carmel, 2,000 E. - Sinai, 5,000 E. - Elburz in the Caucasus, 18,500 T. - Demavend, in Mazanderan, 14,700 B.

#### EUROPE

- Iceland: Snæfell, 6,860 T. - Hekla, 4,960 T. - Solheimas Yökul, 5,590 T. - Oræfa Yökul, 6,240 T. - Akka-Solki, 2,000 E. - Hjalladals Yökul, 2,000 E. - Sneehiltarna, 6,115 B. - Dovrefjeld Range, 4,675 B. - Pass of Jerkin, Dovrefjeld, 4,023 B. - Vorrie, Kielen, Duder Mountains, 3,628 B. - Akka-Solki, Talvig, ditto, 3,392 B. - Sulitlhma, 5,910 B. - Areskutan, Jemtland, 6,180 B. - Sylfiollen, 4,020 B. - Taberg, 420 T. - Brocken, Hartz, 3,600 T. - Bruchberg, 2,500 T. - Ochsenkopf, Fichtelgebirge, 3,920 T. - Summit of Erzgebirge, 3,781 T. - Donnersberg, Mittelgebirge, 2,562 T. - Grosse Rader, 4,972 T. - Schneekopf, Riesengebirge, 4,950 T. - Tafelfichte, Riesengebirge, 3,781 T. - Zobtenberg, 2,825 T. - Mont Blanc, 15,781 T. - Mont Rosa, 15,555 T. - Jungfrauhorn, 13,720 T. - Schreckhorn, 13,397 T. - Furca, 14,040 T. - Finsternarbhorn, 14,116 T. - L'Allié Blanche, 14,775 B. - Nager Horn, 12,217. - Breithorn, 12,800 | Location | Height (ft) | |---------------------------------|-------------| | Bust | 10,112 | | Mont Cenis | 11,460 | | Little St Bernard | 9,594 T | | St Gotthard Pass | 9,975 | | Simplon | 6,463 T | | Rigi | 6,050 T | | Rossberg | 5,154 B | | Hospice Great St Bernard | 8,400 B | | Passice, St Gotthard | 6,817 B | | Tyrol | 15,430 | | Ortler-Spitze | 13,713 | | Glockner | 11,676 | | Hohenwartshöhe | 12,839 | | Orteles | 10,948 | | Muschelhorn | 6,463 T | | Austria | 6,565 B | | Priel | 5,990 B | | Etscher | 5,352 B | | Wechsels, Styria | 5,215 B | | Kasberg | 9,700 B | | Gross Kogl, Carinthia | 9,700 B | | Hungary | 8,640 T | | Lomnitz, Peak of Carpathians | 8,540 T | | Peak of Kesmark | 8,343 T | | Gold-mine of Krivan | 8,500 E | | Greece | 7,000 E | | Orbelus | 6,500 E | | Pindus | 8,000 E | | Olympus | 6,700 T | | Parnassus | 4,000 E | | Albanian Mountains | 1,200 E | | Peloponesian ditto | 4,500 B | | Celene | 7,200 | | Italy | 10,963 T | | Etna, Sicily | 8,455 T | | Le Gran Sasso | 6,307 T | | Monte Velino, Naples | 6,401 T | | Monte de St Angelo, Lipari | 5,260 T | | Vesuvius | 3,975 T | | Soracte | 2,271 T | | Porto Fino, Apennines | 1,920 T | | Spain | 1,439 T | | Gibraltar | 11,673 T | | Mulhacen, Sierra Nevada | 11,393 T | | Pico de Veleta, ditto | 3,900 E | | Sierra Morena | 8,500 E | | Guadarrama | 3,300 E | | Montserrat | 10,285 T | | Pyrenees | 10,657 T | | Le Pic Blanc | 9,700 T | | Tornavacas | 9,290 T | | Canigou | 6,344 T | | France | 14,451 T | | Louceira, dept. Hautes Alpes | 14,444 T | | Olan-en-Valgodmar, ditto | 13,353 T | | Aiguille Noire, Dauphiné | 10,565 T | | Pic d'Autane, ditto | 9,702 T | | Mont d'Or, Auvergne | 6,797 T | | Puy de Cantal, ditto | 6,355 T | | Puy de Sausi, ditto | 6,300 T | | Puy de Dome | 4,759 T | | Puy de Clefrou, ditto | 4,290 T | | Puy de Saudoire, ditto | 3,990 T | | Dole, Jura | 5,412 B | | England | 3,655 T | | Helvelyn | 3,022 T | | Skiddaw | 2,787 T | | Saddleback | 2,901 T | | Grassmerecul | 2,755 T | | Cheviots | 2,658 T | | Bowfell | 2,911 T | | Conistonfell | 2,577 T | | Ingleborough | 2,361 T | | Hedgehope, Northumberland | 2,347 T | | Whernside | 2,334 T | | Highpikes | 2,101 T | | Sheenell, Isle of Man | 2,094 T | | Camelfell, York | 2,245 T | | Holme Moss, Derby | 1,859 T | | Brown-Clee Hill, Salop | 1,805 T | | Axe Edge, Derby | 1,751 T | | Dundry Hill, Somerset | 1,663 T | | Pendlehill, Lancashire | 1,803 T | | Carraton, Cornwall | 1,208 T | | Malvern | 1,414 T | | Pillar, Cumberland | 2,893 T | | Stowhill, Hereford | 1,417 T | | Wrekin | 1,320 T | | Snowden | 3,671 T | | Carnedd, Dafydd | 3,427 T | | Carnedd, Llewellyn | 3,467 T | | Cadair Idris | 2,914 T | | Arran Fowddy, Merioneth | 2,969 T | | Arrenig | 2,609 T | | Caerphilly Mountain | 2,596 T | | Cradle Mountain, Brecon | 2,545 T | | Plinlimmon | 2,483 T | | Radnor Forest | 2,163 T | | Llanddian | 1,898 T | | Llangefnior | 1,839 T | | Moel Famau | 1,845 T | | Tregaron Down | 1,774 T | | Wenmaen Mau | 1,640 T | | Scotland | Ben Macdui | 4,416 T | | Ben Nevis | 4,358 T | | Cairngorm | 4,050 T | | Ben Wevis | 3,729 T | | Ben Lawers | 3,944 T | | Bein More, Assynt | 3,963 T | | Ben More, Perthshire | 3,818 T | | Ben Ledi | 3,651 T | | Scafell | 3,613 T | | Ben Dearg | 3,550 T | | Ben Gloe | 3,724 T | | Ben Ferkinich | 3,482 T | | Scalicroch, Aberdeenshire | 3,400 T | | Ben Ailort | 3,301 T | | Ben Gunly | 3,364 T | | Creuchan | 3,290 T | | Mount Battock, Mearns | 3,450 T | | Ben Veirlich | 3,279 T | | Ben Lomond | 3,191 T | | Stuchachrome | 3,171 T | | Ben Dearg | 3,550 T | | Ben Venach | 3,006 E | | Bednain-brawn | 3,150 T | | Goatfell, Arran | 2,945 T | | Pap of Jura | 2,479 T | | Cobbler | 2,863 T | | Hartfell | 2,635 T | | Black Larg, Ayrshire | 2,890 T | | Tintock | 2,306 T | | Lowther Hill | 2,522 T | | Elden Hills | 1,300 T | | Dollarburn, Peebles | 2,840 E | | Broad Law, ditto | 2,600 T | | Leadhills, house | 1,564 B | | Carnethan, Edinburghshire | 1,700 T | | Arthur Seat, Edinburgh | 810 T | | Macgillieuddy's Reeks | 3,410 T | | Sleibh Dorin, Derry | 3,150 T | | Croagh-Patrick, Mayo | 2,666 T | | Morne Hills, Down | 2,500 T | | Croaghan, Kinshelly | 1,650 T |

From this table it appears that the loftiest pinnacles of our earth are to be found in the Himalaya range. Some of them attain an elevation a mile higher than Chimborazo, which was long regarded as the loftiest mountain on the globe. Of late years our countryman Mr Pentland has measured several mountains in Upper Peru, especially near the Lake of Titicaca, much higher than Chimborazo; and Sorata, one of these, is only surpassed by a few peaks in the great Indian chain. Although those who first attempted to measure the summits of the Himalaya appear to have exaggerated their real altitude, succeeding observers have shown that we must consider that range as the most elevated on the globe, and the peak of Dhawala-giri as the sovereign of mountains.

Human habitations, and even considerable towns, have been erected in very elevated stations on mountain chains. Dr Gerard states, that in the valley of Sulei, in the Himalaya Mountains, there is a village at the enormous elevation of 14,700 feet above the sea, and that rye is cultivated as high as 14,900 feet. Several travellers have found The mines of Potosi = 16,080 feet. Town of Potosi = 13,550 ... Shepherds' huts at Antisana = 13,200 ... Lake and plain of Titicaca = 12,700 ... City of Riobamba = 10,800 ... City of Quito = 9,356 ... City of Toluca, Mexico = 8,818 ... City of Mexico = 7,470 ...

In Europe we have the following. Hospice de St Gothard = 6,790 ... Director's house at Leadhills, the highest habitation in Britain = 1,280 ...

Sect. IV.—General Arrangement of the Solid Materials of the Globe.

A particular description of the solid materials of our earth has been given under the articles Geology and Mineralogy; but it is our business to present here a brief statement of their general distribution.

Persons unaccustomed to examine the structure of the crust of our earth are apt to imagine that mineral masses present but a confused congeries of stony matter, without any order or particular arrangement. Nothing, however, can be more erroneous. Wherever water-courses, artificial excavations, or abrupt precipices, expose the structure of the crust of the earth, we find very striking appearances of the agency of causes which must have acted with great uniformity over vast portions of its surface, and which have produced a general resemblance between the structure of countries widely distant from each other. A large portion of the solid materials is arranged in beds, varying in extent and thickness, but everywhere indicating the operation of one common agent. These beds are sometimes slightly coherent, as when composed of clay or sand; at other times they are consolidated stony bodies, arranged in parallel layers, which are often subdivided into thinner portions by seams or joints, and preserve their parallelism for a great extent, whether their position be horizontal, or at different degrees of inclination. Such beds are termed strata, and the general fact is expressed by the term stratification. Sometimes we find a succession of strata of the same rock in juxtaposition; at other times there are strata of different substances interposed or alternating with the principal rock. The position of strata in plains is generally but little inclined towards the horizon; but as we ascend mountains, the strata are usually inclined, sometimes nearly, or even absolutely vertical. When a mountain, or a chain of mountains, is lofty, it is not unusual to find the strata inclined from the centre or axis of the chain towards either hand (No. 2); and in such cases we generally find the strata reposing on a different kind of rock, which has no appearance of having a seamed or stratified structure. Such rocks may also occur in plains; they have frequently a granular or crystalline structure, and form an important class of stony bodies. Hence the distribution of rocks into two great divisions, the stratified and unstratified. Both divisions present instances of rocks which either appear simple or homogeneous to the eye, or consist of stony bodies, which are evidently composed of two or more materials. Instances of the one occur in clay-slate, limestone, quartz-rock, and basalt; of the other in mica-slate, gneiss, granite, and porphyry. The visible axis of a mountain chain is very often some species of unstratified rock, and stratified materials usually form its flanks. In other instances no unstratified material is visible at the axis, as far as we can trace the structure of a mountain. But whenever the mountain is much elevated, or steep, and its sides ploughed by torrents, it is very rare not to find some unstratified material as the nucleus upon which the inclined strata rest (No. 3, g). We are not, however, to assume, that in the order of superposition the stratified materials are invariably above the unstratified. Though this holds good in a great number of cases, we frequently observe vast masses of unstratified rock directly resting upon regular strata (No. 3, f). In some cases we are able to show such irregular masses connected with other similar materials much lower in the series of rocks, by one or more lines of communication penetrating the strata, which, according to their shape and size, have obtained the name of veins or dykes. Instances of this occur frequently in mountains into the composition of which trap-rocks enter (No. 4).

There are, however, other kinds of veins which we cannot so readily trace to larger masses of mineral substances. The most familiar instances of such occur in metallic veins (No. 5). They may often be observed penetrating both stratified and unstratified matter, in the greatest diversity of forms and directions; sometimes in very minute threads, expanding to the dimensions of many feet, or even yards, and repeatedly contracting again to the thickness of a fraction of an inch.

Sometimes mineral bodies form irregular masses entirely enclosed in the rocks in which they occur, varying in weight from a few grains to many tons, without any perceptible trace of the mode by which they were so introduced. Occasionally long fissures occur, which are not filled up by any solid material; sometimes also cavities are left in the more solid species of rocks, the sides of which are lined with crystalline bodies of various kinds. Sometimes these fissures and cavities are of enormous extent, forming vast caverns in the bowels of the earth, which are either empty or filled with water. Instances of these on the great scale oc- Physical Geography.

car in the limestone strata of many countries, especially of Carniola and North America; and on a less scale in France, Germany, and in our own island. The cavern of Adelsberg, in Carniola, extending to three leagues in its various ramifications, and containing a subterranean lake and river, affords an example of the extent of caverns in limestone rocks; and the no less remarkable Mammoth Cave of Kentucky, which has been traced to the extent of nine or ten miles, is an American example of the same formation. Almost every extensive cavern of Europe is also in limestone, as the interesting ossiferous caves of Gailenreuth and Küloch, in Franconia; of Bize, Somières, and Racogne, in France; the Kirkdale Cave, and the caverns of the Craven and Mendip Hills, in England; but the fine stalactitic cave of the Isle of Skye is in sandstone. We do not recollect any considerable caverns found in other rocks, excepting one in the schist of the island of Thermie in the Archipelago, and the vast Icelandic cavern of Surtsellir, in a current of lava which had flowed from the Bald Yökul. This last cavern, as described by Henderson, is nearly a mile in length, and appears to have been formed in the flowing lava by the disengagement of gaseous matters; as it has all the appearance of being the consequence of an enormous air-bubble, having its arched roof ornamented with coralliform projections of black slag. Perhaps we may also give as an exception the small but remarkable columnar cave at Staffa, which is in trap-rock.

Such is the general structure of the solid crust of our earth. But when we examine the rocks more minutely, we find other striking peculiarities belonging to each, which, besides the order of their superposition, mark different eras in their formation.

The regularity of the lines dividing stratified rocks early suggested the idea of their deposition from water; and this was rendered still more apparent when observers began to remark that the remains of organic beings were often included in them. Plants, shells, and corallines were first noticed as entering into the composition of rocks; and next the remains of fishes, and of other vertebrate animals, were distinctly recognized. Some organic remains were observed to pervade limestone rocks at great depths in the earth, and upon very high mountains; and remains of plants, and even stems of trees, were found fossilized, or converted into stone, in the strata of the coal formation. Some remains of marine genera of animals were detected in slaty strata which do not belong to either of these two kinds of rocks; and, lastly, the remains of quadrupeds were recognised in the sandy marl and gypseous deposits which in some countries lie immediately below the soil, or were imbedded in sedimentary matter in the floors of caverns in various parts of the world. In most instances there is an alternation of marine and fresh-water organic remains; and the general fact appears to be, that the fresh-water shells are most numerous in the upper beds, and those of marine origin in the subordinate strata of the low country, while the former seldom attain any considerable height on mountains; but the marine animal remains are found on the summits of ridges as lofty as the Pyrenees.

Minute examination of the organic remains collected from different strata have established the fact, that comparatively but few of the fossil animals are of the same species with those of the same families now living; and we cannot doubt that whole genera and orders have disappeared from the face of the earth. It is only in the upper deposits that any species identical with those now existing have been detected. Where the organic bodies are completely fossilized, very few if any living species can be recognised; and generally the recent species most nearly allied to the fossils are now no longer to be found in the adjacent seas or regions. Thus the fossil-shells and corallines of northern countries, in both continents, have their congeners (when such are known) generally in tropical climates; and the fossil-trees of the British coal-fields have a greater affinity to the tree-ferns and cycadaceae of southern regions, than to any sort of European vegetation.

These circumstances are too remarkable not to have suggested theoretical speculations. Accordingly, geology has long been conversant with sublime, but hasty, and sometimes unwarranted generalizations; and though she is still remote from the period when a lasting theory shall safely and satisfactorily embrace the explanation of observed phenomena, yet she has begun to assume a more philosophical aspect. Facts are rapidly accumulating, and the deductions of ingenious men, applying the science of number and calculation to this interesting study, have already established a basis on which future observers may erect a philosophical theory of the earth.

The crystalline arrangements of the particles of a considerable portion of the solid materials of the earth's crust, indicate, beyond doubt, that the particles have been once in a state of such freedom of movement as to assume a determinate order, that is, in a certain degree of fluidity; and hence the geologist assumes that such minerals must have either been softened, or melted by heat, or have been suspended in a fluid medium. The phenomena of stratification, and the incorporation of organized bodies in different rocks, are not less certain indications that a large portion of stony matter has obtained its present form by subsidence from water; and this constitutes the grand basis of geological reasoning. The second link in the chain, the relative age of the different strata, is derived from the order of their succession. As the strata bear the marks of deposition from water, it is obvious that the lowest in the series must have been the first deposited, and the upper beds the most recent formations. No such chronological data are afforded by the unstratified rocks, though their relations to stratified materials afford just inferences respecting their true era; but it is from the examination of the strata of any country that we can arrive at a just conception of its geological history.

Relative geological age, then, is to be sought by a careful examination of the superposition of the strata; and as soon as the observation of organic remains has shown a succession of living forms to be peculiar to each series of rocks, the knowledge of such remains forms an important feature in modern geology. Certain fossils are peculiar to certain formations, and we are sometimes able to identify particular rock formations by their included organized remains. It is on such a basis that the most generally received classification of strata has been established; and geologists now usually divide them into three great classes, the primary, secondary, and tertiary.

The primary strata are such as do not include any organic bodies. They are the lowest in the series of strata, and are found to repose on crystalline unstratified rocks, generally on granite, which, as far as we have penetrated the crust, has the appearance of being the general fundamental rock. The name primitive was originally given to these rocks because they were supposed to have been deposited before the creation of organized bodies; but this involves a hypothesis which we cannot prove, as the disappearance of organic remains may be owing to the cause which crystallized such rocks. The term, however, as indicative of the oldest strata, is convenient, although the theory which suggested it may be exploded.

The secondary strata are generally less crystalline in structure, and in their lower beds the organic remains are few, being chiefly marine productions; but these increase in number and variety in the upper beds, which may be said to end with the chalk formation. The organized bodies are chiefly marine productions, as zoophyta, crustacea, shells, fishes; sometimes reptiles, amphibia, and plants are also found in secondary strata. During the deposition of the secondary strata the land appears to have been gradually rising above the level of the waters which once covered it; and during the same period there are marks of more sudden changes or disturbances among them, which are most probably to be accounted for by the forcible irruption of igneous rocks from the bowels of the earth. The land appears to have attained extensive elevations above the waters after the deposition of the cretaceous formation, the newest of the secondary rocks; and another system of deposits now begins to be developed. Stratified rocks appear now also to have been formed by deposition from fresh water, as is proved by the abundant remains of animals inhabiting that element; and this is one characteristic of the tertiary strata.

It is true, that in the coal-fields of the secondary rocks, we occasionally find alternations of sea and fresh water animals; which may be explained by such formations having taken place in extensive estuaries, into which rivers have flowed. But the decided alternations of fresh and salt water deposits are peculiarly characteristic of the tertiary period; and this, with the limitation of such formations to particular basins or circumscribed hollows, enables us to recognise them. Some of the best recognised of these beds are the Paris Basin, the London Basin (No. 6), and that of the Isle of Wight and the opposite coast of Hampshire. Tertiary beds appear to be entirely wanting in Scotland and in Ireland; but they occur in Germany, in Spain, in Italy, and in different parts of Asia and America. In the upper strata of the tertiary period we find many shells similar to living animals in the adjacent seas; and bones of extinct mammalia also occur among their beds. Sometimes the remains belong to genera of which no species now exists; at other times to genera of which we still possess living types.

M. Deshayes has attempted to ascertain the dates of tertiary deposits by the number of living species they contain, considering that which contains the greatest number of existing species the most recent. He distinguishes three tertiary epochs. The most ancient is that of the Paris, London, and Hampshire Basins, in which he finds 3-5 per cent. of living shells, corresponding to the Eocene deposits of Mr Lyell. The middle group includes the basins of the southwest of France, of Baden, Vienna, Hungary, and Moravia. Among these he found 15-2 per cent. of living shells; and this corresponds to Lyell's Miocene period. The most recent strata include the tertiary beds of Italy, Sicily, Spain, of the south-east of France, the Morea, and the eocar formation of England. In it he detected 45-4 per cent. of living shells; and it corresponds to the Pliocene formation of Mr Lyell.

Besides these great formations, there are local deposits of more limited extent, which we may slightly notice. 1. The formations at present going on, or elevated within historical periods, by earthquakes and volcanic agency from the bed of the sea, and added to the dry land. 2. Lacustrine deposits, now forming the rock called travertino, or giving rise to marly sediments. This last deposit, at Big-bone-Lick, in Kentucky, contains bones of the mastodon and other mammalia, with fresh-water shells; and at Market Weighton, in Yorkshire, the bones of the elephant, with those of the lion and the wolf. 3. The incrustations of calcareous matters cementing fragments of other rocks, as in the nagelfluke of the Germans; or including bones of living species, which have fallen or have been washed into caves and fissures. 4. Superficial deposits of gravel and sand, evidently the produce of rivers or of floods, which sometimes contain bones of elephants, the rhinoceros, &c., and are distinguished by the name of diluvian. 5. Erratic blocks of different rocks, which have been removed from their original beds, and scattered over extensive tracts. The most remarkable instances of this transportation are the profusion of large blocks of the zircon syenite of Norway, scattered over all the north of Germany, to the very foot of the Jura chain; of the granites of the Alps on the summit of the Jura, and beyond it into France; and of the granite of Sharp Fell, over the intervening mountains, into Lancashire and Yorkshire. 6. The deposits formed by volcanoes at present in activity, consisting either of ashes, mud, or lava; to which we may add the obsidian, pumice beds, trachyte, and basalts of extinct volcanoes. The best characterized and most remarkable of the former are the ashes which have buried Pompeii, the mud eruptions of Tunguragua in 1797, the enormous lava streams of the Skaptar Yokuil in 1783, and of Vesuvius and of Etna at numerous periods; of the latter, we have examples in the obsidian of Lipari, Ascension Island, and Teneriffe; in the pumice beds of Lipari and of Auvergne; in the trachytes of the latter country, of Germany, and Italy; and in the basalts of almost every region.

The general opinion now entertained by geologists is, that the stratified materials of the earth's crust have been horizontally deposited from water at different eras, but that the unstratified rocks have been produced by the action of a high temperature. The consolidation of the older strata is supposed to have been accomplished by heat, produced either by internal changes among the elements of which they consist, or derived from a source of heat deep seated in the earth. The less crystalline state of the secondary strata, with the more perfect preservation of the forms of these organic remains, are ascribed to their having been subjected to a less intense temperature; while the still more earthy texture of the beds, and less altered nature of the organic bodies among tertiary strata, are ascribed, with seeming justice, to the still less heat they have experienced. The loose beds of clay and sand appear to have been deposited when the crust of our earth was cooled down to its present state.

The unstratified rocks are now generally allowed to be of igneous origin, and believed to have been introduced into the rocks, their present situations after the deposition of the greater part of the stratified materials. These are believed to have been liquefied by heat in the bowels of the earth, and forced upwards by the expansion of elastic matter, either in a softened state, or, in some instances, after they had been melted and again consolidated. In this view, granite, which so often forms the nucleus of mountains, and the lowest basis we can discover of other rocks, is supposed to have been acted on by heat, and slowly cooled after it had been elevated by the agent which liquefied it. This elevation is considered as having produced the fractures, dislocations, and elevations of the stratified rocks; and the heat of the melted material, thus introduced, is believed by most geologists to be the cause of the crystalline state of the older strata which rest on it. In some instances the elevating force has tilted the strata to an absolutely vertical position; at other times it has carried up detached portions of the strata, and separated them from the principal bed, or even inverted the order of their superposition in considerable masses (No. 7). The injection of the fluid matter of granite between some of the strata, has given rise to the rare appearance of beds of this material; and when the force applied has rent or fissured the different strata, some of the liquefied matter has formed granitic veins, penetrating the stratified matter in different directions.

Trap-rocks, in like manner, are now generally considered as the immediate progeny of fire. They are occasionally found as interposed beds, or sometimes alternate with other matter, in distinct but thick beds, though not exhibiting the seams of true stratification. Most geologists regard such appearances as the consequence of subaqueous volcanic eruptions, in which liquid matter had flowed out of craters, spread over the adjacent strata, and been covered by aqueous depositions; after which a repetition of the same process may have given rise to such alternations. The more ordinary appearance of trap-rocks, especially of greenstone and basalt, is either in the form of veins penetrating stratified rocks, or as forming mountain caps, or huge beds superimposed on secondary, or sometimes on tertiary strata. Familiar instances of these veins occur in all coal-fields, more especially in the vicinity of the great coal deposits around Edinburgh and Glasgow. Arthur's Seat, near the former, affords a good example of a mountain capped by trap resting on sandstone strata; and Salisbury Craig, of a vast bed of greenstone overlying the same strata (No.8). The chalk of Antrim is in the same manner covered by greenstone and basalt; and the tertiary strata of Auvergne are often concealed by beds of trachyte or other ignigenous rocks.

The movements which produced these changes are believed to be comparatively of considerable antiquity, that is, before the records of history; but there are some causes in operation at the present day, which may affect the stability of those more ancient arrangements, and which therefore we must now briefly consider.

Sect. V.—Circumstances affecting the present Order of Geological Phenomena.

These may be considered as four: Atmospheric action; the effect of the waters of the ocean and of floods; disturbances occasioned by earthquakes; and the influence of volcanoes.

1. Atmospheric action is slow, but constant. The united influences of winds and rains serve to form detritus, by which the higher portions of the earth's surface are gradually lowered, and the materials thus abraded carried by streams and rivers to the sea. The consequence must be, a very gradual change in the relative level of the land and the ocean. The abrasion of mountains by torrents and impetuous rivers produces deposits of gravel, sand, and mud in plains at their feet. These effects are increased by the sudden bursting of water long pent up in the bowels of a mountain, or dammed up in an alpine lake; thus giving rise to the phenomena which are termed debacles. Instances of the fall of mountains have been frequently afforded in the Alps and other lofty mountains. The fall of the Rossberg was foretold by General Pfeiffer, several years before it happened, from the existence of enormous collections of water in its bowels; and in the recent fall of one of the Aiguilles, which border the southern side of the valley of Chamouni, a vast torrent of liquid mud, bearing before it woods and vast masses of rock, proved the influence which dammed-up water had in the catastrophe. The fall of the mountain of Chiavenna, which in 1618 buried the town of Pleurs and all its inhabitants, appears to have been produced by water insinuating itself among the shaly strata which underlie the limestone beds in the Italian Alps. Among lofty mountains, another cause of disintegration is perpetually at work. The accumulation of snow, and the formation of glaciers on their steep acclivities, where there is a considerable difference between the summer and winter temperature of the climate, is a powerful cause of the degradation of these elevations. The fall of the thundering avalanche, and the movements of the increasing glacier, from its own weight, detach vast masses of rocks, and hurl them into the valleys below. Even the action of minute particles of sand, moved by the winds for a succession of ages, is capable of wearing the hardest stones. Thus the polished surface of the granite of Sinai has been, with great probability, attributed to the perpetual descent of quartz sand over the rock; and we have in our possession nodules of Egyptian jasper, the exterior of which has been worn into smooth and deep channels, apparently by the long-continued action of the sands of the desert of Nitria on the surface of this hard mineral.

The heavier detritus of mountains is accumulated at their bases; but land-floods often convey large blocks of stone to astonishing distances. The finer particles are swept by rivers to the ocean. Humboldt has pointed out how the Gulf of Mexico is rapidly filling up by the sedimentary deposits from the waters of the Mississippi and other rivers which fall into that inland sea. The turbid waters of the Orinoco and the Maranon discolour the ocean to the distance of 300 miles from their mouths; and there can be no doubt that the formation of the flat land of Guiana, now about 100 miles in width, from the most eastern ridge of hills, is principally the alluvial deposition of these mighty rivers. The waters of the Ganges, at the highest inundation, are stated by Rennell to contain an enormous quantity of sedimentary matter; and we can hence conceive how the flat shores of the Hoogly, and the other branches, have originally been formed, and how the Bay of Bengal may, in process of time, be materially altered in its depth. Another striking instance of the effect of rivers in silting up a confined sea is to be observed in the Gulf of Leaotong, or upper part of the Yellow Sea, into which a great part of the detritus of Northern China is deposited by its numerous large rivers, and by the effects of the strong currents in that inland sea. Staunton has calculated that the Yellow River alone discharges into the Yellow Sea more than 2,000,000 cubic feet of earthy matter every hour. The matter carried down by the Nile and the Po is well known to affect the extent and deepness of the adjacent seas; and the distance of Ravenne from the present shores of the Mediterranean (between three and four miles) proves how a comparatively small river like the Montone, pouring its sediment into a still bay, has altered the situation of a port, where, in the ages even of the lower empire, the Roman fleets used to anchor. These facts would lead to the supposition that the land was gaining upon the sea; but there are other circumstances which are at least equivalent to this change.

The natural consequence, as was well pointed out by Playfair in his Illustrations of the Hattonian Theory, and previously noticed by Manfredi, must be to elevate the surface of the ocean, by the deposits of detritus in its bed. This rise is most conspicuous in inland seas. Thus, on the western side of Italy, the ruins of the palace of Tiberius at Capri are now wholly covered by the sea; and the far-famed temple of Serapis, situated on the Bay of Baiae, has its pavement below the present level of the Mediterranean. The same fact is attested by observations along our own coasts. The submarine forest on the coast of Lincoln, first described by Correa de Serra in the Philosophical Transactions, 1799, proves a rise in the level of the sea; and the tradition, that the Godwin Sands once formed part of the fertile domain of the Saxon earl of that name, has probably a fair claim to be arranged with the same phenomenon.

2. The wasting of the land by the waves of the ocean, and the overwhelming of large tracts by the sea, are nearly allied facts. The changes on our own coasts, and on the countries between the Rhine and the Elbe, afford striking instances of the encroachments of the ocean on the land. Even where the rocky boundaries are the most solid, the ceaseless action of the waves has wasted and disintegrated the cliffs around all our shores; but the destruction of the softer strata along the east coasts of England, and the chalky heights of Dover, is still going on, and affords undeniable proofs of the power of this agent. The swallowing up of whole districts along the coasts of Yorkshire is shown in the annihilation of the villages of Auburn, Hyde, and Hartburn, and in the ravages of the sea on the neck of land uniting the Spurnhead with the rest of the country. We have authentic records of the destruction of Dunwich, on the coast of Suffolk; and near Harwich, in Essex, the sea is annually encroaching. The Start islet was within the memory of man a part of the island of Sandey, in the Orkneys.

Still more remarkable changes have taken place on the coasts of Holland and Friesland within historical periods. Well-known instances of this occurred in the devastation which accompanied the formation of the Zuyder Zee in the beginning of the thirteenth century; in the successive encroachments of the ocean, which formed the islands near the Texel in 1251; and in the disappearance of the once fertile district of North Friesland, the only remnants of which, since the disastrous 11th of October 1634, are the three small islets of Noordstrand.

Such changes have occurred in various parts of the earth. Some of them, especially the overwhelming of low sea-coasts, may be owing to the gradual elevation of the level of the sea by detritus washed into it; but probably still more is due to changes in the level of the land by unequal subsidence, the consequence of earthquakes.

3. The terrible effects of earthquakes have attracted attention in every age, and records of some of such convulsions have descended to us from very remote ages. No period of our earth's history has been more noted for the violence and extent of earthquakes than that between the first, third, and the middle of the fourteenth century. China was terribly convulsed for ten years from 1333, when Kiang-si, its capital, was swallowed up; mountains were engulfed, and floods, occasioned by the obstruction of the course of rivers, destroyed vast multitudes of human beings. The succussions extended westwards, and Asia Minor and Egypt were violently shaken in 1346; while in the following year severe earthquakes were experienced in Cyprus, Greece, and Italy. In 1692, the island of Jamaica was visited by a most terrible earthquake, in which enormous masses of earth and rock were detached from the Blue Mountains, and vast quantities of timber, hurled from their flanks, covered the adjacent sea like floating islands. It was during this earthquake that the city of Port Royal, with a tract of adjacent land, estimated at 1000 acres, sunk, in one minute, into the deep. In the succeeding year great earthquakes in Sicily destroyed the city of Catania, and a hundred and forty other towns and villages in that island, where upwards of 100,000 persons perished. In 1746, Lower Peru suffered severely from this calamity. The ocean burst in upon the land with irresistible force, when the barrier of land sunk into the sea, Lima was overwhelmed, and the present port of Callao formed. These convulsions were accompanied by eruptions of water and mud from several volcanoes among the Andes. In 1750, the city of Concepcion, in Chili, disappeared during an earthquake, and the sea rolled over it. In 1755, Lisbon was in a great measure destroyed by one of the most terrible earthquakes that ever visited Europe. The mountain chains between the Douro and the Tagus were most dreadfully convulsed. The new mole at Lisbon, to which multitudes had fled, as to a place of safety, suddenly sank into an hideous abyss, and not one body floated to the surface, nor were any fragments of the vessels sucked into the chasm rendered up, and on the spot there is now an hundred fathoms of water. In this awful convulsion at Lisbon sixty thousand persons perished in about six minutes. A violent shock threw down the greatest part of the city, and the sea retired, leaving the bar momentarily dry; but suddenly a mighty wave, fifty feet high, rolled in on the devoted city. The extent of the terra motus, on this occasion, is very remarkable. The violence of the shocks, which were accompanied by a fearful subterranean noise, like the loudest thunder, was chiefly felt in Portugal, Spain, and Northern Africa; but its effects were perceived over a considerable part of Europe, and were even experienced in the West Indies. Our Scottish lakes, particularly Loch Ness and Loch Lomond, rose and fell repeatedly on that dreadful day. Ships at sea were affected by the shocks, as if they had struck on rocks, and their crews were in some instances thrown down by the violence of the concussions. In 1766, the island of Trinidad, and great part of Colombia, were violently agitated by earthquakes; an islet in the Orinoco disappeared, and land in other parts of the coast was raised above the waters. In 1772, the lofty volcano of Papandayang (the loftiest mountain in Java) disappeared, and an area around, fifteen miles by six, was swallowed up. Most terrible earthquakes desolated Calabria in 1783. This calamity has been admirably described by several writers, among whom we may mention Dr Vivienzo, Grimaldi, Sir William Hamilton, Dolomieu, and the commission of the Royal Academy of Naples. The violence of the shocks was chiefly felt in the Farther Calabria, and in the neighbourhood of Messina. The principal agitation was felt over an area of five hundred square miles. Many sudden sinkings of the land were perceived, numerous fissures were formed, and partial elevations were effected in some places. The greatest depressions happened at Terra Nuova, Oppido, Sinopoli, and Santa Christina; a fissure a mile in length, a hundred and five feet in width, and thirty feet in depth, opened at Plaisano; another about the same length, and a hundred feet in depth, was formed at Cerzalle; and at La Fortuna a chasm a quarter of a mile in length, thirty feet in width, and two hundred and twenty-five feet in depth, suddenly opened in the ground. In some districts considerable mountain-slips, or the separation of huge masses of earth and rock, took place; and along the Straits of Messina, in the neighbourhood of Scylla, the cliff of Gian-Greco, a mile in length, was precipitated on the subjacent houses and gardens. A fragment detached by the earthquake from Monte Jaci crushed multitudes who had fled to the shore for safety; and at the same moment a wave broke on the devoted shore, and swept away the aged prince of Scylla, and numbers of his people.

In the year 1797, Upper Peru was terribly convulsed. The shocks of earthquakes continued with great violence for three months, and the face of the country in the centre of the convulsion was totally changed. In 1811, violent earthquakes shook the valley of the Mississippi, by which lakes of considerable extent disappeared, and new ones were formed; but these were less terrific than the catastrophe which destroyed the city of Caracas in 1812. On the 26th of March there were heard subterranean thun derings; the ground undulated, as if agitated by a boiling liquid, and at one shock this fine city entombed in its ruins 10,000 of its inhabitants. During the earthquake the great lake of Maracaibo had its level lowered, and the riven earth at Puerto Cabello and Valencia poured forth enormous torrents of water. It is remarkable, that the volcano of St Vincent, which had been perfectly quiescent for a century, burst out with prodigious violence on the 27th of April in the same year, and threw out clouds of ashes, which rose to an immense height into the air. Much of the island was ruined by showers of scoriae and ashes; and such was the violence of the eruption, that the decks of vessels 200 miles to windward of St Vincent were covered with an impalpable dust. On the day of this eruption subterranean thunderings were distinctly heard at Caracas, and even on the Rio Apure, 210 leagues in a right line from St Vincent.

In the eruption of Tomboro, about to be noticed, the earthquakes extended throughout an area a thousand miles in diameter, a considerable tract of land at the foot of the mountain disappeared in the waves, and the port of Bima, where ships of war could formerly anchor, was ruined by the uprising of a shoal. On the 19th of November 1822, Chili was visited by a most destructive earthquake. The shock was strongly felt at the same time throughout a line of coast 1200 miles in extent. It is stated, on good authority, that the coast for one hundred miles sustained an elevation of from two to four feet; and about a mile inland from Valparaiso it was raised from six to seven feet. The sudden elevation of the coast was indicated by shell-fish being found adherent to the rocks considerably above high-water mark. The shocks continued until the end of September in the following year; and the area over which the permanent alteration of level extended is believed to embrace the country from the base of the Andes to the sea, a surface of not less than 100,000 square miles. In 1827 Popayan and Bogota suffered most severely from earthquakes, during which vast fissures opened in the elevated plains around the latter city.

The last earthquake in Europe occurred in Murcia in 1829, near Alicante. Several villages in an area of above four square miles were thrown down by vertical movements in the valley through which the Rio Segura flows, and many small fissures were formed in the alluvial soil; while black mud, sand, and marine shells were thrown up from small cavities formed near the sea. Such are some of the severer earthquakes on record; but less considerable shocks are of frequent occurrence in various countries, especially in South America and in Italy. Smart shocks are occasionally felt in Scotland. They have often occurred at Comrie in Perthshire. A smart shock rent the spire on the town-hall of Inverness in the year 1816; and another earthquake was felt at Lancaster in 1834, which shattered chimneys, and alarmed the inhabitants. But all the shocks experienced in our island have fortunately been insignificant compared to those which have been felt in many other countries.

4. The alterations produced on our earth's surface by volcanoes in activity within historical periods appear to be underrated by some writers. Few geologists now will be disposed to doubt that most of the phenomena of earthquakes are the consequences of volcanic fire. The coincidence of those formidable shakings of the earth with volcanic eruptions, and the distance to which such agitations have been felt, show that the causes of both are deep-seated in the earth, and that they are probably the consequences of elastic matter formed by subterranean heat. We shall say nothing of the almost universal diffusion of trap-rocks, which, under the name of trachyte, basalt, greenstone, and trap-tuffa, are now generally admitted to be of igneous origin, nor of the true craters of extinct volcanoes which abound in France, Germany, and many other parts of the earth, but which have ceased to emit any eruption within known periods, and confine our attention to those mountains now either in active combustion, or which have been so within historical periods. Gay-Lussac enumerates 163 volcanoes active at the present time; and Professor Jameson gives the following distribution of 193 volcanoes in a state of present activity:

| Continent of Europe | 1 | |---------------------|---| | European islands | 12| | Continent of Asia | 8 | | Asiatic isles | 58| | African islands | 8 |

To these we may add the gigantic crater of Kiranea, in Hawaii, one of the Sandwich Islands; the volcano of Tomboro, in the isle of Sumbawa; the volcano of Tofan, in the Friendly Islands; that of Barren Island, one of the Andamans, in the Bay of Bengal; to which we may perhaps add the submarine volcanoes which have suddenly risen from the bosom of the deep, but soon afterwards have again disappeared, as that of Nyoe, off the coast of Iceland; Sabrina, off St Michaels, in the Azores; and Graham's Island, not far from the east coast of Sicily.

The substances ejected by volcanoes consist either of melted stony matter termed lava, of scoriae, of ashes, of sand, of muddy eruptions, or of vast quantities of hot and fetid water, always accompanied by earthquakes and subterranean thunderings, and generally by columns of flame or of dense smoke.

The general form of a volcano is a truncated cone, often isolated, and having a circular depression or spiracle on or near its summit, which is named a crater. From this crater smoke is perpetually issuing; sometimes flame appears, or volleys of ignited stones are projected into the air. From the dark cloud of smoke emitted during eruptions, vivid lightnings are usually seen to dart; and the vapours of sulphur, ammonia, or of muriatic acid, are often emitted. Sometimes the boiling lava is elevated from the bowels of the mountain to its terminal crater, fills that cavity entirely, and descends from the cone in streams of liquid fire, which concretes as it cools on the surface, but continues to move on with restless force till it congeals into stone, though it often retains its heat for long periods. At other times the elasticity of the pent-up matter is incapable of forcing the lava to the highest crater; but the liquefied material bursts through the flanks of the mountain, and pours into the plains. Sometimes the volcanic cone is included in an exterior one; a circumstance on which Von Buch has founded his theory of craters of elevation (No. 10). The strata or beds in both cones are inclined from the general axis. The projectile power of volcanoes is prodigious. The dust of the volcano of St Vincent was carried more than 200 miles eastward of that island, and must have attained a vast elevation, to obviate the effect of the trade-wind in its descent. It is stated, on credible authority, that Vesuvius has projected large stones 3600 feet above its summit; and Cotopaxi was ascertained by the French academicians to have hurled a rock, calculated to weigh 200 tons, to the distance of three leagues, or more than ten English miles.

Instances of volcanic eruptions are by no means rare. There are fifty-nine or sixty eruptions of Etna recorded by historians, and about fifty-two of Vesuvius. The first eruptions of Etna are lost in the remoteness of antiquity. Vesuvius was not in existence as an active volcano till the year seventy-nine of the Christian era, when the ancient crater presented a slight circular hollow, the sides of which were overgrown with wild vines, and the exterior of the cone was well cultivated. In the eruption which then took place, Stabiae, Herculaneum, and Pompeii were destroyed, and the elder Pliny lost his life. Stromboli appears to have been an active volcano from 300 years before Christ, and is still in a state of incessant activity, ejecting flame and red-hot stones in quick succession. The most noted eruption of Etna was probably that of 1699, when, from a long rent on its flank, it poured out a stream of lava, which in its course to the sea destroyed fourteen towns and villages, some of which contained three or four thousand inhabitants. This current is fifteen miles in length, about 600 yards at its broadest part, and there it is forty feet deep. Vesuvius has more frequently been active in our own times than Etna. One of its most terrible eruptions was that of 1538, when Monte Nuovo was forced up in two days to the height of 413 feet, and with a circumference of 8000 feet. Another happened in 1631, which covered with lava most of the villages at its foot, and sent forth torrents of boiling water. In little more than a century it has had eighteen eruptions. The eruption of 1794 has been well described by Breislac, who estimates one of its streams of lava on that occasion as containing upwards of forty-six millions of cubic feet. It again burned with great fury in 1805, 1813, and in 1822. We possess lavas of this latter period so strongly impregnated with sulphuric and muriatic acids, that they destroy every species of vegetable material they rest on; and, when recent, the specimens smelled strong of the latter acid. The volcanoes of Iceland have been in activity ever since the island was discovered, in the ninth century. Hekla, though not the most considerable of these, from its position and its former activity, is the best known. It has had many formidable eruptions, twenty-two of which have been noted in about 800 years; and in the same period we have notices of twenty eruptions from five other Icelandic volcanoes. A succession of eruptions of Hekla lasted for six years; but the most severe convulsions of that country happened in 1783, when the dreadful eruption of the Skaptar Yökul burst forth, and did not cease till the following year. About a month before this terrible catastrophe a submarine volcano elevated the crater of Nyöe, seventy miles south-west of Cape Reikianes, and threw out such an immense quantity of scoriae as to cover the sea, to the distance of 150 miles, with a stream which impeded the progress of ships making the island; and portions of this eruption floated as far as the Shetland and Orkney Islands. Nyöe emitted smoke and scoriae from several apertures; but within a year the island disappeared, and a shoal marks its former site. On the 8th of June the Skaptar Yökul threw out smoke; on the 10th an enormous current of molten lava flowed from numerous cones on the Yökul, which, dividing into two main streams, pursued its course to the sea, filling up the beds of two large rivers, and covering an immense extent of once productive country. The horrors of the scene were aggravated by the enormous torrents of boiling water produced by the liquefaction of the glaciers that covered the Yökul, and by incessant showers of ashes which darkened the sun; stream of lava succeeded stream from the 10th of June to the end of August, at short intervals; and noxious emanations destroyed numbers of those whom fire and water had spared. From this calamity Iceland has never recovered; for within the space of two years the island, in consequence of this eruption, lost 9336 persons, 11,460 head of cattle, 28,000 horses, and 190,480 sheep. The extent of the principal stream of lava is fifty miles in length; its greatest breadth is from twelve to fifteen miles; in the plains its general depth is 100 feet, but in the channel of the Skaptar River, which it dried up, it is 600 feet in perpendicular depth. The south-west side of Iceland appears to be one vast focus of subterranean fire; for the several eruptions of the Oraefi, the Skeiderá, Sida, and Skaptar Yökuls, seem but as occasional outbreaks from one immense volcanic fissure, which really belongs to the same chain of icy mountains.

The volcanic formations of Java have been well examined by Dr Horsfield, who describes them as extending throughout Eastern out the centre of that noble island. They form a chain, in Islands. which thirty-eight large mountains, evidently volcanic, are conspicuous, and attain an elevation of from 5000 to 12,000 feet. Most of them are no longer in activity. Tankuban Prahu has been quiescent for a series of ages; but its crater contains a large lake, a mile and a half in circumference, and 250 feet deep, from one side of which sulphurous vapour issues with great violence. Papandayang was once the largest of the Javan volcanoes; but in August 1772 the greatest part of it was swallowed up during an eruption. A luminous cloud at first seemed to envelope the mountain, prodigious quantities of ashes were ejected, which destroyed forty villages, and an area of fifteen miles by six was entirely swallowed up.

The eruption of Tomboro, in the island of Sumbawa, in the year 1815, described by Sir Stamford Raffles, is one of the most violent on record. The earthquakes caused by this eruption were felt over a circle with a radius of 1000 miles; and all around to the distance of 300 miles the quantity of ashes excited terror and dismay. In Java, though 300 miles in a direct line from Sumbawa, the sky at mid-day was darkened by dense clouds of fine dust ejected from the volcano. In Sumbawa it was attended by a whirlwind of surprising fury, which lasted for an hour, sweeping away trees and houses in its career. The explosions commenced on the ceasing of the whirlwind. They began on the 5th of April, and continued until the 15th of July. Whole villages were swept away, and upwards of 12,000 persons perished in this eruption. Vast numbers of trees, torn up by the whirlwind, strewed the ocean to a considerable extent. Raffles states that the explosions of Tomboro were distinctly heard in a circle with a diameter of 1700 miles; and that the quantity of light scoriae and pumice it threw out within a week formed a bed on the ocean two feet in thickness, and many miles in extent, which impeded ships in approaching Sumatra.

The volcanoes of America claim notice, not only from their magnitude and number, but from the peculiarities of their position. Several of the most lofty mountains of South America are active volcanoes. Cotopaxi has an elevation of 16,800 feet above the sea; the volcanoes of Mecas and Antisana equal 17,000 feet, and Tun- guragua 16,500 feet. Humboldt considers these enormous cones as the spiracles of one prodigious volcanic vault, stretching from south to north, which presents an area of 600 square leagues; and he regards them as merely different summits of the same volcanic mass. "The fire issues sometimes from one, sometimes from another, of these summits. The obstructed craters appear to us to be extinguished volcanoes; but we may presume, that while Cotopaxi or Tunguragua have only one or two eruptions in the course of a century, the fire is not less active under the city of Quito, under Pichincha, and Imbabura."

The connection of the volcano of Pasto, in New Granada, with the volcanoes of Quito, was strikingly displayed in 1797. A column of black smoke had continued for several months to issue from the Volcan de Pasto; but, to the surprise of the inhabitants, this smoke suddenly disappeared on the 4th of February 1797, at the precise moment when, at sixty-five leagues from the city of Pasto, the city of Riobamba, near Tunguragua, was destroyed by a terrific earthquake.

The eruption of Tunguragua, on the 4th of February 1797, was of a very extraordinary nature. The country around, to the extent of forty leagues from south to north and twenty leagues from east to west, sustained an undulatory movement of extreme violence, which lasted four minutes. In the district around the mountain every town was levelled with the ground, and the cities of Riobamba and Quero were buried under the ruins of the impending mountains. The base of Tunguragua was riven asunder, and poured out, from numerous apertures, streams of water and mud, which accumulated in the valleys to 600 feet in depth. The mud covered everything, blocked up the channels of the rivers, forming lakes which remained for more than eighty days. Foul, suffocating exhalations were emitted by the lake Quilotoa, and it is said that flames also issued. The covering of mud, and the levelling of elevated portions of the country, totally changed its face. The shocks continued to be violent for upwards of three months, and were felt over a district 170 leagues from south to north, and 140 from west to east. It was during this eruption that the curious fishes, *Pimelodes Cyclopum*, were discovered in the ejected water of the volcano. They are supposed to be generated in subterranean lakes in the bowels of the mountain, and only to be brought to light by such an explosion. There are no less than sixteen active volcanoes in Chili, the eruptions of which are confined to the valleys of the Andes. A line of volcanoes in South America may be distinctly traced from 5° to 6° of south latitude, running parallel to the ocean, and occupying the highest ridges of the Andes.

In North America, Mexico presents a chain of active volcanoes crossing the general direction of the cordillera of Anahuac from east to west. It commences on the coasts of the Mexican Gulf, with the small but energetic Volcan de Tuxtla, which lies a little to the southward of a line joining the rest; next is the vast cone of the peak of Orizaba (No. 9), which attains an elevation of 16,365 feet; and then the still loftier Popocateptl, rising to 17,060 feet. These lie to the east of the city of Mexico; and on its west are the cones of Jorullo and Colima. They are all active; but at no very distant period the snow-clad summits of Iztaccihuatl, of Toluca, and of Tancitara, lying in the same line, were also burning. This chain extends for 140 leagues; and perhaps the volcanoes should be considered as spiracles, all connected with an huge ignited fissure, passing throughout the table-land of Mexico. Five of these mountains penetrate far into the region of perpetual congelation; and from the inaccessible summits of Orizaba and Popocateptl are pinnacles of frozen snow, from which smoke continually issues.

The extent of these eruptions is attested by the prevalence of volcanic rocks over the whole table-land; and the present form of this magnificent plateau would seem to be derived from the accumulation of ignigenous formations in a series of valleys among primary rocks. Tuxtlia had a strong eruption in 1793, which covered the roofs of the houses at Oaxaca, Vera Cruz, and Perote, with ashes; and at the latter, which is 196 miles distant, the subterranean noises resembled heavy discharges of artillery.

The formation of the volcano of Jorullo dates not farther back than the year 1759. Until that period, the present site of the volcano was a fertile plain, well cultivated, and producing indigo and the sugar-cane. In the month of June in 1759 subterranean thundersings commenced, accompanied by frequent shocks of earthquakes; and these alarming phenomena continued to terrify the inhabitants for fifty or sixty days. About the beginning of September everything seemed to announce the re-establishment of tranquillity; but on the night of the 28th the horrible subterranean explosions were renewed, and an area of four square miles rose up, like an enormous bladder, to the height of above 500 feet. Those who gained the mountains around saw flames shoot up, from a surface of a square league in extent, and huge fragments of ignited rock hurled into the air through a dense cloud of ashes. The softened surface was seen to undulate like an agitated ocean. The waters of two streams precipitated themselves into the burning chasm, and this seemed to add fresh fury to the flames. Thousands of small burning cones, from six to eight feet high, issued from the softened and inflated surface; and in the midst of these hornitos or ovens, which still pour out steam, and emit the sounds of internal ebullition, six vast mountain masses arose, to a height varying from 300 to 1690 feet above the former plain. The most elevated of these is the Volcan de Jorullo, which is perpetually burning, and has ejected a vast quantity of ashes and scoriaceous lava, often mingled with fragments of primary rocks. The ashes of this eruption were scattered to a distance of 160 miles. The eruptions of the Jorullo continued till February 1760 without intermission, but they have since become less and less frequent. On this catastrophe Humboldt justly remarks, that an event, "by which so considerable an extent of country entirely changed its appearance, is perhaps one of the most remarkable physical revolutions in the annals of our globe." Geology indicates parts of the ocean where, within the last two thousand years, several small volcanic islands have been formed; but it gives no other example of the formation, from the centre of a thousand burning cones, of a mountain of scorie and ashes 1695 feet above the level of the adjoining plain, upwards of thirty-six leagues from the sea, and forty-two leagues from every other active volcano;" and he adds, that though within six days' journey of the Mexican capital, this wonderful event had remained unknown in Europe until the period of his journey (No. 11).

In the works of the South Sea missionary Ellis there is an interesting account of the volcano of Kiraua, in the Sand-in-the-Islands. The devastating effect of volcanic fire in the South island of Hawaii had been noticed by Captain Cook, but it has been particularly described by Mr. Ellis. The route to the present volcano is over a vast waste of naked lava. Instead of a mountain-crater, however, Kiraua presents a chasm in a plain, perhaps occasioned by the engulfing of the previous elevation. He describes it as a flat area of fifteen or sixteen miles in circumference, sunk from 200 to 400 feet below the level of the country by which he approached Kiraeua. In the bottom of this area yawns an immense abyss, in the form of a crescent, about two miles in length from north-east to south-west, and about a mile in breadth at its widest part. One portion presents a flood of glowing matter in a state of terrific ebullition. Fifty-one conical islets, containing as many craters, rise around the edges, or from the surface of the burning lake. Some of them emit flame and smoke, and others occasionally vomit streams of lava, which roll in blazing torrents down their black sides into the boiling flood below. The sides of the gulf just described are perpendicular, and rise from a wide horizontal ledge of solid lava of irregular breadth, but extending completely round the chasm; and beneath this ledge the sides slope to the burning lake, the surface of which appears to be from three hundred to four hundred feet lower.

Besides this volcano, there are several others now extinguished in the island. Of these, one is Mouna Roa, which Captain King calculated, from the position of its perennial line of snow, to be 16,020 feet in height; and the same navigator assigns to another, Mouna Kaah, the vast elevation of 18,000. In a word, the researches of Mr Ellis would lead us to infer that the whole island of Hawaiah, an area of about 4000 square miles, is covered by lavas and other volcanic matters in various stages of decomposition.

The whole islands in the Pacific are divided by Kotzebue into two classes; the low and the elevated. The former are coral reefs, of which the formation has been so well described by Captains Flinders and Beechy. Their bases would appear to be submarine mountains, which are brought to the surface of the waters by the depositions of the coral animalcules. Captain Beechy examined thirty-two of these islands, the largest of which was thirty miles, the smallest one mile, in diameter. Twenty-nine had lagoons within their centres, which seem once to have existed in them all, until they had been filled up by the operations of the zoophytes. These low coral islands present a singular appearance. (No. 12, a section.) They often form chains, extending for hundreds of miles with little interruption. Their exterior is a border of dead coral rock, and stands a few feet above the waves, on which the seeds of plants, wafted by the sea, soon fix their roots, and speedily a verdant wood encircles the island, whilst its central lagoon is still the habitation of living zoophytes. The elevated islands of the Pacific very generally abound in volcanic products, and numbers of them are still the seats of active volcanoes. Many of them have the appearance of having been raised from the deep by submarine volcanoes; and Mr Lyell has with much force of reasoning contended, that numerous coral islands ought to be considered as the cones of extinguished submarine volcanoes covered over by coralline formations. If we adopt this idea, and extend it to the enormous coral reefs described by Flinders, King, Horsburgh, and Chamisso, the surface of the Pacific must repose upon a prodigious magazine of volcanic energy, which is from time to time destined to increase the surface of the habitable globe. A very extensive line of active volcanoes exists in the Great Eastern Ocean, which has been noticed by Lyell, and forms too remarkable a feature on our globe to be omitted in a sketch of physical geography. It commences in Barren Island, off the coast of the Malayan peninsula; is continued by a chain of active volcanoes through Sumatra, Java, Sumbawa, and Flores; it then turns northward through the Moluccas, the Philippines, Formosa, and Loo-Choo; it passes through Japan, Physical Jesso, and the Kuriles; reaches the peninsula of Kamtschatka, and terminates in the Aleutian Islands, after a course of between 9000 and 10,000 miles, besides the several branches which are sent off to the eastward in the directions of Papua and the Ladrones.

The cause of volcanic phenomena is involved in obscurity; but various conjectures have been proposed by geologists. The Neptunian hypothesis has ascribed them to the inflammation of beds of coal, or of other combustibles, in the burning mountain; regarding them as local, or very limited in their range. Lemery's experiment on the heat extricated by moistening a mixture of iron filings and sulphur, and burying it in the earth, was supposed to afford a satisfactory explanation of the manner in which the fire might originally be kindled; and the extinction of volcanoes was supposed to give probability to the local nature of the heating cause. Abbé Breislac modified this hypothesis, by supposing that volcanic fires were fed by collections of petroleum collected in caverns, and set on fire by the chemical action of several other substances. Both these views seem to afford too narrow a basis for the extent of volcanic energy, the apparent connection between distant volcanoes, and the great length of time that some of them are known to have remained in activity. The Huttonians consider volcanoes as spiracles connected with the great source of central heat. (Playfair, 86.)

Another hypothesis has been suggested by the discovery of the metallic bases of the alkalis and earths. Their bases have been supposed to constitute the interior of the earth; and when water finds its way to them, through fissures in its crust, it will be rapidly decomposed, with the extrication of intense heat and the evolution of elastic gases, and thus the phenomena of earthquakes and volcanoes may be produced. This view, originally suggested by Sir Humphry Davy, has been illustrated by Humboldt, and especially by Daubeny, with great ingenuity; and although it has not yet arrived at the state in which, perhaps, we are entitled to designate it a theory, it is an hypothesis which affords a satisfactory explanation of the observed phenomena.

All the enumerated causes have produced and are still producing changes upon the surface of the globe; some of which are general, though slow in their operation, whilst others act much more violently, but are more local in their effects.

Mud and air volcanoes, as they have been termed, appear to owe their existence partly to the disengagement of gases by subterranean fire, but in a great measure to water from a higher level, compelled to pursue a subterranean course, by a thick bed of tenacious clay suddenly escaping by a perpendicular fissure, with a force proportional to the height of the aqueous column. An ingenious explanation of the phenomena of the mud eruptions of Sicily and Italy, on this principle, is given by Dr Daubeny. One of the most noted eruptions of this sort is that of Macaluba in Sicily, which in 1777 threw up a vast column of mud, mixed with stones and naphtha, and the disengagement of sulphurous vapours, with much hydrogenous gases. Similar eruptions occur near Catania, and also near Modena. The gaseous exhalations of the Grotta del Cane and of the lake of Anzano in the kingdom of Naples, are probably disengaged from mineral bodies by heat. The inflammable air of Pietra Mala near Bologna, and of the petroleum wells of the country around Baku, on the western shores of the Caspian, are evidently connected with subterranean fire. In the latter district there are strong proofs of volcanic agency still active in the bowels of the earth. The singular eruption which M. Figalgo saw at the northern extremity of Prince William's Sound in North America, belongs to the same class. "The Indians conducted him into a plain covered with snow, where he saw great masses of ice and stones..." Sect. VI.—Distribution of the Waters of the Globe.

Natural water never exists in a state of absolute purity. When collected as it descends from the atmosphere, it always contains air; and when derived from springs, lakes, or rivers, it invariably contains more or less of saline ingredients. Dr Henry found in spring-water between four and five per cent. of its bulk of gas, of which nearly three and a half consisted of carbonic acid, and the rest of atmospheric air. Besides the gases, natural waters contain the following salts: muriate of soda, muriate of lime, sulphate of potassia, sulphate of lime or of magnesia, carbonate of lime, and sometimes carbonate of soda. Hail and snow water, when collected at a distance from habitations, contains scarcely any trace of these salts. Rain-water is next in purity; but dew-water is generally contaminated with vegetable matter, as well as with salts. The water of springs usually abounds with these salts, and often with other ingredients. River-water differs much in its purity; but when it flows through a siliceous rock it is often very pure. Lake-water is subject to much contamination; but often, when the lakes are deep and extensive, the water is depurated by subsidence and rest; though stagnant water, when shallow, becomes contaminated with vegetable and animal matter. The water of the ocean contains about three per cent. of saline contents, or one thirty-third part of its weight. The quantity of saline matter appears to increase slightly from the highest latitudes towards the equator. The ocean, from its extent, claims our first attention.

1. Ocean.—From data already given in section second, we shall probably not err if we estimate the surface of the whole ocean and its branches at 144,478,000 square miles. It is not so easy to arrive at a safe conclusion with regard to its mean depth. Laplace and later investigators seem to believe that the depths of the ocean may be in proportion to the elevation of the land, and the ratio of their respective areas. Mrs Sommerville states, that although it is probable that profound cavities of small extent exist in the bed of the ocean, its mean depth does not exceed the mean height of the continents and islands above its level. According to Lagrange, the velocity of the tidal wave is a function of the depth of the channel through which it moves; and recent investigations respecting the velocity of the tidal wave in these seas, would lead us to infer that the mean depth of the sea around the coasts of England is about 120 feet, on those of Scotland 360 feet, on the western coast of Ireland 2000 feet, and in the main Atlantic 50,000 feet, or a little more than nine miles and a half. The examination of the co-tidal lines on Whewell's charts would lead to the conclusion that no considerable extent of the ocean has a greater depth than the Atlantic. If the depths of the ocean are indeed proportional to the elevation of the land, in the ratio of their respective surfaces, the greatest profundity of the oceanic abyss would be about fifteen miles, and its mean depth, at a distance from the land, would vary from one to five miles. If we assume four miles as its mean depth, this would give 577,892,000 cubic miles as the volume of the whole waters of the globe; or, if we reckon it five miles in depth, the mass of waters would equal 722,365,000 cubic miles.

The waters of the ocean are in perpetual movement, from the effects of winds, currents, and the tides. The effect of winds in agitating the ocean is chiefly confined to its surface. The most powerful waves in the free sea only rise to the height of a few feet; and it is highly probable, that in the profound depths of the ocean, there reigns a perpetual stillness, unruffled by any movement, except in as far as general currents may disturb the tranquillity of the waters. The surface, acted upon by strong gales, is thrown into long ridgy waves, which rise with an uniform swell, and maintain the same velocity for considerable distances, the top curling in foaming surges, which are precipitated on the undulating surface below, often to the great danger of the navigator. No region of the ocean is subject to more violent oscillations of this nature than that portion of the Atlantic termed the Bay of Biscay, when agitated by a smart gale of westerly wind. Geometry has been applied to measure the velocity of waves, and Lagrange estimates, that where the depth is not very great, the velocity will be the same as a heavy body would acquire in descending from a height equal to half the depth of the water, and that the velocity of the wave is in the duplicate ratio of the depth of the channel in which the water moves. It is believed, however, that the wildest tempest in the open sea does not agitate the water at the depth of an hundred feet; but when waves meet with resistance, as when they dash upon lofty cliffs, we have known them rise apparently in one unbroken mass to the height of two hundred feet.

There are other movements of the waters of the ocean, termed currents. The most general currents are produced by the movement of the waters perpetually setting from the polar regions towards the equator, and the progression of the tropical seas towards the west. Both are chiefly caused by the centrifugal force of the earth's diurnal rotation. This produces a culmination of water towards the equator, and thus a current is generated towards the part of the earth in most rapid motion; but the water accumulated there passes off before the influence of the trade-winds; and the strength of the western current is increased by the mobility of the particles of water preventing the fluid immediately acquiring the velocity of that portion of the earth over which it arrives in its progression towards the equator, by which it will appear to have a motion from east to west, or in an opposite direction to the earth's rotation. The higher density of the sea, when it has been cooled by the polar ices, will increase its tendency to displace the lighter waters, which are forced westward by the constant pressure of the trade-winds, and the other causes already mentioned. The Polar currents from the north have been well described by Marren, Scoresby, and other navigators. The set of the current from the antarctic regions has been illustrated by the voyages of Cook, La Pérouse, Flinders, and others. The Equatorial current becomes very apparent in the Atlantic and in the Pacific, between the parallels of 30° on each side of the equator. According to the best observers, it has a mean velocity of nine or ten miles per day in the open sea. In the Atlantic it produces that remarkable movement of the waters termed the gulf-stream. This current passes between the West Indian islands, and sweeps along the northern coasts of South America, from Cumana to the isthmus of Darien; from thence it stretches toward Cape Catoche in Yucatan, and, after wheeling through the Mexican Gulf, it rushes through the channel between Cuba and Florida, passing between the Bahamas and the United States of North America towards Newfoundland. In the latter part of its course it can be distinctly traced, not only by its effect on the dead reckoning of ships, but by the superior temperature of its waters, which betray its tropical origin, and by its greater depth; for it appears in a long succession of ages to have hollowed out for itself a submarine channel. In the Bahama Channel it has a breadth of fifteen leagues, and its waters have, in latitude 42°, a temperature of 71° Fahrenheit, whilst that of the adjacent sea beyond its influence is only 63°. The gulf-stream flows with diminished velocity as it proceeds northward, until it is lost on the banks of Newfoundland, where it encounters the great current from the arctic regions. This vast sub-marine bank, indeed, probably owes its origin to the deposition of the solid particles borne along by both currents, at the point where their opposing forces meet; and there can be little doubt that the eastern movement of the waters of the North- ern Atlantic is the resultant of the compound motion of these mighty oceanic streams. It has been computed from the average velocity of both, that the waters of the Atlan- tic, between the parallels of 10° and 43° north, perform a circuit of about 8800 nautical leagues in thirty-four months. The retrograde current preserves a general easterly direc- tion after leaving Newfoundland; but it reaches consider- ably farther north, for its influence frequently throws on our western shores the trees and ligneous fruits of the An- tilles; but the great body of it is deflected to the south- ward at the Azores, partly by the influence of the polar cur- rent from the north; and then sending a branch to enter the Straits of Gibraltar, gives rise to that dangerous cur- rent which has been described by Major Rennell as the cause of the frequent shipwrecks on the African coast within the Canary Islands, when the mariner, by reckoning, con- siders himself as far to the westward of Tenerife.

Another branch of the great western current of the At- lantic appears to be deflected by the peculiar form of South America, from the latitude of Pernambuco, along the east- ern coasts to Cape Horn; and, doubling that promontory, it finds its way into the Pacific. There is also a general western current in the Great Pacific. Ships are carried by this with great celerity from Acapulco to the Philippine Islands; but, in returning, it is found advisable to sail north- wards, for the benefit of the variable winds and of the po- lar currents. In the Southern Pacific, the polar currents, being little interrupted by land, proceed with less deviation from their course than those in the northern hemisphere; and hence they carry icebergs nearer to the tropical regions than we ever find to the north of the equator. The west- ern motion of the waters of the Pacific is interrupted and broken by the vast chain of islands, shoals, and sub-marine banks which stretch from China to New Zealand. The gen- eral direction is changed or modified by the form of these lands; and the vast mass of New Holland is one cause of those dangerous currents around its shores which were noticed by Cook, La Pérouse, and Flinders. The weaken- ed force of the general current in the Indian seas permits its interruption by the variable direction of the monsoons; but the current which always sets from Australia towards the Bay of Bengal seems to be produced by the modifying influence of the south polar current on the general equa- torial current when weakened by the interruption of islands. The direction of this current, finding obstacles to its pro- gress in a north-west direction, and being resisted by the banks and islands which stretch between Cape Comorin and Madagascar, is deflected to the south-west, forming the current in the Mozambique Channel, and thence sweeping round the Cape of Good Hope in a powerful stream, not less than forty-three leagues in breadth. The entrance into this current may be ascertained by the sudden in- crease in the temperature of the water to eight degrees be- yond that of the adjacent ocean. It is probably to this current that the vast shoal off the Cape, known by the name of the Aguilllas Bank, is to be attributed. It appears to round off upon both sides, forming a bank forty leagues in breadth, and 160 leagues from east to west.

The strong current through Bass's Straits, separating New Holland from Van Diemen's Land, is but a part of the general western current; but in its passage through the intricate channels between the islands, reefs, and shoals of the ocean north of Australia, it is divided and modified in a thousand different ways. There is another current less easily accounted for. On the western coast of Africa, about thirty leagues from the land, a strong current appears to set eastward in the Gulf of Guinea, and often draws ships far off their southern course. It seems to be a species of eddy, produced by resistance to the polar current caused by the sudden contraction of the Atlantic from the form of the opposite coasts of America and Africa. These currents may be considered as general; but there are other currents which, though special, are remarkably constant. Of these we may notice the perpetual current which sets into the Mediterranean through the Straits of Gibraltar. It occu- pies the middle of the Straits. On either shore a less pow- erful and less extensive current alternates with the tides; but there is a constant and powerful influx from the Atlantic into the Mediterranean. A perpetual current sets out of the Baltic to the Northern Ocean; and here also smaller coun- ter-currents are found on the coasts of Sweden and Jut- land. Another considerable local current perpetually sets from the Black Sea in the Mediterranean. The causes of these peculiarities we shall afterwards explain.

Besides the motions already described, the waters of the Tides, ocean are subject to alternations in the level of its surface in any particular place several times a day. The rising of the water is termed flood, the fall is named ebb-tide. The con- nection between the tides and the moon were long observed before any theory was proposed to account for them. Des- cartes ascribed the flood to the pressure of the moon on the earth's atmosphere; but Kepler first suggested that it might arise from the attraction exerted by the moon; and this theory was confirmed and enlarged by Newton, who showed how well the general phenomena might be explained on the principle of universal gravitation. The theory and the phe- nomena are given under the article Astronomy; to which we refer for the explanation of the two floods and two ebbs daily, how the united influence of the sun and moon causes the two spring and two neap-tides during each lunation; and why the highest tides of all take place about the equinoxes, when the sun and moon are on the equator, and in perigee.

The phenomena of the tides accord well with the theory in the open sea; but in the vicinity of land, amongst nume- rous islands, or in deep inland basins, their regularity is af- fected by various causes, as the occurrence of strong gales in particular directions, the form of points of land, of banks, and shoals, and the mechanical influence of narrow chan- nels. Thus the tides of the German Ocean take twelve hours to reach London bridge; and consequently the time of high-water there is just as a new flood is commencing in that sea. The usual rise of the tides in mid-ocean is about three or four feet; but on the coasts of Britain, espe- cially in the mouths of rivers, the attraction of the land raises them far higher. Thus, at Chepstow, the tide rises from forty-five to sixty feet; and it is recorded that once, from the influence of a strong gale blowing at the same time into Bristol Channel, the rise was seventy feet. In the river of Annapolis, which falls into the bay of Fundy, on the coast of America, the spring-tides have been known to reach the surprising height of 120 feet. In such cases the flood-tide often rushes in with a vast mass, like a huge wave, several feet above the general surface, which is term- ed a bore, and is dangerous to ships that may have been stranded on the receding of the tide. Something similar is often seen in our smaller rivers, and on such extensive flat sands as those in Morecambe Bay on the coast of Lancashire, and in the Solway Frith. In narrow straits the tide runs with great velocity, such as no ship can stem even with the most favourable breeze. In the embouchure of the Mersey, the ebb-tide has a rate of five or six miles an hour. In the Pentland Frith, between Caithness and the Orkney Islands, the spring-tides have a velocity of nine miles an hour.

Whirlpools appear to be occasioned by currents meeting Whirlpools with sub-marine obstacles, which throw them into gyration. When the movement is rapid, the centre is the most de- pressed portion of the rotating circle, and objects drawn within it are submerged in that point. Several small whirl- pools, capable of whirling round a boat, are seen among the Orkney Islands. That of Coryvreckan, in the narrow chan- nel between Scarba and Jura, in the Western Islands, is caused by a rock of a conical form rising abruptly from the bottom, where the depth is 600 feet, and reaching to within ninety feet of the surface. This obstruction, in a tortuous rocky channel, causes a succession of eddies; and when the flood-tide sets in, with a fresh breeze in the opposite direction, the eddying waters rise in short heavy waves, which are highly dangerous to boats, and even to decked vessels.

The Malström on the coast of Norway, near the island of Moskoe, is a whirlpool of a similar kind, the perils of which are probably much exaggerated. The flood-tide setting from the south-west amongst the Laffoden Isles, especially when it meets with a strong gale from the north-west, produces a great agitation of the waves, and a whirlpool is formed, the roaring of which is heard at the distance of many miles. Its agitated vortices are dangerous to vessels, and it is said that seals and whales, when caught within its eddies, are unable to extricate themselves from destruction.

It is now well ascertained that Charybdis, in the Strait of Messina, owes its terrors to the imagination of seamen in the infancy of navigation, and all its celebrity to poetic fancy.

The sea, when viewed near land, has its colour affected by the nature of the bottom. Where the bottom is sandy, and the depth not very considerable, it has a greenish tint; and this is the general hue of the German Ocean; but when viewed out of soundings, the colour of the ocean is generally of a beautiful deep blue. This may be considered as the natural hue of a deep column of water, from which only the most refrangible colours reach the eye of the spectator placed on a ship's deck; and the greenish tint is produced by the mingling of this hue with the yellowish tint of the sandy bottom. Sometimes partial colours are observed in the ocean. Captain Cook and other navigators describe a brown colour of the sea, produced by myriads of minute mollusca and crustacea. In the Greenland seas, the waters in which the whales chiefly delight are streaked with green. This colour appears in broad streams in the sea; and, when examined by Scoresby, was found to be produced by innumerable meduse and minute crustacea, the olive-green water containing sixty-four in one cubic inch. Captain Horsburgh mentions a milky appearance of the sea, observed in a passage from China to Australia, which was occasioned by minute globular bodies linked together, and doubtless forming some species of berce or medusa. The occasional red colour of the sea on the coast of Brazil and of China is believed to proceed from minute mollusca which float in myriads in the waters at certain seasons of the year. Ehrenberg asserts that the name of Red Sea has been given to that gulf from the prevalence of a species of oscillatoria.

The phosphorescence of the sea is a very brilliant appearance which the ocean and arms of the sea frequently assume. It is best seen when a vessel is in brisk motion through the waves, or when a boat is impelled by oars through a calm sea. We have seen this appearance so brilliant that we could distinguish letters in a book by its phosphorescence in the wake of a ship, where it will sometimes form a line of pale lambent light several hundred yards in length. We have also seen from the deck, by its light, the hour on a watch, when in other situations on the deck we could not distinguish the features of the seamen. When the hand is immersed in luminous water, we have found the shining particles adhering to the fingers; and on examining such water in a globular glass vessel by transmitted light, we scarcely ever failed to observe in it several minute animals of the genera medusa and berce. These animals appeared to have the power of emitting light by their own efforts, as the water, when left at rest, shone by fits. On counting their number in a glass of water, and suddenly dashing it on the deck, in a dark night, it was observed that the number of luminous points generally coincided with that of the animals in the water, and never exceeded it. We have found these animals in sea-water, from 60° to 37° north latitude, in the Atlantic, and in innumerable bays; and believe that they are very general in the ocean.

The dependence of the phosphorescence of the sea on living animals was long ago asserted by Captain Cook and Sir Joseph Banks. In the voyage from the Canaries to Rio de Janeiro they drew a net through luminous water, and caught a large medusa, which shone brilliantly when taken on board; they obtained from it also three minute crustacea, about one tenth the size of the glow-worm, which also were very luminous in the dark. Banks named the first Medusa pellucens, and the most brilliant of the second Cancer fulgens. Other naturalists have enlarged the list of marine luminous animals, which includes occasional instances amongst the larger fishes.

A considerable number of experiments have been made on the temperature of the ocean by Ellis, Piron, Flinders, Parry, Ross, and others; but the results are not uniform. Ellis found in the Atlantic that the temperature remained stationary at 50° at depths a little exceeding a mile. Piron, who had sounded to greater depths, found it to decrease with the depth. The mean temperature of the ocean at the surface, and at a distance from land, is higher than that of the atmosphere with which it is in contact; but it is generally lower than the air at mid-day, and higher than the atmosphere at midnight. The ocean over a bank is colder than where it is of great depth. This is ascribed to cold currents being forced upwards by the obstruction; but whatever be the cause, the fact is important, as it may enable a seaman to recognise his approach to land when the atmosphere is obscure and his reckoning doubtful, as was first remarked by Mr Williams.

The Southern Ocean, receiving less fresh water than the Atlantic, is rather saltier, as is also the Mediterranean; but the Baltic, White and Black Seas, from the size of the rivers they receive, are fresher. According to Marcet their specific gravities are as follows:

- Mediterranean: 1·02980 - Southern Ocean: 1·02882 - Northern Ocean: 1·02829 - Arctic Frozen Ocean: 1·02664 - White Sea: 1·01901 - Baltic: 1·01523 - Euxine: 1·01418

Sea-water has a mean specific gravity of 1·0280 to distilled water as 1·0000. The late Dr John Murray published an admirable analysis of sea-water, 10,000 parts of which afforded him:

- Muriate of soda: 245·04 - Muriate of magnesia: 28·63 - Muriate of lime: 0·00 - Sulphate of magnesia: 17·04 - Sulphate of soda: 2·66 - Sulphate of lime: 9·72

Dr Murray detected no trace of muriate of lime, which Lavoisier has stated as high as 20·38, and Pfaff at 31·25. In reflecting upon these discrepancies, he was led to conclude that several of the products obtained may be the results of the mutual decomposition of the salts during the processes for separating them from the water; thus the sulphates of magnesia and of lime would be decomposed by muriate of soda, and when both are obtained from sea-water, we should regard them as results of decomposition. The real ingredients of sea-water would thus be,

- Muriate of soda: 220·01 - Muriate of magnesia: 33·16 - Sulphate of soda: 42·08 - Muriate of lime: 7·84 This doctrine he has happily applied to explain the activity of several mineral waters, the analysis of which only affords inert ingredients.

The saline contents of the ocean do not appear to vary considerably; they, however, increase as we approach to the equator, from \( \frac{1}{2} \) to \( \frac{3}{4} \), as is proved by the increase of density. The numerous experiments of Bladh of Sweden proved that the specific gravity of the waters of the Atlantic on both sides of the equator increase as they recede from the poles. This table was republished by Kirwan in the Geological Essays, with a reduction of the specific gravity to temperature 62° Fahrenheit. The longitudes of Bladh are from the meridian of Teneriffe.

| Latitude | Longitude | Sp. Gr. at 68° | Sp. Gr. at 62° | |----------|-----------|---------------|---------------| | N. | E. | | | | 59° | 39° | S° 49° | 1-0266 | 1-0272 | | 57° | 18° | 18° 48° | 1-0263 | 1-0269 | | W. | | | | | 57° | 1° | 1° 22° | 1-02669 | 1-0272 | | 54° | 0° | 4° 45° | 1-0265 | 1-0271 | | 44° | 32° | 2° 4° | 1-0270 | 1-0276 | | E. | | | | | 44° | 7° | 1° 0° | 1-02705 | 1-0276 | | 40° | 41° | 0° 30° | 1-0270 | 1-0276 | | 34° | 40° | 1° 18° | 1-0274 | 1-0280 | | 29° | 50° | 0° 0° | 1-0275 | 1-0281 | | W. | | | | | 24° | 0° | 2° 32° | 1-0278 | 1-0284 | | 18° | 28° | 3° 24° | 1-0275 | 1-0281 | | 16° | 36° | 3° 37° | 1-0271 | 1-0277 | | 14° | 56° | 3° 46° | 1-0269 | 1-0275 | | 10° | 30° | 3° 49° | 1-0266 | 1-0272 | | 5° | 50° | 3° 28° | 1-0268 | 1-0274 | | 2° | 20° | 3° 25° | 1-0265 | 1-0271 | | 1° | 25° | 3° 30° | 1-0267 | 1-0273 | | S. | | | | | 0° | 16° | 3° 40° | 1-0271 | 1-0277 | | 5° | 10° | 6° 0° | 1-0271 | 1-0277 | | 10° | 0° | 6° 5° | 1-0279 | 1-0285 | | 14° | 40° | 7° 0° | 1-0278 | 1-0284 | | 20° | 6° | 5° 30° | 1-0279 | 1-0285 | | 25° | 45° | 2° 22° | 1-0275 | 1-0281 | | E. | | | | | 30° | 25° | 7° 12° | 1-0273 | 1-0279 | | 37° | 37° | 68° 13° | 1-0270 | 1-0276 |

Dr Traill has published the results of some experiments on water taken up in different parts of the Atlantic, from 53° to 62° N., which also show a progressive increase in the specific gravity of the ocean towards the equator, and likewise as the depth increases.

Lat. 47° 47' N. Long. 10° 40' W....surface.............1-0277 Ditto................Ditto..........................40 fathoms......1-0280 37° 0'................9°................surface.............1-0281 32° 0'................16°................surface.............1-0284 Ditto................46 fathoms.............1-0286 26°...................64°................86 fathoms...........1-0287 22° 11'................surface.................1-0289 5° 20'................56°................surface.............1-0267

This last was apparently affected by the waters of the Orinoco, though out of sight of land.

The phenomena of sea-ice have been admirably described by Scoresby, who divides it into two kinds, field and mountain ice. Field-ice has a thickness of from two to three or four fathoms. He mentions single fields 100 miles in length and fifty in medial breadth, with a general elevation above the water of from four to six feet. When the ice is of smaller extent it is termed a floe. Mr Scoresby found the specific gravity of ice to sea-water to be as 8 to 8-97, or nearly as 8 to 9, so that ice floats in sea-water with \( \frac{1}{3} \)th above water; and a field which has six feet above water will have forty-eight feet below it, if the ice be of uniform thickness. When a field or floe is exposed to the dashing of the waves, especially to what is called a ground-swell, it is speedily broken up; but the pieces are apt to collect into what is termed a pack, when from the mast-head no opening can be discovered in it. In smaller collections it forms patches. The ice is loose or open when ships can pass between the fragments of floes or packs. By the collision of pieces of ice, the fragments are often heaped on the larger portions, and form hummocks, which are thus sometimes raised thirty feet above the general level of the ice. Some ocean-ice is whitish and porous. This affords only a brackish water; for though freezing separates the salt from the ice, the salt water is retained in its pores, and the water yielded is not potable; but ice which looks blackish whilst swimming has a solid consistence, a beautiful beryl-green hue in the air, and when melted affords a potable water, a fact of great importance to navigators in high latitudes. Sea-ice assumes different appearances, according as it is formed in smooth or in rough water. The first process in freezing is the formation of minute prisms; when these form in smooth water, they speedily unite into sheets of ice, which in twenty-four hours of keen frost will attain a thickness of two or three inches, and in forty-eight hours will be able to bear the weight of a man. This sort often forms in the interstices of fields, and is termed bay-ice. It sometimes increases to three feet in thickness. When the congelation takes place in rougher water, the crystals of ice accumulate, rendering the water sluggish; and this is termed by sailors sludge. When the motion of the waves prevents the concretion into larger masses, it forms what sailors term pancakes, which generally attain the thickness of a foot. Both these sorts of ice are termed young ice, to distinguish them from the more solid field-ice. They often rapidly form around ships engaged in the whale fishery, till no open water can be seen from the mast-head; and they often disappear, from the effects of gales or currents, with an almost magical celerity. The heaviest ice is liable to disruption by the waves, especially when detached from the shores, and when the fields are carried by currents into the open sea. Through the openings thus produced, Scoresby penetrated in one voyage as far north as 81° 30' N. between Spitzbergen and Greenland; and at another time reached the long-lost eastern coast of Greenland, which he surveyed to the extent of 800 miles. The drifting of fields of ice is a proof of the existence of currents from the circumpolar regions. Scoresby mentions that he has seen a field drift 100 miles to the south in a month, although the wind had been very variable during that time. When ice-fields drift they often acquire a rotatory motion, which brings them into collision with other fields; the crushing of the weaker ice is attended with an awful noise; and the utmost presence of mind in the mariner is often required to extricate his vessel from the encounter of such irresistible forces. Mountain-ice or Iceberg is the term applied to those enormous masses of ice that are more common in Baffin's Bay than in any other part of the northern seas, and are also very frequent in the Great Southern Ocean. It is now generally believed that they are glaciers originally formed on the cliffs of lofty islands or in abrupt valleys, where their lower part being undermined by the waves, in process of time they thus become detached from the land, and float about, as impelled by currents. Some of the icebergs seen in Davis' Straits have an area of five or six square miles, rise 100 feet above the surface, and are aground in water of 100 fathoms; from which data we calculate, that such enormous masses weigh more than two thousand millions of tons. Icebergs assume most fantastic shapes, as may be seen in Cap- Physical Geography.

tain Ross's and Captain Parry's Voyages. Large icebergs can only be produced where there is deep water close to the cliffs on which they are formed; and this is probably the reason why they abound in Davis' Straits. In the Southern Ocean icebergs have been said occasionally to be 200 feet above the surface, and Forster describes them as composed of layers of different coloured ice. It is dangerous to approach some icebergs, still more to make fast to them; for sometimes they suddenly split into a thousand pieces, with a thundering explosion, as if they were of unannealed glass; at other times they suddenly lose their equilibrium, and are overturned by the wasting of the submerged parts, whilst the upper portion has lost little of its weight. Icebergs have been wafted by currents and storms far from their native seats. Occasionally they have been encountered in the Atlantic, in latitude 40° north; but in the southern hemisphere they have been frequently seen as low as 35°. The Ice-blink is one of those singular effects of extraordinary refraction, by which the existence of open water is often recognised, though it really be beyond the limits of direct vision.

The polar ices appear to be subject to periodical increase and diminution, depending on causes not yet perfectly understood. One of the last and most remarkable disruptions of the arctic ice is that described by Scoresby. That able philosopher has traced the usual boundary of the firm ice in close and open seasons; and during one of those periodical disruptions, in which more than two thousand square leagues of ice had broken up, he was enabled to penetrate through the boundary, which had seemingly remained undisturbed for four centuries. Large, however, as this quantity disrupted seems, it is a very small part of the whole polar ices. Leslie computed that a total eclipse of the sun for a single hour deprives our planet of as much heat as would be capable of melting a circle of ice 500 miles in diameter and 150 feet in thickness, which he estimates to be sixty times the extent of what is annually melted in the Greenland Seas; yet the effect of a total eclipse of the sun does not cool the atmosphere more than 3° of the thermometer. We cannot, then, ascribe any important influence on our climate to the melting of this portion of the polar ices.

Inland Seas.—A few of the inland seas yet remain to be noticed. We begin with the Baltic. This sea differs from the arms of the ocean already mentioned, by its comparative freshness, by its want of tides, and by its sending a current continually into the German Sea. Its freshness arises from the number and size of the rivers it receives, in proportion to its extent. It is a narrow, irregular sea, the extreme length of which is about 1200 miles, with a mean breadth of about 140 miles. This will not give an area of 70,000 square miles, and its mean depth is only about sixty fathoms. Yet it receives forty considerable rivers, and the waters of innumerable lakes, which are the drainage of a country five times as extensive as its whole surface. The immense influx of fresh water into such a small basin, from which evaporation is but slow for a large portion of the year, sufficiently accounts both for its deficiency of salt and for the out-going current. The want of regular tides in the Baltic is a consequence of the same causes that operate in all confined collections of water having narrow communications with the ocean, viz. that the moon must act almost equally on all parts of its surface at the same time, and the oceanic tidal-wave, obstructed by the sinuosities of the narrow channel, only penetrates a little way into the three passages which lead into the Baltic.

The Mediterranean is the noblest and most remarkable inland sea on the face of our earth. A strong current is perpetually setting into the Mediterranean through the Straits of Gibraltar, especially in the centre; and although there be an eddy on each side, this reflux is far inferior to the quantity flowing from the Atlantic. This inland sea, which is 2350 miles in length, with a breadth varying from 100 to 650 miles, covers an area of about 1,000,000 of square miles. Its depth is much more considerable near the Straits than that of the adjacent Atlantic. The fine hydrographical chart of Don Vincente Tofino de San Miguel, republished in this country with additions by Captain W. H. Smyth, R. N., shows, that from the longitude of Cape Trafalgar, where the Atlantic is from fifty to sixty fathoms, the channel deepens as we proceed upwards, until, a little within Gibraltar, it is a thousand fathoms deep, and soon after is beyond soundings. It is also very deep towards its eastern extremity. Between Sicily and Africa its depth varies from a hundred to thirty fathoms; so that there exists in that part of the sea a submarine chain or shoal, which may be considered as an extension of the rock-formations of Italy and Sicily, with deep water on either hand. Besides the perpetual current from the Atlantic, it receives the surplus waters of the Euxine, by the constant stream passing through the strait of the Dardanelles. These immense additions appear necessary to supply the enormous evaporation from its surface. It has been remarked, that the rivers which fall into it are the contributions of a smaller extent of territory than the river-domain of any other inland sea, in proportion to its area; for they are to each other rather less than one and a half to one. Some have contended that there is a current below that setting in from the Atlantic. The only fact offered in proof is stated to have happened in 1712. A Dutch merchantman, laden with wine and brandy, was sunk by a broadside from a French privateer in mid-channel, and a few days afterwards the ship and cargo were cast on shore near Cape Spartel, twelve miles below where the accident happened; but the nature of the cargo probably prevented the complete sinking of the vessel, and she may have drifted within the range of the outgoing current on the African shore. The eastward slope of the bottom of the strait would certainly favour the ingress of the cooler waters of the Atlantic into the Mediterranean, and afford an argument against the existence of this alleged under-current.

There is, however, no need of the hypothesis of a counter-current to explain the preservation of the level of this sea, notwithstanding that it receives several large rivers, and a perpetual supply from the Euxine and the ocean; since Dr Halley has shown, that the amount of evaporation from its extensive surface, in so warm a latitude, is sufficient to preserve its level unchanged.

Tides are but little felt in the Mediterranean. They are only sensible in certain parts of that sea, and seldom rise to six inches above the mean level; but, in the Straits of Messina, the form of the land, and the peculiar set of the currents, give them a more considerable range. These tides, however, are irregular, and are liable to be affected greatly by the direction and force of the winds. The want of considerable tides in the Mediterranean kept the Romans ignorant of the extent of ebbs and floods, and caused them several disasters when they carried their arms to the shores of Western Europe. Even in the time of Tacitus they were imperfectly acquainted with the currents produced by the tides around the northern coasts of Scotland, the difficulty of stemming which the historian ascribes to a "mare pigrum, remigantibus infestissimum."

That large inland sea the Euxine, with its branch the Sea of Azof, has somewhat of the characters of the North American lakes in its comparative freshness, which is one cause of the duration of ice on waters that have the same latitude as the finest regions of Italy and France. The sur-

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1 Scoresby's Arctic Regions, and Ross's and Parry's Voyages. face of the Euxine and the Sea of Azof is estimated as equal to 170,000 square miles; but as they receive some of the largest rivers in Europe, and smaller streams from Asia, the drainage of an extent of country equal to five times their own surface, we see a strong reason why there should be a constant efflux through the Bosporus. This current in the Thracian Bos- porus has generally a velocity of from three to five miles an hour, according to the direction and force of the wind. Be- fore this outlet was formed, the Euxine probably united with the Caspian, and constituted an hyperborean sea on the plains of Russia, and the provinces on the Lower Danube; and most probably the floods of Deucalion and Ogyges were connect- ed with débâcles arising from disruptions of the barrier of this sea.

According to Pallas, there are traces of the former union of the Caspian with the Euxine, in the saline marshy ground and numerous lakes which intersperse the flat coun- tries of Ulagann-Ternik, Alabuga, and Byeloe. The pre- sent Caspian is below the level of the Euxine. From baro- metrical measurements, Parrot stated its surface to be 300 feet below the level of that sea; but the recent trigonome- trical levelling of Fuss, Sabler, and Sawitch, which has been conducted with the utmost care, reduces this depression to 1012 feet. Its length is about 700 miles, and its breadth about 210, which would give it an area of 147,000 square miles. It has a very unequal bottom. Hanway states, that in some places 450 fathoms of line could not reach the bottom; but in other places it abounds in shallows, which render its navigation perilous, independently of the sudden storms to which it is liable. It is without tides; but its shores are subject to irregular floods, from the size and number of its rivers, and the direction of the winds. It is extremely salt, and abounds with fish; amongst which the most important is the isinglass sturgeon, and the sterlet (Ac- cipenser huso, and A. Rutilus). Several species of Sal- mo and Cyprinus, and a species of herring, are numerous; and there are extensive fisheries of several kinds of seals. How seals found their way into this inland sea, unless at one time it communicated with the ocean, we cannot easily ex- plain.

The Sea of Aral seems once to have communicated with the Caspian, from the similarity of their fishes, and the nu- merous salt lakes and marshes that still crowd the inter- vening space, especially towards the northern extremity. The Sea of Aral is 200 miles by 100, and therefore has an area of about 20,000 square miles. It is extremely salt, though it receives several very large rivers, particularly the Sibon and Gihon, the ancient Jaxartes and Oxus.

The inland sea or lake called Baikal differs from those already mentioned in being perfectly fresh. Its length is 860 miles, and its breadth forty; so that its area is about 14,400 square miles, including the islands which it contains. This lake or sea is remarkable for its green tint, and the very great inequalities of its bottom. In some parts its depth is asserted to be more than 3000 feet, but then the water suddenly shoals to 150 feet. Considering the mass of water it receives from the rivers Bargazin, Selenga, Angara, and many less considerable tributaries, and that its only visible outlet is the Lower Angara, which is not capable of dis- charging above a tenth of the water poured into this sea, it is conceived by some geographers, that evaporation, in a climate where the whole surface is annually frozen for five months, could not carry off the surplus, and that there- fore the Baikal must have some secret outlet, by which its superfluous waters may reach the ocean. The same diffi- culty occurs here as with the Caspian and the Aral. How could the seals which it contains have been introduced? They are extremely numerous in its northern part, and as many as two thousand have been killed in one year. Of the fishes, the omul, or Salmo migratorius, is the most im- portant in an economical point of view; but it also con- tains S. salvelinus, S. oxyrhinchus, the carp, the tench, the sterlet, and the isinglass sturgeon. One fish is peculiar to this sea, Calognathus Baikalensis, which occurs in such quantities as to corrupt the air by its putrefaction on the shores after storms, to which this sea is very liable. There are many appearances of volcanic fire in the vicinity of the Baikal. It is probable that the surface of the waters of this inland sea once stood on a higher level than at present, and that thus the salmon and the seal might have been able to surmount the rapids of the Yenisei, or have found their way into the Baikal by the channel of the Lena.

The Red Sea is the only other Asiatic sea which we shall here notice particularly. It is a long arm of the ocean, ex- tending between Asia and Africa for nearly 1500 miles, whilst its mediastinal breadth is not above 100 miles. Until Lord Valentia's voyage, our charts of this sea were very imper- fect, and much still remains to be done; but his lordship has established the important point of the superior depth of the western side, although the general depth is inconsider- able. These peculiarities of its form give it more the char- acter of a river than of an arm of the ocean; a circum- stance which, with the position of its strait in the direction of the tidal wave from the Arabian Sea, makes it subject to very considerable tides, especially in its lower part; whilst their rise is not prevented by the influx of any river. The less powerful influence of shape, and the influx of the united streams of the Euphrates and Tigris, tend to dimi- nish the height of the tides in the Persian Gulf, another in- land sea, about half the length of the Red Sea.

The Tchad, an inland sea of fresh water, is one of the most considerable of the kind in the world; and constitutes that Baler el Soudan, so long considered as the common re- ceptacle of the rivers of Central Africa. The discoveries of Denham and Clapperton prove that it is one of the grandest features in the kingdom of Bornou. It seems to be not less than 200 miles in length by 150 in breadth; but in the rainy season it becomes greatly enlarged, and encroaches on its flat shores. It is principally fed by the rivers Shary and Yeou. The former flows from the south into the lake, and is a noble, rapid river, half a mile in width. The lat- ter, once supposed to be the Niger, comes from the west, but is not considerable.

We have already noticed the general direction of the Mexican equinoctial current into the Gulf of Mexico, from which Gulf, it has obtained the name of the Gulf Stream. The ed- dying of this current in the great inland basin, the preva- lence of the trade-winds, and the enormous quantity of mud brought down by the Mississippi and the less impor- tant tributary streams descending from the Mexican Sierra Madre, are, according to Humboldt, rapidly filling up the gulf. The eastern coast of Mexico, from 18° to 26° north latitude, abounds with bars, over which large vessels can- not pass; and, far inland, that distinguished traveller found banks filled with sea-shells but little changed. Were the Isthmus of Darien annihilated, the equinoctial stream would pass round the globe; and this change would probably, ere long, place the feeble governments of Japan and China at the mercy of some European conqueror.

Hudson's Sea, improperly called a bay, is during a great part of the year encumbered with ice. This basin is almost three times larger than the Baltic, comprehending an area of about 500,000 square miles. It is inhabited by some kinds of cetacea, especially the Greenland whale and the beluga.

Baffin's Sea is now found to communicate with the po- Baffin's lar sea by several openings. Its extent, if we include Da- Sea. vis's Straits to the latitude of Cape Farewell, is only inferior, as an inland sea, to the Mediterranean. It is locked up for a considerable period of the year in impenetrable ice; but in the summer months the ice is broken up, and partially carried off by a strong current, which is continually setting down through the strait. The centre of that sea, however, is occupied either by a series of rocky islands, or by shoals, on which vast icebergs are perpetually grounded. Its desolate shores have been much frequented by our whale fishers, on account of the multitudes of the *balaena mysticetus* that resort to this inlet.

Buffon has justly termed North America the country of lakes or of fresh-water seas. Commencing in the north with Great Bear Lake, we may trace a connection by means of minor lakes to Slave Lake, a vast sea of fresh water, about 200 miles in length and 100 in breadth. This last lake communicates with the Arctic Ocean by Mackenzie's River, and with Athabasca Lake by Slave River. A chain of smaller lakes conducts thence to Winnipeg, which is scarcely inferior to Slave Lake in extent of surface. Advancing to the south-east, we arrive at Lake Superior, which, with the Michigan and the Huron, may be considered as forming the great inland sea of Canada. Lake Superior has an extreme length of 380 miles, a breadth of 161, and a circumference of 1525 miles; the coasts are rocky, and its depth, in some points, is 1200 feet, but its mean depth may be about 900 feet; and its surface is 641 feet above the sea. The Huron communicates with the Michigan by a channel four miles in length; it is 250 miles in length, varies from sixty to 180 miles in breadth, and has a circumference of 1100 miles. The Michigan is 262 miles in length by 55 in breadth, and has a circumference of 915 miles. The mean depth of the three lakes is 900 feet, which shows that their bottom is 300 feet below the level of the ocean. Their whole surface is computed at 72,930 square miles. We have already pointed out the danger to the whole valley of the Mississippi, should earthquakes ever burst the rocky barrier at the southern extremity of the Michigan.

Lake Erie is connected with the Huron by a narrow channel, which expands into Lake St Clair, and again contracts ere it reaches the lower lake. Erie is 230 miles in length, about sixty in breadth where it is widest, and about thirty-five at the mean. It differs from the upper lakes by its comparative shallowness, the mean depth not exceeding 120 feet. From many observations, it would appear that Lake Erie is rapidly filling up. Long Point, at the Big Creek, is stated to have advanced three miles in three years, in consequence of the alluvion carried into the lake by its numerous rivers. The connection of this lake with that of Ontario is by the river Niagara. The difference in level between the lakes is 330 feet; and, after the river has passed down twenty-one miles, it suddenly precipitates itself over the celebrated cataracts of Niagara, in a fall with a depth of 160 feet. It then flows for seven miles through a deep rocky chasm to Queenstown, where the country suddenly sinks; and it flows seven miles farther, through a champagne country, to Lake Ontario, another fresh-water sea, 171 miles in length and about sixty in breadth. The lakes above the falls abound with many kinds of fish, as salmon, sturgeons, several species of cyprinus, &c. The five last-mentioned seas or lakes are computed to have an area of 70,000 or 80,000 square miles.

3. Lakes.—Several of the large collections of water already mentioned have been considered as lakes; but that designation properly belongs to those less considerable bodies of water which we are now to notice. Many lakes are salt; and of such, a considerable number exist in Asia. The most celebrated salt lake in that quarter is the Dead Sea, or Lake Asphaltites. Independently of the account in the sacred Scriptures relating to the destruction of the cities of the plain the site of which is now occupied by this lake, the nature of the surrounding country bears evident traces of the ravages of fire in remote ages. The frowning cliffs on its eastern shore are evidently volcanic, thermal waters occur near them, the country around is often to this day shaken by earthquakes, and asphaltum is yet collected on its shores. Some have supposed it the crater of a volcano. The lake is much more saline than the ocean; and although the waters are limpid, their specific gravity is 1:2110. Mr Gordon attests their great buoyancy, and an analysis of the waters by Dr Marcet shows that they contain one fourth of their weight of salts. An hundred parts of water yielded,

| Substance | Parts | |--------------------|-------| | Muriate of lime | 3-920 | | Muriate of magnesia | 10-246 | | Muriate of soda | 10-360 | | Sulphate of lime | 0-054 |

This saltness of the Dead Sea is evidently owing to the abundance of salt in its vicinity, which all the streams falling into it perpetually deposit in this lake. Although the waters of the Jordan, the most considerable of these, are nearly tasteless, and do not contain above one three-hundredth of their weight of salts, the latter are of the same kinds as are obtained from the waters of the Dead Sea; and it is easy to imagine how this constant supply of saline matter to a lake of narrow dimensions, without any visible outlet, must produce an accumulation of salt, which cannot pass off by evaporation. On the western shores of this lake are the ruins of considerable edifices, and still more ancient remains are said to be covered by its waters.

Messrs G. Moore and Beek have very lately proved, by the temperature at which water boils, that the surface of the Dead Sea is 500 feet lower than that of the Mediterranean; and the correctness of this estimate they confirmed by the barometrical results of Professor Schubert of Munich, which give 500 feet as the depression of the Lake of Tiberias, and 598 feet as that of the Dead Sea.

The no less celebrated Lake of Gennesareth, or Sea of Galilee, is still smaller. According to Ali Bey, it is only seven leagues in length by two in breadth. It is embosomed in lofty mountains of a volcanic character, amongst which Mount Thabor is pre-eminent. Ali Bey considers this lake as the crater of an ancient volcano; and the nature of the rocks around, and the thermal springs of Tiberias, favour the idea of the volcanic nature of the country.

The want of any outlet, and the influx of rivers passing through a very saline soil, are the causes of the saltness of the considerable lakes of Van in Armenia, and of Ournia in Persia. The former has a circumference of 168 miles; and its waters, though palatable, are brackish. Ournia, or Ourumia, is not less than 300 miles in circuit, and is probably the saltiest of any known lake. Its waters are stated to contain thirty-two per cent. of salt; and the sinking of the waters, during the dry season of the year, leaves a thick crust of salt on its shores, which at some points extends a considerable distance into the lake. There are many other lakes of considerable size in Central Asia; many of them are salt; but the most singular is that in Thibet, which produces tincal or borax, a substance which is imported in flat prismatic crystals into Europe; where it undergoes purification.

Lakes of America.—If North America be the country of lakes, South America is remarkably deficient in them. That of Titicaca, in Upper Peru, already noticed, is the only considerable one in the southern division of the new world. It is true, that in the rainy season, the mighty rivers which water the plains of central South America inundate the low country to an immense extent, converting it into seas with a prodigious area, but of small depth; and it appears that inundations of this sort gave rise to the fable of the Lake of Parima, or El Dorado.

The Mexican lakes are more remarkable for their elevation of 7450 feet above the sea than for their extent. They are essentially saline, being more salt than the Baltic. The specific gravity of the waters of Tezcuco is 1:0215, and the impregnation is derived from a soil containing muriate of soda and carbonate of soda. These lakes have diminished since the Spanish conquest, and the capital is no longer in the lake, but connected with it by a canal. The natural decrease of these lakes has been accelerated by the felling of trees, and by a tunnel driven with immense labour through a mountain, to carry off the superfluous water that formerly often flooded part of the city. Between the inland seas of North America and the Icy Ocean there are innumerable lakes; a circumstance in which the northern portion of the new continent resembles the boreal regions of the old. A lake amongst the defiles of the Rocky Mountains, about the fiftieth degree of latitude, we were assured by the late Mr D. Douglas (a botanist, who several times crossed that chain, and was afterwards unfortunately killed by a wild bull in the Sandwich Islands), presents a remarkable peculiarity. From each extremity it discharges a considerable stream, one of which, falling into the Columbia, finds its way to the Pacific, whilst the other forms a branch of the Saskatchewan, and therefore may be said to fall into the Atlantic, through Lake Winnipeg, the inland seas of Canada, and the St Lawrence.

European Lakes.—Amongst these some of the most celebrated are the Lakes of Constance and Geneva. The first receives the Rhine, which was fabled to pass through it without mingling with its waters. Like other alpine lakes, it uniformly increases in summer, by the melting of the mountain snows; and its lowest period is in winter, when its sources are locked up in ice. Its length is thirty-five miles, its greatest breadth is eighteen miles, and its mean depth is 600 feet, but in some places it requires a line of 2130 feet to reach the bottom. Its waters thus fill a chasm about 1000 feet below the level of the ocean.

The Lake of Geneva has an area less than that of Constance; its extreme length being fifty-four miles, and its greatest width fifteen. Its depth is very variable; in some parts it was found by Deluc to be 960 feet, which shows that its bottom is only 200 feet higher than the level of the Mediterranean. Like the Lake of Constance, it receives a river, the Rhone, at one end, and discharges it at the other extremity, near to which the river precipitates itself into a subterranean channel termed Perle de Rhone. The beauty of the banks of this lake, and the prospect of the distant glaciers, have given it its great celebrity. The other Swiss lakes, though eminently beautiful, are not of sufficient importance to be described here.

Italian Lakes.—The beautiful scenery on the Lago di Como and the Lago Maggiore have been the theme of general praise; but some of the smaller Italian lakes, which seem to be the craters of extinct volcanoes, deserve notice. The lakes of Nemi and Averno are most evidently so; and the poetical representations of the sulphureous exhalations from the latter may possibly have had a real origin in former periods; but now birds rest with impunity on its surface, and fishes inhabit its waters.

British Lakes.—Our lakes are in general of small extent. The lakes of Westmoreland and Cumberland are more remarkable for their picturesque beauty than their size, and appear but as insignificant specks in a map of Europe. A few of the Scottish lakes are on a larger scale. Some of them are remarkable for their depth, and probably on that account Loch Tay and Loch Ness are never frozen. Loch Ness in some parts is 800 feet deep; so that its bottom is not less than ninety or a hundred feet below the bottom of the deepest part of the German Sea, on the eastern coast of Great Britain. The depth of Loch Tay varies from eighty to 600 feet. Loch Lomond is the largest of the Scottish lakes, being thirty miles in length by nine in its widest part, and at its northern extremity it is 600 feet deep. In the terrible earthquake at Lisbon, Loch Lomond repeatedly rose and fell several feet. The same took place in Loch Ness. A huge wave rolled with impetuosity from east to west, and the waters sunk and fell repeatedly in an hour. Loch Tay was remarkably agitated in 1783 and 1784, during the violent earthquakes in Calabria. On 22d September of the latter year, two large waves were observed to rush from each extremity of the lake, and coalesce near its centre, leaving about 100 yards of its banks at the eastern extremity momentarily dry; but the water returned and overflowed its usual boundary. The same appearances were repeated for several successive days with less violence; and the agitations of the loch did not cease till the 15th of October.

The agitation to which some alpine lakes are subject when the air is perfectly calm, in which even whole breakers have been observed, is a very obscure phenomenon. It has been often observed on the lakes in the north-west of England, in the Highlands of Scotland, and in the Lake of Geneva. It may possibly be explained by the compression of air in caverns existing in the adjacent mountains, the roofs of which are on a higher level than the surface, and communicating with the bottom of the lakes. If water much charged with air find its way into such caverns, the air separated may be compressed in their roofs, until its elasticity becoming superior to the pressure of the column of water, forces its way into the lake, and thus gives rise to what in England is termed a bottom wind.

The Irish lakes of Killarney and Lough Erne are highly picturesque; but the petrifying qualities of the waters of the more extensive Lough Neagh, in Antrim, ought rather to be attributed to the qualities of the soil in its vicinity. We possess specimens from that locality of wood converted into siliceous petrifications.

Periodic Lakes.—The inundations of rivers may be said periodic to form periodic lakes; but there are other instances where the cause is more occult, although the effect be no less conspicuous. The best-known instance of this is the celebrated Lake of Cirknitz, in Carolina. It is only about eight miles in length by two in breadth. In the beginning of June its waters disappear through several fissures in its basin, and the peasants immediately begin to cultivate its bottom, or to pasture their herds on the rich herbage which its oozy bed soon produces. The crop is removed, and about the end of September or middle of October the waters return, spouting up through several apertures in the earth with great force. With these waters various species of fish appear; and sometimes is seen amongst them that curious animal, so puzzling to systematic naturalists, the Anguina Proteus of Laurenti, the Syren Anguina of Shaw. This lake is placed in a valley amongst limestone hills, which are well known to be hollowed out into vast caverns. Of these the most considerable is Mount Javornick. The caves are supposed to be the receptacles of subterranean rivers, which, when augmented by the autumnal rains, overflow into the channel which communicates with the basin of the Cirknitz; and when the sources of these floods fail, the waters retreat to their summer level. Other lakes of a similar kind are said to occur in Dalmatia.

4. Rivers.—The size and course of rivers are chiefly determined by the height and direction of the mountain chains in which they originate. Thus the Rhine, the Danube, and the Rhone, the largest rivers in Europe, take their rise in the Swiss Alps. The great rivers of Asia have their origin in some of its lofty central chains. The northern rivers, the Irtysh, Ob, Yenisei, and Lena, may all be traced to the Altai; the Ho-yang-Ho and Yang-tse Kiang of China arise in the mountains forming the eastern abutment of the central table-land of Asia; whilst the southern ramparts of that table-land contain the sources of the great river of Cambodia, the Irawaddy, the Brahmaputra, the Ganges, and the Scind or Indus. In Africa, the Nile has one of its sources in the lofty mountains of Abyssinia, and the other in the more distant central chain; the Niger rises in the western chain, runs eastward, is deflected in the kingdom of Houssa. towards the south and west, and finally pours its waters into the Atlantic in the Gulf of Guinea. In America, the Magdalena and Marañon spring directly from the Cordilleras of the Andes, which also contain the principal sources of the Orinoco; whilst the more sluggish streams of the Paraguay and Parana derive their waters from a lower transverse chain, stretching to the coast of Brazil. In North America, the Arkansas, Red River, and Missouri, the great feeders of the Mississippi, spring from the Northern Andes, where also are the sources of the Columbia to the west, and of the Saskatchewan, the principal stream, which, flowing into Lake Winnipeg, becomes thus a part of that vast chain of lakes or seas, the outlet of which is the St Lawrence.

The channels of rivers are partly produced by the action of their own waters; but undoubtedly also by some of the causes which we have indicated as disturbing the original arrangement of the solid materials of the earth. Thus earthquakes have often opened passages for rivers through barriers of rocks and mountain chains. It may be admitted, that the continual action of water will excavate the hardest stone; but the ravines through which rivers flow frequently bear no traces of having been formed by the action of water; and the depth of some channels, compared to the scantiness of the stream, scarcely permits us to ascribe the whole to the excavation produced by the water. Thus, it is impossible to suppose that the small stream of the Lauricocha, in South America, has cut its way through the chain of the Andes, which it traverses in a long mural chasm, only twenty-five fathoms wide, but of astonishing sublimity. In other instances, the bottom of the channel itself is even below the level of the ocean; thus, the bed of the Marañon, above its junction with the Rio Negro, and more than 1000 miles from its embouchure, was found by Condaminé to exceed 620 feet; which, by the estimate of Humboldt, is more than 100 feet below the level of the Atlantic. This may be a chain produced by an earthquake, like that which, in 1752, suddenly opened, and received the whole waters of the Rio del Norte (a river which has a course of 1000 miles), so as to lay dry its channel for a distance of fifty leagues; and it was several weeks before the river assumed its usual level. Sometimes the channel of a river near its mouth is much below that of the adjacent sea. It is so in the Marañon; and the same has also been proved with regard to the Mississippi, which, just above New Orleans, is 153 feet deep, that is, at least fifty feet below the mean depth of that part of the Mexican Gulf. In such circumstances it is obvious, that the lower part of these hollows in river channels must be filled with salt water; which, by its superior gravity, will insinuate itself below the descending current. Indeed this takes place in the mouths of all rivers communicating with the sea by considerable channels of small declivity. Thus the lowest stratum of water at London Bridge, at the highest flood, is salt, though the upper current flowing over this stratum is quite potable. The tides in the Marañon are sensibly felt 600 miles from its mouth; and probably the sea-water forces itself up at least thus far beneath the mighty current of the river.

Rivers occupy the lowest parts of valleys through which they flow, and when they are very large their declivities are generally small. The surface of the Marañon, 3000 miles from the sea, has only an elevation of 1235 English feet, which would give less than five inches per mile for its mean declivity; but, from the point to which the tides of the ocean reach, the declivity is considered as not above 0·2 inch per mile. The Ganges, from Hurdwar, where it issues from the defiles of the Himalaya, has a mean declivity of four inches per mile; that of the Volga is about five inches.

The general form of the channels of rivers, however, especially in a campaign country, indicates, that if not wholly formed, they are greatly modified, by the action of their waters. In fact, their beds are usually proportional to the force of the stream. On the banks of many large rivers we can still trace the different heights at which the waters have formerly flowed. Playfair mentions the existence of four or five successive terraces on the Rhine, each of which has evidently been formed in succession by the river; and hence he concludes that the Rhine once flowed 360 feet above its present level. Similar remarks were made on the Upper Rhine by Saussure. The general effect of the action of the water must be the erosion of the channels in the lapse of ages; and even in the rocky beds of rivers this action of water is very perceptible. Thus the waters of the Niagara have apparently worn away the limestone rock of the falls, and formed a deep ravine through the stony bed, six miles in length from Queens-town, where the cliffs terminate abruptly, to the present site of the Falls. The undermining of the cliffs by this stupendous cataract has caused a recession of the Falls equal to about eighteen feet in thirty years; but we cannot thence infer the length of time that this cataract has existed, because we have no certainty of the equability of the disintegration, nor of the other causes which may have aided or facilitated the process. When rivers which have their sources in lofty mountains flow through extensive plains, they are subject to periodical inundations at the rainy season, or when the snows which feed them are melted by the summer heat. Under these circumstances, the swollen river bears along with it mud, sand, and gravel. A portion of these is deposited on the banks of the river, and thus tends to elevate them above the level of the adjacent country; whilst a portion is swept to the sea by the current, and the materials are there collected into sand-banks, or, by the repellent action of the waves, serve to augment the triangular plain often formed at the mouths of rivers, to which the name of delta has been applied. This evident and gradual increase of a delta has given rise to the idea of attempting to estimate the antiquity of existing geological arrangements by such a natural chronometer. Thus, if the increase of a delta at the mouth of any river be a certain number of feet in a given time, it has been supposed that we could thence obtain a measure of the age of the whole alluvial land at that point. But for the accuracy of such deductions several data are required. We must ascertain that the supposed delta is really the formation of the river, and that the rate of increase at distant periods is uniform. In every attempt to solve such problems there is danger of mistaking the effects of general for local causes; and there is strong reason to disbelieve the alleged uniformity of the increase of the alluvial soil at the mouths of rivers. The Po, of late years, appears to have gained upon the Adriatic, at the rate of 228 feet per annum, at its two principal embouchures; but had the annual increase always equalled this, the mouth of the Po at the commencement of the Christian era should have been seventy-eight miles from the present shore, or extended beyond Mantua. But towns fifty miles nearer the Adriatic, in the valley of the Po, have existed from before the birth of Christ. The calculations of Volney, on the advance of the delta of the Nile, are founded on very insufficient grounds; and as yet it must be conceded, that we have no determinations of this sort worthy of confidence.

The currents of large rivers may frequently be traced, by their colour, to great distances in the ocean. Thus the Ganges, when in flood, discolours the sea in the Gulf of

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1 Works, vol. i. Bengal to the distance of sixty miles, and its mud covers the bottom at eighty miles from the land. The mud of the Orinoco is carried far into the Atlantic; and on examining water drawn from the sea in latitude 8° 20' north, that is, in the parallel of the Orinoco, and 200 miles from its embouchure, Traill ascertained that the specific gravity of the water suddenly fell below what it was to the north and to the south of this parallel. The enormous mass of water discharged by the Marañon, according to Sabine, discours the ocean, and has even considerable rapidity in a direction inclined to that of the equinoctial current, at the distance of 300 miles from the American coast. Fresh water may be obtained from the surface of this current, at a great distance from the shore.

The periodical inundations of rivers depend on great falls of rain in mountainous regions, or on the melting of snows in the neighbourhood of their source. The period depends on the return of these seasons in different places. Within the tropics the rainy season is usually about the time when the sun passes the meridian towards the tropic; and continues until his return to the same place. The floods of the Marañon and La Plata cover a vast extent of unexplored country. The rise of the Orinoco commences in May, its inundation begins in June, and the waters return to their channel in September; from which time they decrease until April of the succeeding year. The inundations of the Lower Mississippi begin in March, are at their height in June, and the river is lowest in October. The waters, at their height, 1000 miles from the sea, attain a rise of fifty feet; at 300 miles, of twenty-five feet; and at 100 miles, of twelve feet. The western bank of the Mississippi forms a vast series of lakes, which are dried up in autumn into arid plains, interspersed with swamps; but the delta below the junction of Red River is a dismal swamp, scarcely elevated above the sea. In the Red River, the Arkansas, and the Lower Mississippi, are found those enormous rafts of drift-wood, formed during the river floods, which sometimes extend for ten or twelve miles in one mass, rise and fall with the stream, yet have a luxuriant vegetation on their summits. The inundations of the Ganges follow a nearly similar course. The waters begin to increase in April, when the rains commence in the mountains in which it has its sources; the rate of its increase is about three inches daily until the month of July, when the rains descend in torrents in the plains. The increase of the river is then more rapid, being about five inches daily; and the country for 100 miles along its banks presents in the end of July the appearance of a vast lake, interspersed with insulated villages and woods. The general height of the inundation-waters in Bengal is about twelve feet, but in some places they have a depth of more than thirty feet. The great river of Ava, the Indus, the Euphrates, and the Tigris, have also their periods of inundation, depending on the circumstances determining the setting in of the rains on the mountains in which they originate.

The inundations of the Nile have long been celebrated. Shortly after the commencement of the rains on the mountains of Abyssinia, in June, the river begins to rise, and attains its greatest height in August. At Cairo the greatest rise is twenty-eight feet; at Rosetta it is no more than four about the middle of August, when the valley of Egypt, with a mean breadth of three or four leagues, and the greatest part of the delta, are covered with one sheet of water. Its increase is irregular; and it decreases gradually until the following May. As soon as the waters are within their usual channel, the soil, moistened and enriched by the sediment deposited from the inundation, is diligently cultivated by the natives.

By such inundations the banks of rivers are usually elevated above the adjacent plains; and when the stream carries down much mud and gravel, the whole bed of the river may become considerably raised above the general level of the country through which it flows. This natural process has been considerably accelerated by art on the Po in Italy. To obviate the evil of sudden overflowings of this river, its channel, and that of the tributary streams, have for many ages been carefully embanked; but the quantity of mud and gravel carried down by its waters is so great, that the bottom of the river is gradually raised, further embankments become necessary, and the repetition of these processes have elevated the whole bed of the river below its junction with the Mincio, until it is now actually thirty feet higher than the general level of the adjacent plains; a circumstance fraught with the utmost danger to the whole country between that point and the sea, should any accident happen to the main embankment. Where the evil is of less magnitude, the Italian engineers have long followed a remedy, devised, it is said, by Torricelli. When streams bring down much mud, and their channels have been suffered to become elevated above the level of the adjacent plains, the lower ground is subdivided by embanked enclosures; the turbid waters of the river are admitted by flood-gates into the subdivisions, and are there allowed to deposit their sediment. The addition of fertilizing mud thus made, in many situations amounts to four inches in depth by each irrigation, which may be repeated several times in the year. By such means, in five or six years, the soil is raised to a level with the river, and the risk of extensive inundations greatly diminished. The process is termed la colmata. It has been very advantageously applied in Tuscany and Lombardy; and if steadily and judiciously directed, under the auspices of a just and vigorous government, might possibly still avert a catastrophe which seems to impend over the districts of the Lower Po.

When river-courses lie amongst mountains, they are subject to sudden breaks, which, according to their depth, give cataracts rise either to rapids or to cataracts. There are many picturesque waterfalls in Scotland, as the Cascade of Fyers and that at Lochleven-head in Inverness-shire, Bruar Cascade in Athole, the Devon Lynn in Clackmannanshire, and the magnificent Falls of the Clyde near Lanark. The most stupendous cascade in Great Britain is the Fall of Glencoe, in the district of Applecross, in the parish of Kintail, and county of Ross. At the head of a wild and solitary glen, seven miles from the inn of Shealhouse, the river Girsaic is precipitated in one unbroken fall of more than three hundred feet. At the distance of about fifty feet from the bottom, the water impinges on a shelf, whence it falls into a dark pool, amidst naked perpendicular rocks. When the river is in flood, it descends in one unbroken sheet of above 380 feet in height. Wales and Cumberland can boast of many beautiful cascades on a smaller scale, particularly the Pistil y Cain and Pistil y Moudach, in Merionethshire. The cataracts of the Dahl in Sweden, of the Staubach in the Alps, of Tivoli and Terni in Italy, the less considerable cascades of Marraneria, the far-famed Styx of Homer, Hesiod, and Pausanias, in Arcadia, and the Fall of the Rhine at Schaufhausen, are amongst the most celebrated in Europe.

A cataract may exist in a comparatively flat country, from of America a sudden sinking in its level. The stupendous cataract of Niagara, in which a large and rapid river, 1650 feet in width, precipitates itself, by two channels, in one vast leap of 160 feet perpendicular, is an instance of such an occurrence. The celebrated cataract of Tequendama, near Bogota, though found by Humboldt to be far less lofty than had been supposed by Bouguer, is still of stupendous height, and one of the most magnificent in the world: a river half the breadth of the Seine at Paris is precipitated into a rocky gulf, at two bounds, to a depth of 540 feet.

The cataract of the river Shirawati, in the Indian province of Canara, exceeds in beauty and sublimity every Physical Geography.

The country around the village of Haliati, about three miles north-west of the fall, presents the richness of a tropical forest, mingled with cultivation. The traveller comes suddenly on the river. "A few steps more," says Dr Christie, "over huge blocks of granite, bring you to the brink of a fearful chasm, rocky, bare, and black, down into which you look to the depth of a thousand feet." The bed of the river is one fourth of a mile broad, in a direct line; but the edge of the fall is elliptical, with a sweep of about half a mile. This body of water rushes, at first for about three hundred feet, over a slope at an angle of 45°, in a sheet of white foam, and is then precipitated to the depth of eight hundred and fifty feet more, into a black abyss, with a thundering noise. It has therefore a depth of 1150 feet. In the rainy season the river appears to be about thirty feet in depth at the fall; in the dry season it is much lower, and is divided into three cascades of varied beauty and astonishing grandeur; but the smaller streams are almost dissipated in vapour before they reach the bottom. Dr Wilson succeeded in viewing this magnificent cataract from the bed of the river, from which it appears to much advantage.¹

The number of considerable rivers which fall into the sea in different parts of the old continent amounts to 430, and those of the new continent may be estimated at 140, but this seeming disproportion is amply compensated by the vast dimensions of the latter.

Several attempts have been made by philosophers to compute the quantity of water which rivers discharge into the ocean; but it is a problem scarcely admitting of any solution, beyond a probable approximation. From the observations of Father Riccioli on the discharge of the river Po, it has been calculated, that the water poured out by all the rivers of the globe is equal to forty-one cubic miles daily, or to 14,965 cubic miles annually. This computation is probably too high. The greatest estimate of the mean annual fall of rain, dew, &c. is not above thirty-four inches for the whole earth; and the superifices of the globe being 196,816,658 square miles, this will afford a mass of water equal to 105,614 cubic miles as the whole that is annually precipitated on the earth. But from this must be deducted all that falls on the ocean, which, if we consider the precipitation in the ratio of their surfaces, will give 38,271 cubic miles as the water that falls on the land. Of this quantity, however, at least two thirds are expended in irrigating the soil or in sustaining vegetable life, and are restored to the atmosphere by the process of evaporation, so that no more than 12,757 cubic miles will become the annual tribute of the rivers of the globe to the ocean. This estimate differs less from that founded on the tables of Cotte than some later speculations on this subject, in which we suspect that the mean annual rain, and the quantity returned by the rivers to the sea, are considerably underrated.

The length of the course of twenty-two of the principal rivers, and the area of their respective domains or basins, were calculated with much care, and given in a tabular form in the essay on Physical Geography in the last edition of the Encyclopaedia Britannica. These elements we here insert, with the addition of a reduction of the proportional lengths into English miles, which has been obtained by multiplying them by 180, the distance between the remotest sources of the Thames and its embouchure at the Nore, that river having been assumed as unity in the first column of the table. The proportion of water discharged by each has been omitted, because the data on which that column was calculated did not seem to be sufficiently established.

¹ Jameson's Journal, vol. xxiv. for January 1838. good spring-water generally vary from $\frac{1}{3000}$th to $\frac{1}{5000}$th of their weight; and such water has a specific gravity of from $1-00016$ to $1-00030$. Such springs are termed soft, and answer well for culinary and other domestic purposes. When the solid ingredients exceed this proportion, the water becomes hard; and is less fitted for cooking or dissolving soap, in proportion to the quantity of foreign impregnation it contains. The springs of Upsal, analysed by Bergman, afford a good instance of a very soft water. The three purest contained no more than $\frac{1}{3000}$th part of their weight of solid matter, and had a specific gravity of $1-0002$. From a kanne (about 57,000 grains troy) of the Sandvik spring, he obtained 9-65 of solid contents, in the following proportions:

| Grains | Grains | |--------|--------| | Carbonate of lime | Muriate of lime | | 5-5 | 0-5 | | Muriate of soda | Sulphate of potassa | | 2-25 | 0-25 | | Silica | Carbonate of soda | | 0-5 | 0-25 |

besides four cubic inches of carbonic acid gas, and two cubic inches of oxygen gas.

The same ingredients are found in most potable waters, and many also contain sulphate of lime. The springs which supply Liverpool have a specific gravity of from $1-0026$ to $1-0027$, and contain $\frac{1}{3000}$th of solid matter. They are rather hard; but when the ingredients are in more considerable quantity, the spring becomes a mineral water.

Mineral springs, besides the substances already indicated, may contain one or more of the following substances: sul. waters, phuretted hydrogen, or nitrogen; sulphates of soda, ammonia, magnesia, alumine, iron, copper; nitrates of potassa, lime, and magnesia; muriates of potassa, ammonia, and baryta; more rarely free sulphuric acid, boracic acid, and borate of soda. Muriatic acid uncombined has been detected in the waters of the Pusambio in South America, which arise from the volcanic mountains of Purace, and are strongly impregnated with sulphuric and muriatic acids; and in Java sulphuric acid has been discharged by volcanoes in such quantities as to destroy vegetation. Boracic acid is deposited from several of the Tuscan springs, especially from the thermal waters of Sasso. The borate of soda is found in some of the lakes of Persia and Thibet. Mineral waters may be, 1. acidulous; 2. chalybeate; 3. sulphureous; or, 4. saline. The first generally owe their sparkling appearance and acid qualities to carbonic acid. The second contain either the carbonate or the sulphate of iron. The third owe their fetid smell to sulphuretted hydrogen, or to sulphures of lime, or of some other alkaline substance. The fourth contain a notable portion of neutral salts, from which they have a disagreeable taste, and are hard waters.

The acidulous waters almost always contain some alkali-line or earthy salt. Such are the waters of Tunbridge, Spa, and chalybeate Pyrmont, Seltzer, and others. The chalybeates generally contain from one to two and a half grains of oxide of iron, or some salts, in each English pint, often combined with other salts. Of this kind are the British springs of Peterhead, Brighton, and the chalybeate sources of Harrowgate and Cheltenham, which last, however, are much mixed with other saline ingredients. These vary from $1-0007$ to $1-0010$ in specific gravity.

Our sulphureous waters are potent, and owe their qualities to sulphuretted hydrogen. The principal are, Strathous waters, near Dingwall; St Bernard's Well, at Edinburgh; Hartwell, near Moffat; Harrowgate, Leamington, and a less known well at Kilburne, near London. Their specific gravity ranges from $1-0042$ to $1-0132$, especially of those which, like Harrowgate and Leamington, contain much saline matter. Moffat spring contains very little saline impregnation, and its sulphuretted hydrogen amounts to $1-20$ cubic inch in each English pint. The old well at Harrowgate contains much saline matter, and $1-75$ cubic inch of that gas in the same quantity of water. The thermal wa- Saline mineral waters contain a notable quantity of some of the salts already indicated as found in springs. The most noted in Great Britain are the waters of Dunblane, Pitcaithly, Cheltenham, and Leamington. Their specific gravity varies from 1.0047 to 1.0119. The waters of the last contain respectively 80-5 and 110-38 grains of salts in each English pint. All these mineral springs are reputed remedies for various diseases, but are evidently unfit for culinary purposes. The source of their peculiar impregnations is the mineral beds through which they flow.

Thermal Springs.—The heat of thermal waters has been ascribed, by Hutton, Laplace, and Bischoff, to the agency of a central heat, which is conceived to exist in the bowels of the earth. Some indeed have supposed that their temperature may be derived from the mutual chemical action of local accumulations of certain substances at more moderate depths in the earth. But the long-continued uniformity of their heat has given rise to a modification of this doctrine, which is ably supported by Dr Daubeny. He considers thermal springs as deriving their temperature from the same source as volcanoes, viz. the infiltration of water through the external crust of the earth, to the regions where he conceives the metallic elements of earthy and alkaline bodies to exist. The intense heat extricated during the oxidation of these elements will convert a portion of the water into steam, which, under compression, will attain a high temperature, act on various earthy matters, and communicate its heat to the subterranean waters which issue in thermal springs. The position of such springs in the vicinity of disturbances or upheavings of strata by igneous rocks, seems to favour this idea. Whatever may be the cause, we can now no longer doubt that there is within the earth a source of heat, the effect of which is perceived in the increased temperature as we descend in deep mines, and in the distances to which the effects of volcanoes are known to be propagated. The hypothesis of a central heat, unconnected with chemical changes, has been strongly insisted on, from the uniformity of the increments of temperature with the depth of borings for Artesian wells; and the result would lead to the conclusion, that the heat of the earth augments, at an average, one degree of Fahrenheit for every 100 feet of perpendicular depth; so that at the depth of three miles water would be boiling hot. But if we adopt the views of Daubeny, the temperature of particular springs will depend on the variable profundity of the unoxidized materials of the globe, and the supply of water. The temperature of thermal waters seems to be liable to little change in each spring; but in different waters it occurs in every degree, from a temperature very little exceeding the mean of the place, to the boiling point of water.

Table of the Temperature of some Thermal Springs.

| Location | Temperature | |---------------------------|-------------| | Matlock | 66° | | Bristol | 74 | | Buxton | 82 | | Bath | 116 | | Alhama, Spain | 118 | | Argos, Portugal | 142 | | Gurgitello, Ischia | 88 | | Coquinas, Sardinia | 198 | | Nero's Bath, Naples | 121 | | Albano, Padua | 121 | | Thermopylia, Greece | 113 | | Balkan, Turkey | 163 | | Vichy, France | 113° | | Cauterets, do | 130 | | Bourbonne les Bains, do | 131 | | Neris, do | 145 | | Plombieres, do | 146 | | Chaudes Aigues, do | 174 | | Aix-la-Chapelle, Germany | 135 | | Weisbaden, do | 158 | | Carlsbad, Bohemia | 167 | | Borarts, Lower Rhine | 172 | | Geysers, Iceland | 212 |

Iceland abounds with boiling springs, some of which are remarkable for their issuing out of a small mass of rock in the midst of a large river, as that figured by Sir George Mackenzie in the Reikjadalssæ; others are celebrated for their intermissions and magnificent jets of boiling water, as the Geysers. These fountains are well known by the writings of Von Troil, Stanley, Mackenzie, Hooker, Henderson, and by two Danish publications, the first by Olafsen and Povelsen, the latter by Ohlsen. The two principal of these singular fountains are in a valley near Haukadal, about sixteen miles from Skalholt, the bottom of which contains numerous cavities filled with hot water. In the midst of a little mount formed by siliceous deposits from its waters, six or seven feet in height, opens a circular basin fifty-six feet in diameter in one direction, and forty-six in the other. The depth of this basin is three feet; but in its bottom is a round hole about ten feet wide, the termination of a pipe or fissure which sinks into the earth to the depth of seventy-eight feet. By this pipe the water occasionally retires, leaving the basin quite dry, and no steam is then perceived to issue from the pipe. Periodically the water begins to rise in the pipe, and again fills the basin to overflowing; the ground is shaken by hollow subterraneous explosions; suddenly a prodigious column of boiling water is shot into the air with astonishing violence, and clouds of steam obscure the atmosphere. This is followed by successive jets, sometimes to the number of sixteen or eighteen in five minutes, the last of which is generally the most energetic, and attains an altitude of from 90 to 100 feet. On a second visit to the Great Geyser in 1815, Mr Henderson saw it rise to the height of 150 feet; and Lieutenant Ohlsen says that in 1804 he took the altitude of one eruption with a quadrant, and ascertained it to be 212 feet. These efforts are attended with a loud noise, and the ground trembles beneath the feet, whilst the velocity with which the jets and the accompanying steam are hurled into the air is astonishingly sublime. When stones are thrown into the pipe, they remain there until the succeeding jet projects them with great violence into the air, and they may be seen descending amid showers of boiling water. The intermissions of this extraordinary fountain are not absolutely regular; but there are generally four great series of eruptions from its orifice in twenty-four hours.

The jets from the Strokr, or New Geyser of Stanley, though smaller in diameter, are scarcely less magnificent; and, taking place more frequently, and with surprising violence, afford a high gratification to the traveller.

The mechanism of these Geysers must be simple, to have lasted for many ages. They are mentioned by Saxo in his History of Denmark; and this shows that some of them have existed for about 600 years. It is not easy to imagine an application and removal of a heating cause within the earth in such short intervals; but an arrangement somewhat resembling that represented in No. 13 would be capable of producing a Geyser. A cavity similar to that supposed by Mackenzie or by Lyell, in which water performs the office of a valve, from the form of the lower part of the canal communicating with the surface, might suffice; or its effect might be increased by a simple valve formed by a mass of stone accidentally getting into the position represented in the figure. Indeed, an experiment of Mr Henderson on the Stroeker proves the dependence of the force of its jets on some such mechanical obstruction. That gentleman found that the eruptions were accelerated by throwing a quantity of stones into the pipe, and the violence of the succeeding jets seemed to be in proportion to the load so applied; just as loading the valve of a steam-boiler will increase the elasticity of the steam. Henderson asserts that he was thus able to double the effect of the fountain, and to make it project the water to a height of 200 feet. The steam is no doubt produced by the agency of that volcanic fire which everywhere pervades Iceland, and is evidently in great activity under this secluded valley.

If the cavern in the imaginary section, No. 13, be supplied with water from a subterranean source, whilst steam is introduced into it from some more distant volcanic focus, the fountain will remain quiescent until the accumulation of steam, and the pressure of the vertical column of water, aided by the rude valve of rock, will augment its elasticity, until it become sufficient to overcome the resistance, and to project the water remaining in the pipe into the atmosphere.

The Geysers are not confined to the valley near Haukadal; several on a smaller scale exist at Reykum, issuing, at intervals of about an hour and a half, from a bank of hot sulphureous clay.

The waters of the Great Geyser hold a notable proportion of silex in solution, which no doubt is owing to their alkaline impregnation, and their high temperature. According to the analysis of Dr Black, in the Edinburgh Philosophical Transactions, vol. iii., an English gallon contains—

| Gr. | Gr. | |-----|-----| | Silex | 31·58 | | Muriate of soda | 14·42 | | Sulphate of soda | 8·57 | | Caustic soda | 5·56 |

Argil | 2·80 | And a little sulphuretted hydrogen.

The silex is deposited in beautiful stalactitic forms in many of the adjacent pools, and the Geysers petrify plants, like other petrifying waters.

Cold intermitting springs have been well described by the late Mr Gough of Kendal, who divides them into two kinds; reciprocating, or such as have a constant but irregular flow; and true intermitting springs, in which the issue is entirely suspended for a time, and the water returns at intervals of greater or less regularity. Of the former sort we have a good example in Gigglewick Well, in Yorkshire, in which the reciprocations are well marked. In it the water will sometimes rise and fall a foot in ten or fifteen minutes; at other times it will fall as much in four minutes, but usually requires double that time to regain its former height. The water bubbles out with force, so as to carry with it particles of sand. Another reciprocating well occurs near Torbay, called Lay-well. It is described, in an early volume of the London Philosophical Transactions, as ebbing and flowing sixteen times in an hour. A similar well, but on a far less scale, is St Anthony's Well on Arthur Seat. The cause seems to be in both instances the confinement and compression of air in tortuous cavities within the subterranean channel of the spring, which, from the position of the aperture, is alternately compressed by the accumulating fluid, until it acquire sufficient elasticity to drive before it the resisting column of water. It is easy to conceive a simple mechanism capable of this alternation, without any species of valve, except that formed by the water itself, which may be represented by the diagram No. 13, without its valve. Of the perfectly intermitting spring the most remarkable is the Bolder-Born, in Westphalia, near Paderborn. After flowing for twenty-four hours, it entirely ceases for the space of six hours; it then returns with a loud noise, and mixed with air, in a stream sufficiently powerful to turn three mills very near its source. In Lowthorpe's Abridgment of the Philosophical Transactions (vol. ii.), it is said to disappear twice in twenty-four hours; and possibly its intermissions are not regular, but may depend upon the quantity of rain which has recently fallen. Dr Boué has described a powerful intermitting spring which occurs at Bihar, in the commune of Maybugyer, in Hungary. It issues, many times a day, from the foot of a mountain, after hollow subterranean noises, in such quantity as in two minutes to fill the channel of a considerable stream. Its returns are most frequent from Christmas to Midsummer, when they take place every quarter of an hour; but they are rarer during the other half of the year.

The most probable manner in which such powerful effects are produced, seems to be the descent of streams of water, much mixed with air, into a cavern, the more direct communication of which with the surface is through a channel so connected with the cavern as to confine the air in its upper part, in the manner of a water-blast, until the increasing elasticity, from compression, enables the imprisoned air to dislodge the water accumulated between it and the outlet.

Sect. VII.—The Atmosphere.

The earth is surrounded by an invisible and highly elastic fluid, termed its atmosphere, which accompanies it in all places, its movements; being retained by the influence of gravitation toward the earth, though, from the repulsion between its own particles, it extends to a considerable distance from the surface. Its density diminishes as it recedes from the earth, and its expansibility is in the inverse ratio of the force by which it is compressed. Its density is further affected by the influence of heat and cold, by the admixture of watery vapour it contains, by the decreasing force of terrestrial gravitation, and the increasing attraction of the heavenly bodies as its distance from the surface of the earth is augmented. Experiment and calculation have proved that the rate of diminution is such, that the atmosphere at the height of seven miles would only have a density 1/4th of what it has at the surface; and that if the altitudes be taken in an arithmetical series, the decrease in density will follow a geometrical ratio. Thus, if the density at the surface be assumed as unity, at seven miles it will be 1/4th, at fourteen miles it will be 1/16th, at twenty-one miles it will be 1/64th, and so on. This property of the air would lead to the idea of an indefinite extension of the atmosphere; but there is evidently a limit to this, probably arising from the gravitation of the particles of air; and the nicest observations on the duration of twilight, as well as some other phenomena in astronomy, almost warrant the inference, that the utmost limit of the atmosphere cannot exceed forty-five miles in altitude. See the article Atmosphere.

Air is ponderable; 100 cubic inches of air at the temperature 60° Fahrenheit weigh 30·5 grains troy, at a mean barometrical pressure; and a perpendicular column of the whole atmosphere is just balanced by a mercurial column of about thirty inches in height. Hence the length of such a column is employed to ascertain the weight or pressure of the atmosphere; and the oscillations of the barometer have taught us, that this pressure varies in different stations on the earth's surface, and in the same place at different times. See the articles Barometer, Atmosphere, and Meteorology. The height of the barometer forms an important part of a meteorological journal. Observation has made us acquainted, not only with the occasional, but also with the diurnal oscillations of the column; and exhibited the connection of the more considerable variations with changes in the weather. The trouble of continual attention to obtain the mean heights of the barometer at any place, have given rise to several contrivances for registering the maxima and minima during the absence of the observer. A very ingenious instrument for this purpose is described by Mr Keith, in the Philosophical Transactions of Edinburgh (vol. iv.).

The open end of the instrument is a larger cylinder than the rest of the tube. The cistern is placed horizontally at the upper part, and an ivory float, supporting a kned wire, is placed on the surface of the fluid, as in No. 14. The scale is above the opening of the tube, and the kned wire moves two light indices, sliding on a fine wire, with sufficient friction to retain them in the position which they receive from the movements of the ivory float. The same gentleman caused the instrument, by a slight modification, to record the whole course of its observations through a given period. A perpendicular cylinder, divided into thirty-one compartments, is moved round its axis once a month by clock-work; a pencil attached to the float wire records the whole course of the oscillations; and a proper scale, marked on the cylinder, gives the value of the irregular curves thus traced. Once a month the cylinder is covered with a fresh paper, ruled into thirty-one columns, for another monthly series.

Professor Traill has given a simple and cheap form of a register barometer for maxima and minima.

A diagonal barometer registers the maximum height of the column. Before the tube is bent, a piece of iron wire is introduced into the closed end of the tube, which is then filled in the usual way. The wire being lighter than the mercury, will rise to the top of the column, is pushed before it on the rise of the barometer, and, on the fall, remains, by its own weight, at the point it had attained. A slight modification of Cassini's rectangular barometer marks the minima, by means of a piece of iron wire introduced at the open end of the tube, as in No. 15. To make the indications of both instruments as nearly equal as the construction will allow, the lower part of this tube is bent to less than a right angle.

It is obvious that the barometer affords the means of ascertaining the weight of the whole atmosphere. The pressure of the atmosphere varies from twenty-eight to thirty-one inches of the barometric column. A column of pure mercury, whose base is a square inch, and its height the mean between twenty-eight and thirty-one, or 29.5 inches, is found to weigh 14.5 pounds avoirdupois; therefore, the pressure of the whole atmosphere will be 14.5 pounds on each square inch of the earth's surface. And the superficies of our globe is equal to 790,116,426,647,758,800 square inches, if we multiply this number by 14.5, we obtain, as the absolute weight of the whole atmosphere, 11,456,688,186,392,473,600 pounds; or, if we adopt the more common estimate of the mean barometric height at 30 inches, and the weight of that column at 15 pounds on the square inch, this will give 11,851,746,399,716,352,000 pounds, or 5,290,958,214,159,085.7 tons, as the mass of our atmosphere.

The variations of atmospherical temperature are ascertained by the thermometer. (See the articles Meteorology and Thermometers.) The instrument, for this purpose, should be placed in the shade, protected against reflected heat, and also against that transmitted through walls, or other bodies to which it is attached. It is by no means easy, in a town, to obtain an eligible position for a measure of atmospheric temperature, on account of the many causes which affect the instrument. It ought to be protected by a glass case against rain; yet the air should circulate freely around it, and it ought to radiate its heat freely to the sky. The importance of obtaining accurate information on changes of the temperature of the atmosphere has given rise to numerous observations on the annual, monthly, daily, and hourly variations of the thermometer.

The labour and minute attention required in such investigations have induced ingenious men to devise methods of ascertaining mean temperatures from a few observations. It has been found that the mean of the thermometer during the month of April will, in our climate, nearly indicate the mean annual temperature; and that the mean of observations made about half past 8 A.M., at 2 P.M., at 9 P.M., and an hour before sunrise, will give the mean temperature of each day. To abridge such fatiguing observations, register thermometers have been devised. The best known of these are the thermometers of Six and of Rutherford. (See the article Meteorology, Plate CCCLIV, fig. 6 and 7.) A modification of Six's thermometer, rendering springs to the indices unnecessary, has been constructed by Professor Traill, in which the indices slide in slightly inclined tubes, as will be easily understood from No. 16.

As in Six's thermometer, the expansions of alcohol are the measure of the temperature; the indices, of iron wire, enclosed in glass tubes, are moved by a short column of mercury; and they are adjusted for a fresh observation by bringing them to the extremities of the mercurial column with a magnet.

The atmosphere itself is invisible; but the column of cold air interposed between the eye and distant objects invests them with an azure hue. This tint depends upon the greater refrangibility of the blue rays of light than of those towards the other end of the spectrum. When a ray of white or undecomposed light enters our atmosphere, the red and yellow rays pass with little deviation from a rectilinear course; but the blue rays are dispersed in the air, and affect the colour of objects beheld through a long column of the atmosphere. As we ascend lofty mountains, the refractive power of the air diminishes with its density, and hence the blue tint is lost in the blackness produced by the want of refraction; until a traveller, on the ridges of the Alps, the Andes, or the Himalaya, will perceive the sky to assume a hue approaching to deep blue or black. Saussure has described this phenomenon, and invented the Cyanometer, an instrument for measuring the intensity of the colour of the sky. (See the article Cyanometer.) The form will be best understood from No. 17. Atmospherical air was long considered as a simple substance, one of the elements of other matter; but the discoveries of Black, Cavendish, Priestley, Scheele, and Lavoisier, have proved it to be a compound, of which the principal constituents are oxygen and nitrogen gases, usually estimated at twenty-one volumes of the former to seventy-nine of the latter. Dr Black discovered that the atmosphere always contained carbonic acid, which was found to exist in slightly variable proportions; and it holds water suspended in the state of vapour, in proportions which are still more fluctuating. If we average the two last ingredients, the composition of the atmosphere may be stated as follows:

| Nitrogen | Oxygen | Carbonic acid | Aqueous vapour | |----------|--------|---------------|----------------| | 77.50 | 21.00 | 0.08 | 1.42 |

The manner in which these different ingredients are united has given rise to some controversy. The opinion once generally entertained was, that they were united by an affinity between their particles, like that which forms chemical compounds. The celebrated Dr Dalton first suggested the theory, that of the various elastic fluids constituting the atmosphere, the particles of one are neither retractive nor repellent of the particles of another; but that the pressure upon any one particle of such a mixture is entirely derived from particles of the same kind of matter. Each fluid, on this view, is considered as occupying the same space as it would in a vacuum; and each constituent of the atmosphere, under a mean barometrical pressure of thirty inches, as exerting its own separate pressure in the following proportions, depending on its own quantity and specific gravity.

Thus, the

| Nitrogen | Oxygen | Carbonic acid | Aqueous vapour | |----------|--------|---------------|----------------| | 23.36 | 6.18 | 0.02 | 0.44 |

Soon after the composition of the atmosphere was detected, and the qualities of oxygen in supporting combustion and respiration were understood, it was conceived that important modifications of disease might depend on or originate in the varying proportions of this essential ingredient in the air; and various methods were devised for analysing atmospheric air, and ascertaining the quantity of oxygen it contained. 1. Dr Priestley devised a method of doing this by the eudiometer, or measure of the purity of the air. His method consisted in adding, in a graduated tube, fifty-five volumes of nitric gas to a hundred of atmospheric air. The property of this gas is to abstract oxygen from the air, and to form nitrous acid, which is absorbed by water; and the amount of the diminution marked the quantity of oxygen in the air. 2. Gay-Lussac improved Priestley's eudiometer. Instead of adding the nitric gas in the graduated tube, he mixed a hundred volumes of the gas with as much of the air to be analysed, in a wide jar, and transferred this mixture to the graduated tube to ascertain the diminution; on dividing the residual air by four, he obtained the amount of the oxygen present in the air. 3. Scheele employed the sulphuret of potassa for the same purpose. 4. Seguin used the rapid combustion of phosphorus; and, 5. Volta exploded a mixture of hydrogen and air by means of an electrical spark. This is one of the most elegant and accurate of the eudiometers. He recommends a mixture of 200 volumes of hydrogen with 300 of air. On passing an Physical electric spark through the wires in the top of the jar, the hydrogene is fired, instantly forms water with the oxygene of the included air, and, in ordinary cases, the diminution of volume of the mixture amounts to 195, of which one third is oxygene, or sixty-five parts of oxygene have been lost by 300 of air, or about twenty-one per cent.

6. Dr Hope's eudiometer consists of a graduated tube exactly ground to a small tubulated phial. The bottle may be filled, either with hydrosulphuret of potassa, or with a liquid proposed by Davy (which is formed by saturating sulphate of iron in solution with nitric gas.) The air is in the tube, which is inserted in the neck of the phial; the instrument is inverted; the air, now in the phial, is agitated with the liquid for a few minutes, and on opening the tubulation of the phial, below the surface of the liquid, the amount of the oxygene absorbed will be indicated on the graduated tube. This very neat instrument has been modified by Dr Henry, who substituted a bottle of caoutchouc for the phial. Any of these instruments, especially 18 or 19, will show the quantity of oxygene in any given air; but it has been found that the quantity of oxygene of the air is little subject to variation, and that gas is equally present on mountains and in plains, in the town or in the country, except in confined situations, where fuel is burning, or many animals respiring. Air which is very deleterious to man has no sensible difference in the proportion of the oxygene from the purest atmosphere; and we must probably look to other causes than this for the unwholesomeness of the atmosphere.

The quantity of carbonic acid in the atmosphere is subject to slight variation, except in confined apartments. Dr Dalton found it only varying from $\frac{1}{400}$th to $\frac{1}{1000}$th of the whole air, an uniformity which is surprising, considering how much carbonic acid is constantly formed during respiration and combustion. Perhaps this may be partly owing to our imperfect modes of ascertaining very minute changes in its quantity. If we are to give credit to the experiments of the younger Saussure, the quantity in the atmosphere is greater in summer than in winter, by 2-3 volumes in 10,000; but his experiments are not generally considered as satisfactory, in a case where the difference appears so great, and the point so difficult of determination. Even the experiments of Dalton are not considered as decisive; for lime-water does not appear to be capable of abstracting all the carbonic acid from air, as was remarked by Cavendish; and philosophers have generally considered the quantity as ranging from $\frac{1}{300}$th to $\frac{1}{1000}$th of the whole atmosphere. In confined apartments, however, the quantity of carbonic acid is sometimes much increased, while the oxygene is consumed in the process of combustion and respiration; and this double vitiation of the air may give rise to fatal consequences, as was too well illustrated in the catastrophe of our unfortunate countrymen in the dungeon of the Black Hole in Calcutta in 1756, and in one which happened to persons confined in St Martin's Roundhouse in London in 1742. In the celebrated Grotta del Cane, the carbonic acid which issues from crevices in the floor fills the bottom of the cave to the level of the entrance, and then flows out by its superior gravity, so that the upper part is free of the noxious impregnations. A man enters with impunity, because his head is above the stratum of carbonic acid; but the air of the cave is speedily fatal to a dog, because the prone position of his air-passages brings his respiratory organs into contact with the gas. It is remarkable that these gases do not appear to vary considerably in their relative proportions in any part of the accessible atmosphere. Air collected on the summit of Mont Blanc was analysed by the younger Saussure, and agreed in the proportions of the gases with that of Geneva; and the same uniformity was found in air from Spain by De Marti, from Egypt and France by Bertholet, from the coast of Guinea and England by Davy, from the summit of Teneriffe and the ridges of the Equinoctial Andes by Humboldt.

The quantity of aqueous vapour in the atmosphere is liable to far greater variations than that of the other constituents. Water in the state of vapour is invariably present; but the proportion is modified by the temperature, and by changes in the barometric pressure of the air. The process by which water finds its way into the atmosphere is one of the highest importance, and has obtained the name of Spontaneous Evaporation.—If water be exposed to the air, it gradually disappears; and if this be carried on in closed vessels, the air is found to have taken up water, which may again be obtained by the action of certain substances that have a great affinity for water. The principal substances which have been employed for abstracting water from air are, sulphuric acid, quicklime, chloride of lime, and carbonate of potassa. If given proportions of any of these be enclosed in a vessel containing air, they absorb the moisture of that air; and the weight they thus acquire has been taken as the measure of the quantity of its water. The same is estimated by the quantity of water which is absorbed by a given volume of air thus artificially dried. Some very careful experiments on this subject were made by Saussure, who inferred that a cubic foot of air, at temperature 65° Fahrenheit, when fully saturated with moisture, holds eleven grains of water dissolved. Notwithstanding the acknowledged accuracy and patience of this philosopher, we are inclined to prefer the method of Dr Dalton (founded on the suggestion of Le Roy of Montpellier), of observing the temperature at which moisture begins to be separated from the air. This is termed the Dew-point of that atmosphere, and forms an important element in several interesting problems in meteorology. Dalton had shown, that the water existing in the atmosphere has precisely the same elasticity as it would assume in a vacuum at the same temperature; hence it is obvious that it exists in the air, not as water, but as elastic vapour. But the elasticity of vapour is demonstrated to depend on temperature; and, therefore, if we could measure the elasticity, we could determine the quantity of water in the atmosphere. This elasticity is measured by the Manometer, a barometer the cistern of which is enclosed in the vessel containing the air, and therefore only affected by the variations of elasticity of the included air. Deductions from these experiments have been given in a tabular form in the Manchester Society's Transactions.

In the article Hygrometry there is another table of great value, showing the elasticity, weight, and rate of expansion of aqueous vapour, in air saturated with moisture, at each degree of Fahrenheit's thermometer, from 0° to 100°, which was intended by the author of that treatise to facilitate hygrometrical investigations. In this table the elasticity of the vapour is calculated on a formula deduced by Biot from Dalton's experiments.

The quantity of water suspended in the air is liable to great variation, from several natural causes; and it is chiefly liable to be deposited or separated, on a diminution of the capacity of the air for moisture, by a reduction of its temperature. Warm air is capable of taking up, in the state of invisible vapour, more water than cold air. Thus a cubic foot of saturated air, at 32°, contains of watery vapour 2-350 grains; at 60°, 5-825 grains; at 70°, 7-941 grains. When the temperature of the air is diminished, as, for instance, by mingling with colder currents in the atmosphere, a quantity of water is separated; and, according to the sud- denness of the change, will be suspended in the form of visible clouds, or precipitated in fogs or rain.

Changes in the density of the air also affect its capacity for retaining moisture. Evaporation is quicker, everything else remaining unchanged, in rare than in dense air. Saussure found, that water evaporated quicker on the Col du Géant, which is 11,275 feet above the sea, than at Geneva, which is but 1824 feet above that level, in the proportion of eighty-four to thirty-seven. Light air then speedily becomes charged with moisture, which slight changes in temperature are ready to precipitate; and hence a falling barometer is often the immediate precursor of rain.

When atmospheric air approaches its point of saturation with moisture, evaporation goes on slowly, at length ceases, and a slight diminution of temperature is sufficient to precipitate a portion of water on contiguous bodies; we then term the air damp or moist. This change is not always to be immediately recognised by our sensations; and as the change is important to the success of some processes in the arts, and also to health, philosophers have invented instruments for indicating the approaches of the air to saturation, which have been named

Hygrometers, or measures of moisture. The change in bulk which some vegetable and animal substances undergo, in moist air, has been employed for this purpose. The most common are the twisting and untwisting of a cord of flax or hemp by moisture and dryness; the movements of the natural twist in the beard of the wild oat, *avena fatua*; in the Indian grass, *andropogon contortum*, proposed by Captain Kater; or the arista of the seed of *stipa pennata* (a common inmate of our gardens, and often found in our fields), proposed by Dr Cumming of Denbigh. Unfortunately these substances, though indicating minute changes in the air, are incapable of forming comparable hygrometers, and gradually become less and less sensible to the influence of humidity. The same may be said of the animal substances which have been employed in the construction of such instruments. The most simple of the animal hygrometers is Wilson's, formed of a rat's bladder fixed to a glass tube, and filled with mercury to a certain point on the tube when the bladder is moist. As it dries, the contraction of the bladder raises the mercury in the tube. Still more delicate hygrometers were formed by Saussure and De Luc, on nearly the same construction. (See the article Hygrometry.) Saussure employed a fine human hair, freed from unctuous matter by boiling in a weak alkaline solution; and De Luc used a slip of whalebone, scraped extremely thin. These animal bodies lengthen by moisture; and, in both instruments, one end of the hygrometric substance was fixed to the frame, while the other was attached to a fine flat wire lapped round an index made to move along a graduated arc by the varying length of the hygrometric substance. In Saussure's instrument, the index was brought back by a small weight as a counterpoise; and the same was effected in De Luc's by a spiral gold-wire spring. Both instruments were graduated by placing them in a jar filled with air saturated with moisture, and then in air dried by quicklime, or some similar body; and the space between these extreme points was divided into 100°, or any given number of degrees.

The imperfection of these instruments, when made by the hands of the inventors with every care, rendered it difficult to have two that gave the same indications under similar circumstances; and the still more fatal objection, that they become altered by time in mobility and delicacy of indication. Although both Gay-Lussac and Biot have endeavoured to show the relation between the indications of the hair hygrometer and the quantity of moisture in the air, we cannot consider the instrument as entitled to confidence.

A very elegant hygrometer was invented by Daniel, founded on the determination of the temperature at which dew begins to be deposited; but for the description of it we must refer to the article Hygrometry, as well as for an ingenious substitute devised by Mr Adie of Edinburgh, and another by M. Pouillet. All these depend on the dew-point; and all are liable to the objection, that it is difficult to determine the precise degree at which dew begins to be formed, especially at low temperatures. The original idea of obtaining a measure of the hygrometric state of the air from comparing the indications of a thermometer with its bulb moist and dry, is certainly due to Dr James Hutton, the celebrated geologist. This eminent philosopher made use of a single thermometer, which he carried enclosed in a glass case. This form of hygrometer, neglected for thirty years, has at length become an object of philosophic interest in this country and Germany; and two thermometers are now mounted on the same scale, and the indications of the wet and dry bulbs seen at the same time. This instrument has been termed a psychrometer. The most elegant form of an instrument on this principle is the hygrometer of Sir John Leslie, or double air-thermometer; one of the balls is covered with tissue paper, defended by a thin covering of blue silk, while the other is naked; but in order to obviate the effect of light, the dry ball is formed of pale-blue glass. This instrument is described in his original paper, and fully in the article Meteorology of this work. When the covered ball is moistened with water, the evaporation produces a degree of cold in proportion to the dryness of the ambient air; and the air within that ball contracting, the elasticity of the air in the other ball pushes the coloured fluid in the stem towards the covered ball. This transference is marked on a scale, each degree of which is equivalent to $\frac{1}{10}$th of a centigrade thermometrical degree. This hygrometer does not indicate the absolute dryness of the atmosphere, but only the degree of dryness it has after being reduced to the temperature of the humid ball. In the article to which we have just referred, there are tables given for correcting the indications of Leslie's hygrometer, and showing the point of saturation at different centesimal degrees, and the corresponding degrees of the hygrometer.

The instrument gives excellent comparative results; and two or three minutes are sufficient to bring the fluid to equilibrium, which marks the full effect of the evaporation; and no farther sinking of the hygrometer is produced by keeping the ball bedewed with moisture. In winter, in our climate, it ranges from $5°$ to $25°$; in our summer from $15°$ to $55°$. It has occasionally been observed as high as $80°$ or even $90°$, and in thick fogs it falls to the beginning of the scale: it rises when the weather clears up after rain, and in long tracts of clear warm weather it remains high. It always falls before rain, and remains low during wet weather. When as low as $15°$, the air feels damp; when it is between $30°$ and $40°$, we should term the weather dry; from $50°$ to $60°$, very dry; and from $70°$ upwards, it would be called intensely dry. In a warm inhabited apartment the hygrometer would not be below $50°$; but when below $30°$, that room would be felt to be uncomfortably damp.

The mean annual evaporation is an interesting subject, for an approximation to which we are mainly indebted to the experiments of Dr Dalton and Mr Hoyle, at Manchester, in 1796, 1797, and 1798. A cylindrical vessel of tinned iron, three feet deep and ten inches in diameter, was filled with soil; it was provided with one pipe near the bottom, and another one inch from the top. This was left in this state for a year, until it became covered with grass. Bottles were now applied to the end of each pipe, so as to collect both the surplus water which soaked through the earth, and that which ran off at the upper pipe. The soil was soaked with water on commencing the experiment; a rain gage of similar dimensions was placed near the cylinder of tin; and the evaporation was estimated by subtracting the quantity which passed into the bottles from the whole rain.

| Year | Inches | |------|--------| | 1796 | 30-629 in. | | 1797 | 38-791 in. | | 1798 | 31-259 in. |

Evaporation: 23-725 in., 27-857 in., 23-862 in.

The mean evaporation for each month, deduced from the three years, was:

| Month | Inches | |---------|--------| | January | 1-008 | | February| 0-528 | | March | 0-623 | | April | 1-485 | | May | 2-684 | | June | 2-184 | | July | 4-095 | | August | 3-386 | | September| 2-954 | | October | 2-672 | | November| 2-055 | | December| 1-484 |

giving 25-158 inches for the annual evaporation at Manchester. Dr Dalton considers that we ought to add five inches to this for the annual dew, which will raise the evaporation to about thirty inches.

From the nature of our climate, the mean annual evaporation over the globe cannot be estimated at less than thirty-four; and this would make the whole mass of water raised from the surface of the earth equal to 105,614-72 cubic miles, or equal to all the rain that falls upon the globe. The evaporation of any place may be ascertained by exposing freely to the air a given quantity of water in a shallow circular dish, accurately turned, so as to have 100 inches of superficiality. It should be so placed as not to receive any rain; on measuring the remainder once a week or once a month, we shall obtain the evaporation.

There is reason to believe that the evaporation from the foliage of trees and from herbage is somewhat greater than from an equal area of water, and that the quantity from the surface of new-ploughed land is almost equal to what arises from water; but we have got no good comparative experiments on this subject.

Having considered the conversion of water into an elastic fluid, let us turn our attention to the mode by which it is re-converted into moisture.

1. **Clouds**.—Some have accounted for the suspension of so dense a fluid as water in air, on the supposition that the vapour in clouds existed in the shape of hollow spherules, filled with a lighter fluid than the atmosphere; but this suggestion would seem to be devised rather to meet the difficulty than to be supported by observation; and it is possible to explain the considerable durability of clouds without recourse to any such gratuitous assumption.

The experiments of Volta and Cavallo have shown, that when water is converted into vapour, not only is caloric absorbed, but electricity is also developed. The vapour acquires positive electricity, and the remaining fluid possesses the opposite electric state. By the process of evaporation much electric fluid must be carried into the atmosphere; and when the capacity of the vapour for electricity is diminished by its partial condensation by cold in the upper regions of the air, it may be conceived that the spherules of vapour may become surrounded with atmospheres of electric fluid, the mutual repulsions of which may prevent the coalescing of the particles of vapour into drops so heavy as to descend by their gravity on the earth, especially as a stratum of air charged with moisture is specifically lighter than dry air at the same temperature. The form of clouds, and their different degrees of aggregation, may depend on the electric tension of the atmospheres surrounding the particles of which they are composed; or on a balance between the electric repulsions and the gravitation of the particles for each other. This view is favoured by the generally positively electrified state of the atmosphere when clouds are forming, the heavy falls of rain which accompany electric explosions when the discharge of the accumulated electricity allows the coalescing of the moisture into heavy drops, as well as by the peculiar odour of some kinds of fog, and the highly electric state such fog often exhibits. Beccaria, Romayne, Bennet, and Volta have remarked, that the atmosphere exhibits signs of electricity on the appearance of clouds and fogs. Beccaria always found the electricity of the air *positive*; and Romayne states, that mere diminution of volume in any substance is constantly attended with increased electrical tension. The various forms of clouds, their aggregation in deep masses piled on each other, or their dispersion in long radiated filaments, may be more easily explained on an electric theory than on any other supposition; and the accidental manner in which solar light is reflected from them to the eye of the spectator gives an infinite diversity to their appearance.

The vulgar have long remarked the association of certain forms of clouds with the state of the weather; and cloud meteorologists have begun to appreciate the value of a descriptive language for indicating their varied appearances. No system of nomenclature has yet been applied to clouds superior to that proposed by Mr Luke Howard. He divides them into seven species. (See Plate CCCCXI.)

1. **Cirrus**.—A cloud resembling a lock of hair or a feather, composed of parallel, flexuous, or diverging fibres, unlimited in the direction of their increase.

2. **Cumulus**.—A cloud which increases from above, in dense, convex, or conical heaps.

3. **Stratus**.—An extended, continuous, level sheet of cloud, increasing from beneath.

4. **Cirro-Cumulus**.—A connected system of small roundish clouds, placed in close order or contact.

5. **Cirro-Stratus**.—An horizontal or slightly inclined sheet, attenuated at its circumference, concave downwards, or undulated. Groups having these characters.

6. **Cumulo-Stratus**.—A cloud in which the cumulus is mixed with cirro-stratus or cirro-cumulus; the cumulus flattened at top, and overhanging its base.

7. **Nimbus**.—A dense cloud, spreading out into a crown of cirrus, and passing beneath into a shower.

The cirrus is generally formed of white radiated streaks, pencilled on an azure sky. It is the most elevated species of cloud. Its motions are slow. It generally indicates a breeze, and often precedes a storm. Horizontal sheets of cirrus, with streamers pointing upwards, often indicate rain; while the depending fringes are the precursors of fine weather.

The cumulus is generally a dense cloud, moving near the surface of the earth. When large, this species often exhibits large intervening spaces of clear sky. It begins in the morning, and obtains its maximum about two p.m., and usually decreases before sunset, breaking up and disappearing before nightfall. It is the prognostic of settled weather.

The stratus is the lowest modification of cloud, often creeping along the ground, in calm evenings, from lakes and rivers, and rising toward the higher grounds. It often, at night, travels over plains, invests the summits of moderate elevations, but usually melts away before the morn-

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1 Philosophical Magazine; Rees' Cyclopædia; Nicholson's Journal, xxx. Cirro-cumulus consists of dense rounded masses, in close position. It often forms beds at different altitudes in the air. It is transient in winter, and occurs in the intervals of summer showers. A frequent repetition of it is amongst the best prognostics of fine weather.

The cirro-stratus is narrow in proportion to its horizontal length. It seldom retains long the same form. It is often seen very high in the air, especially that variety which is composed of grouped masses, and is chiefly seen in summer evenings. When seen overhead, it has an uniform hazy appearance; but viewed on the horizon, it often seems very dense, from being viewed edgewise. It indicates rain or storms of snow when it is stationary. It often forms the seat of rainbows and halos when descending from the zenith. It often shrouds mountain summits, and in cold weather often descends into plains as soaking dense mist. The cumulo-stratus usually appears in an overcast sky, and may be regarded as a prognostic of rain. It appears to be formed by the successive blending of cumuli, inclosed by streaks of dark stratus. The thunder-cloud is one modification of this species. It often assumes a portentous size, and is contorted into singular forms. Its indications are not uniform. When it appears in the morning, the day is often dry, though overcast; but when much mixed with stratus, it indicates approaching rain, after some interval.

The nimbus is a shower seen in profile as it approaches the spectator from the horizon. The thunder-cloud, on the discharge of its electricity, generally passes into the nimbus.

Mists or Fogs are but the condensation of invisible vapour into minute drops of water. They are generally more stationary on the sea than on the land. They are less frequent in the day than the night, in hot than in cold climates; and they are frequent in approaching the poles. One of the most remarkable stationary fogs is on that part of the Atlantic called the Banks of Newfoundland. The warm waters of the gulf-stream, after sweeping along the shores of the United States, and warming the superincumbent air, are encountered at Newfoundland by the polar currents, and the colder air over them. The consequence of this intermixture of the aerial currents at different temperatures, is the diminished capacity of the mixture for moisture, and the consequent deposition of a portion of its vapour in the form of mist. When the cold producing condensation of atmospheric moisture is intense, instead of mist, a deposition of minute particles of ice may take place, and will cover with delicate crystallizations the hair and clothes of the traveller, or the twigs and branches of trees. When the air is saturated with moisture, sudden condensation assumes the form of rain.

Rain.—Amongst the numerous speculations on the production of rain, one of the most ingenious is that proposed by Dr James Hutton. He evidently considered the water as dissolved by the air; but if we merely consider the term "complete saturation with moisture" as meaning the dew-point, and the quantity suspended by air as indicating the quantity of vapour existing in the atmosphere at a given temperature by its own elasticity, this phraseology will produce no error. Hutton ascribed rain to the intermingling of air saturated with moisture at different temperatures. His views were communicated to the Royal Society of Edinburgh in 1787; but, from his contenting himself with a mere enunciation of his theory, it has been scarcely noticed by meteorologists, until illustrated in the articles Meteorology and Hygrometry. Sir John Leslie has shown, that the mere commixture of air at different temperatures would produce but a small part of the effect perceived. It is proved by experiment that the capacity of air for moisture, or the quantity of vapour in a given space, increases in a much higher ratio than the temperature; and hence, if the space occupied by the mixture of masses of moist air, at different temperatures, have the mean temperature of the mixture, or one below it, a deposition of moisture or of rain will follow; but the whole effect would be smaller than what is often observed to take place, unless we suppose that two currents of air are moving, with considerable velocity, in opposite directions.

The objection to this simple view of the question is, that there are often heavy falls of rain when the clouds emitting it are evidently in very slow movement; and we have long believed, that without calling in the aid of electricity, as already stated when treating of the formation of clouds, no theory of rain will be satisfactory.

The hygrometric water in the air seems to owe its suspension to the repulsions among the electric atmospheres of its particles: when that electricity is withdrawn by any cause, the particles of water, no longer kept asunder by their electricity, coalesce, and descend by their gravity with a force proportional to their quantity and the reduction of their electric tension. If the electric fluid be slowly withdrawn, there is neither lightning nor any explosion; but if the accumulation has been great, and the abstraction sudden, we have the phenomena of thunder and lightning.

On this view, evaporation is to be considered as the grand mode by which the atmosphere acquires electricity, and the accumulation seems in general in proportion to the intensity and duration of the evaporation. Hence lightning is most common and vivid in warm latitudes; and there is seldom any considerable electric discharge from the atmosphere, without a concomitant fall of rain, which is usually proportional to the violence of the thunder-storm.

Rain falls in drops of a size varying from $\frac{1}{4}$th to $\frac{3}{4}$d of an inch in diameter; and it has been calculated that their ultimate velocity is in the duplicate ratio of their diameters. When formed high in the atmosphere, or passing through very cold strata of air, they congeal into hail or into snow.

To ascertain the quantity of rain which falls in any place in a given time, various species of ombrometers or rain-gages have been employed. The most usual form of the gage is that represented in No. 21. It consists of a cylindrical vessel of copper, usually $\frac{3}{75}$ of an inch in diameter. Upon its upper part, a funnel of the same metal, with an upright brim exactly one foot in diameter, is fitted; a hollow vessel of metal, or a piece of cork, serves as a float, to support a rod of boxwood graduated in tenths of an inch. The water which falls into the funnel finds its way into the cylinder, and raises the float. The quantity is read off the rod at a stay placed across the funnel, and from the relative proportion of the cylinder and funnel, each $\frac{1}{100}$th of an inch is equivalent to $\frac{1}{100}$th of an inch of water entering the mouth of the funnel.

Another very convenient form is represented in No. 22, where the height of the column of water in the cylinder is seen in a glass tube connected with the cylinder below. A scale is applied, as in the figure; and when the observation is made, the water is let off by the stop-cock.

Mr Luke Howard's rain-gage is seen in No. 23. The funnel is fitted on the neck of a glass bottle, by means of an exterior tube, which is soldered to the funnel, and prevents the entrance of water into the vessel, except through the mouth of the funnel. The quantity of rain is ascertained by a graduated glass measure, the divisions of which correspond to 100ths of an inch, falling on the mouth of the funnel. This same measure will serve to ascertain the quantity evaporated in a given time, provided the mouth of the vessel, No. 20, is exactly equal to the mouth of the rain-gage.

There are considerable practical difficulties in the proper position of a rain-gage. It is found, that if one be placed on the ground, and another on the top of a building, that which is in the lowest position will almost always indicate more rain than the upper. Thus, the quantity collected in a gage on the top of York Minster, from February 1833 to February 1834, only equals 14-968 inches; while perfectly similar instruments placed on the top of the museum of that city, and on the ground, gave relatively 19-852, and 25-706 inches. The elevations of the first and second stations are 212 feet 10½ inches and 43 feet 8 inches above the surface. These differences are too considerable to be attributed to any thing but some imperfection in the instrument when much exposed to gales of wind; and it probably arises from eddies being formed round the rim of the funnel, which divert part of the water. On the whole, the most accurate estimate of the quantity of rain will be obtained from instruments placed on the ground, or near it. The larger the recipient surface, the more accurate will be the indications; and on this principle, General Sir Thomas Brisbane, president of the Royal Society of Edinburgh, collects the rain from the roof of his observatory (the area of which has been accurately ascertained), for measuring the quantity of rain which falls in his vicinity.

When the quantity of rain which falls at any place has been regularly kept for several years, it is easy to obtain the mean annual quantity at that place. The multiplication of such observations has proved, that the mean annual quantity is greatest in tropical climates, and diminishes as we recede from the equator. Thus, the mean annual rain at

| Location | Quantity | |-------------------|----------| | Island of Granada, lat. 12° | 126 | | Kingston, Jamaica, lat. 18° | 83 | | Calcutta, India, lat. 22° 23' | 81 | | Rome, lat. 41° 54' | 39 | | Liverpool, lat. 53° 22' | 33 | | Edinburgh, lat. 55° 57' | 24 | | Petersburg, lat. 59° 56' | 16 |

In general the number of rainy days is least where the fall of rain is the greatest, and the dampness of any climate must not be inferred from the annual quantity of rain.

A country may be involved in almost perpetual fogs, or have many days of drizzling rain, in which the number of inches of rain throughout the year may be very small.

The quantity of rain, however, depends upon other circumstances than the latitude. It is much influenced by the vicinity of mountains in every climate. Thus,

| Location | Quantity | |-------------------|----------| | At Kingston, on the shores of Jamaica, it... | 83 | | In Carib country, among the mountains there... | 97 | | In Demerary, in the swamps of Guyana... | 73 | | In the lofty island of Granada... | 126 |

Yet in Demerary as much as six inches of rain have been observed to fall in twelve hours. Similar observations occur in our own country.

| Location | Quantity | |-------------------|----------| | At Carlisle the annual rain... | 34 | | At Whitehaven... | 52 | | At Kendal... | 56 | | At Langside, near Glasgow... | 37 | | At Carbally, in Stirlingshire... | 46 | | At Ardincaple... | 50 | | At Lochlomond... | 52 | | At Castle Toward, Argyleshire... | 56 |

In Great Britain, it is generally found that much more rain falls on the western than on the eastern coasts. Thus,

| Location | Quantity | |-------------------|----------| | At Edinburgh the rain... | 34 | | At Elgin... | 24 | | At Garnet Hill, near Glasgow... | 32 | | At Greenock... | 39 |

The latter town lies in a sheltered nook, or perhaps its annual quantity would have been higher; for,

| Location | Quantity | |-------------------|----------| | At Catrine House, in Ayrshire, it... | 45 | | And at Lanfine House, ditto... | 52 |

It is scarcely necessary here to dwell on various extraordinary substances which have sometimes fallen from the air. Showers of sulphur have been mentioned by different observers. Wormius records such a shower as occurring at Copenhagen in May 1646; but the same phenomenon occurred in May 1804; and the philosophers of our own age, on analyzing this deposit, recognised it to consist of a vegetable pollen, resembling the powder of lycopodium. This substance fell in the night time, and, being phosphorescent when observed, gave considerable alarm. At Bordeaux, in 1761, a shower of a yellow powder was observed, which, however, was recognised as the pollen of the pine forests in the neighbouring landes, carried up into the air by a violent gale. Of a similar nature, in all probability, was the yellow rain which covered the Lake of Zurich in 1677.

The supposed showers of blood which have been recorded in some ancient chronicles are now ascribed to the same source as the red snow of Greenland and the Alps, that is, to the red globules or seeds of the uredi natalis; or to minute red insects, which are sometimes carried down in great quantity with rain. The appearance of drops of blood falling from the air has occasionally been produced by the red excrement of insects. That small frogs and fishes have occasionally descended with rain has been admitted, as such animals, and even much more ponderous matter, have been elevated into the atmosphere by whirlwinds.

The showers of stones which have been noticed by several ancient writers as happening in Italy, Greece, and Asia Minor, are now believed to belong to the class of facts described under the article Meteorolite. These are mi-

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1 See also Chladni, Mémoire, in Journal de Physique; Howard, in Phil. Trans., 1802; Bibliothèque Britannique, 1803; Nicholson's Journal, 1803, 1805, 1809, 1810; Annales du Muséum d'Histoire Naturelle, 1803; Izard's Lithologie Atmosphérique, Edin. Phil. Journal, vol. i. The phenomena and cause of this deposition have engaged the attention of philosophers from the time of Aristotle; but their speculations present little that is satisfactory until the experimental investigations of Dr Wells. One of the most remarkable circumstances attending the deposition of dew is, that the bodies on which it is found have invariably a lower temperature than the ambient air, as was first pointed out by Dr Patrick Wilson of Glasgow, who ascribed the coldness to the deposition of dew; but Dr Wells has proved that the cooling of these bodies below the temperature of the air always precedes the formation of the dew, and is, in fact, the cause of this aqueous meteor. His theory is founded on Leslie's view of the heat which is sent off from the earth toward a clear sky; and on this simple principle Wells has been enabled to give a satisfactory explanation of the phenomena of dew.

The essential requisites for the deposition of dew are five in number, viz:

1. An atmosphere replete with moisture. As dew is a deposition of water from the air, it obviously must be in excess ere it is deposited. In Egypt, the south winds, blowing over extensive tracts of sandy desert, deposit no dew; but when the Etesian or north wind blows, loaded with moisture from the Mediterranean, the deposition of dew in lower Egypt is sufficient to soak through the garments of the inhabitants; and in the arid plains of New Castile the dews are far less copious than on the coasts of Andalusia.

2. There must be a considerable difference between the temperature of the earth in the night and the day. Other things being the same, the deposition is greatest when a cool evening succeeds a sultry day. In our climate, the dews are most abundant in spring and autumn, when the difference of temperature is greatest; but as the ratio of the increase of moisture in the air is greater than the increase of heat, hot climates have more copious dews than Great Britain, even though the difference between diurnal and nocturnal temperature be less in the former. Thus we have found the dews in the south of Spain, and in Morocco, far more copious than here; a short exposure about sunset in those countries completely soaking one through the clothes.

3. But the most essential requisite for the deposition of dew is, that the bodies upon which it forms should have their temperature considerably lower than that of the ambient air. In repeating Wilson's experiments, Dr Wells found that all bodies on which dew formed were colder than the air by $10^\circ$ or $15^\circ$. Thus, in clear weather, he observed that a thermometer laid on grass was $12^\circ$ lower than one laid on mould, and sixteen and a half below one on a gravel walk. He always found that the coldness of the body preceded the deposition of dew; and this is illustrated by the deposition of moisture which immediately takes place on pouring into a glass cylinder, placed in a warm, moist room, very cold water.

4. A serene and cloudless sky is also very favourable to the formation of dew. Wells found, that although the atmosphere be in other respects favourable to the deposition, yet little or none is formed if the sky be veiled in clouds; and, in such cases, the temperature of objects on the surface was near that of the air. He points out how clouds act as a screen to intercept the emanations between the earth and the upper regions of the air, which are well known to be the abodes of perpetual congelation.

The same effect was produced by interposing screens of an opake material between the sky and the surface of the earth. Thus a thermometer, laid on a table, indicated a lower temperature than one placed on the ground under the table; and a lock of wool laid on the table collected ten grains of dew, whilst an equal weight of wool placed under the table gained only two grains. Fogs, acting also as screens, intercept the influence of the sky; and though in themselves moist, they are unfavourable to the deposition of true dew, which may be regarded as water separated from the inferior stratum of air; but fogs, like rain, are precipitated from the general mass of the superincumbent air.

The effect of clouds in preventing the radiation of heat is beautifully illustrated by carrying Professor Leslie's elegant invention, the Æthroscope, into the open air. Every cloud that passes the zenith affects this delicate instrument, which rapidly loses its heat when exposed to the clear sky; or the effect is still more conspicuous if the operator hold a parasol, and alternately extend the æthroscope beyond the shade and withdraw it under the parasol.

5. Serene and calm weather is favourable to the formation of dew; a fact which had been noticed by Aristotle. The influence of wind seems to depend on the rapid succession of aerial particles preventing the full influence of the cooling medium on the general temperature of an inferior stratum of the atmosphere. Every circumstance which favours radiation is favourable to the deposition of dew, such as a rough surface, and dark colour; proving the dependence of dew on the cooling of the objects on which it is deposited.

Dew often partakes of the sensible qualities of the honey and dies upon which it is found; hence dew-water is very aptly dew to putrefy. Some substances found on plants have been erroneously confounded with dew. What is termed honey-dew generally owes its qualities to the saccharine exudation from the bodies of the insects called Aphides. The jelly-dew is believed to be the original form of a cryptogamic vegetable production, the Tremella nostoc of Linnæus; a membranous, pellucid, greenish-yellow matter, about one or two inches in width, which is at first moist and soft to the touch, but dries into a blackish membrane.

Snow.—When the congelation of moisture takes place slowly, the flakes of snow assume very symmetrical figures, the varieties of which have been well represented in the work of Scoresby. They usually present modifications or combinations of the hexangular prisms, often consisting of a star of six rays, formed of prisms united at angles of $60^\circ$; and from these other prisms shoot, at similar angles, giving the whole a plumose appearance, of exquisite beauty, and often of surprising regularity.

The quantity of snow which falls in any place is not proportional to the height of its latitude. In many temperate countries, the successive falls of snow, during a winter, may often greatly exceed in depth what falls in the same period in countries between the 65th and 80th degrees of latitude. In fact, it bears some proportion to the evaporation in countries where the winter temperature is below the freezing point of water. Snow is extremely useful, in cold climates, as a loose and spongy covering of a white colour, in preventing the loss of terrestrial heat by radiation, and in defending the surface from the frigorific influence of very cold winds.

Hail differs from snow chiefly in its state of aggregation, and its far less regular crystalline form. It appears, indeed, to be the production of sudden and intense cold in the Physical higher regions of the atmosphere, and therefore often occur in warm climates, where snow is utterly unknown. A hail shower is often ushered in by a rattling noise, heard over head, before almost any hailstones have reached the ground. The ordinary size of hailstones with us varies from \( \frac{1}{10} \)th to \( \frac{1}{3} \)d of an inch in diameter. Occasionally they are much larger; as in the remarkable hail-storm described by Dr Neill, which occurred during a thunder-storm in the Orkneys on the 24th of July 1818. Mingled with ordinary hail were enormous masses of ice, some as large as the egg of a goose; by which animals were killed, and individuals wounded. An enormous hailstone is recorded to have fallen, amongst many very large masses, at Handsworth House, near Birmingham, during a thunder-storm, on the 8th of June 1811. It consisted of a cuboidal mass 6\( \frac{1}{2} \) inches in diameter; and, from the engraved sketch, it resembled a congeries of frozen balls about the size of walnuts. On the Continent of Europe, the large hail of summer is frequently exceedingly destructive to vineyards; and in some districts of France, where the damage has often been great from thunder-storms, accompanied by large hailstones, the ingenious device of the Paratonomerie has been employed to prevent the accumulation of electricity over these valleys. The contrivance consists in the erection of pointed electric conductors on the surrounding hills, by which the lightning is silently and gradually abstracted from the air, and dangerous accumulations over the included districts generally prevented. In hot climates, the descent of large masses of frozen water is still more common than in Europe, especially within the Calms off the western coast of Africa.

Winds.—The influence of mechanical causes in producing those aerial currents termed winds has been generally admitted, from the ages when they were poetically imagined as viewless forms, imprisoned or let loose from the caverns of Æolus, to the present day, when they are considered as the consequences of partial changes in the density of the atmosphere, arising from the unequal distribution of heat over the surface of the globe, or from alterations on its hygrometric and electric states. But the manner in which their force and direction are affected by these circumstances, in those sudden and local storms which frequently agitate the air, is difficult of explanation, although the general phenomena may admit of a very probable solution.

The dependence of the weather on the direction and force of the winds has given rise to many inventions for indicating their variations. The direction is shown by the well-known vane, or simple anemoscope, the movements of which are referred to the cardinal points by a fixed index, or by a plate marked with the rhumbs of the mariner's compass. To save the trouble of frequent daily observations, the labour of the meteorologist may be materially abridged by the following methods of obtaining maxima and minima of the direction of the winds.

In the first, No. 24, the vane b, fixed to the axis a, causes the latter to revolve in a socket, in the centre of a writing slate marked with the rhumbs of the compass. The arm c, jointed to it, carries a pencil d, which records the progress of the vane on the slate. The pressure of the pencil may be regulated by sliding the ball e on the arm; and, to diminish friction, the socket consists of a flint, with a smooth hollow cavity, and the axis is kept perpendicular by moving between friction rollers.

In the second, No. 25, the same effect is shown on a vertical dial, by converting the horizontal motion of the vane into a vertical movement of the index. This is produced by one bevelled wheel e on the axis of the vane, and a similar one f on the axis of the index. The maxima and minima are shown by two moveable arms gg, concentric with the index, but pushed to either side by a stud on the lower surface of the index. These instruments, devised long ago by Trail, and published in Professor Jameson's Journal for 1837, sufficiently answer the object for which they were intended.

The varying force of the wind is ascertained by the anemometer. The principle of several such instruments is the direct pressure of the wind on a given surface, kept by a vane perpendicular to the direction of the breeze. Sometimes this pressure is measured by raising a weight; in some instruments by the degree of compression of a spiral spring; or, as in the ingenious anemometer of Mr Waddell of Leith, by the compression of a slender cylindrical bag filled with quicksilver, and connected with a vertical tube one foot in length, with a bore of \( \frac{1}{20} \)th of an inch. The force of the wind is estimated by the height of the column of mercury. A practical difficulty arises from the quickness of the oscillations, which renders it not easy to read off the indications of the instrument during a smart gale.

The same objection in some measure applies to the elegant pocket anemometer of Dr Lind. It consists of two equal glass tubes, united below by a narrow connecting piece, as in No. 26, and having between them a scale of equal parts. The tubes are filled to the middle with water; and as the instrument is made to revolve freely round the iron spindle a, its body acts as a vane, to keep the orifice of the kned brass tube always directed to the wind. The breeze received at that aperture depresses the liquid in that limb, and raises it in the other to a certain point on the scale, according to the force of the breeze. When it is very strong, the detached piece of brass b is joined to the top, and an allowance made for the effect on the column. The sum of the two columns is the height of a column of water which the wind is capable of sustaining; and every body opposed to that wind will be pressed on with a force equal to the weight of a column of water of that height, and with a base equal to that of the surface exposed to the wind. In the original instrument the bore of both legs was \( \frac{1}{20} \)ths of an inch, the connecting tube was \( \frac{1}{10} \)ths, and the length of the whole was six inches. Lind gave the following table as its indications.

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1 Edinburgh Philosophical Transactions, vol. ix. This instrument is, upon the whole, the most convenient anemometer we possess, and has the advantage of portability. The oscillations of the liquid might be farther checked by twisting spirally the tube of communication, as in the marine barometer.

A very ingenious anemometer has been constructed by Mr Richard Adie, late of Edinburgh, now of Liverpool. An index E is moved by the rising and falling of a cylinder A, closed at one end, and inverted in a larger cylinder G, containing water, on the principle of a gasometer for chemical experiments. The inverted vessel is suspended by a cord passing over a pulley B fixed on the axis C of the index. The counterpoise is a weight W, the cord of which, wound on a fusee or snail D, equalizes the counterpoise in the different positions of the internal cylinder. The influence of the wind on the instrument is produced through a tube, as in the figure. One end terminates in the interior cylinder, and the other in a kned tube moving freely round by means of the attached vane. A very gentle breeze is sufficient to elevate the cylinder, and the revolution of the index marks the comparative force of different winds. This anemometer has been found to act well, and has been put up at several of the northern lighthouses.

The last anemometer we shall notice is that invented by Professor Whewell, who exhibited it to the British Association. It consists of a small wind-wheel of eight vanes, directed to the wind by an attached vane. By a train of Geography-toothed wheels and endless screws, the rapid revolutions of the wheel are converted into a slow vertical motion, by which a pencil is made to trace a downward line on the surface of a vertical cylinder having the same axis as the vane. The length of this line records the whole amount of the aerial current that passes over the place of observation, and the part of the circumference of the cylinder on which the trace appears indicates the direction of the winds. Three of these instruments are, by direction of the Association, on trial in different places, and the results will be communicated in the present year (1838).

In Smeaton's valuable paper upon the moving power of wind and water (Philosophical Transactions, vol. li.) there is the following table of the force of the wind, calculated by his friend Mr Rouse; which the celebrated engineer considers as entitled to confidence, especially in velocities below fifty miles per hour.

| Miles per Hour | Feet per Second | Perpendicular Force on an area of One Foot, in lbs. avoird. | Common Designations of the Winds | |----------------|-----------------|----------------------------------------------------------|--------------------------------| | 1 | 1-47 | 0-005 | Hardly perceptible. | | 2 | 2-93 | 0-020 | Just perceptible. | | 3 | 4-40 | 0-044 | Gentle breezes. | | 4 | 5-87 | 0-079 | Pleasant brisk gales. | | 5 | 7-33 | 0-123 | Very brisk gales. | | 10 | 14-67 | 0-492 | High winds. | | 15 | 22-00 | 1-107 | Very high winds. | | 20 | 29-34 | 1-968 | A storm or tempest. | | 25 | 36-67 | 3-075 | A great storm. | | 30 | 44-01 | 4-429 | A hurricane. | | 35 | 51-34 | 6-027 | | | 40 | 58-68 | 7-973 | | | 45 | 66-01 | 9-963 | | | 50 | 73-35 | 12-900 | | | 60 | 88-02 | 17-715 | | | 80 | 117-35 | 31-490 | | | 100 | 146-70 | 49-200 | |

The third column is calculated according to the squares of the velocities of the wind, which Smeaton considers as generally correct.

Trade-winds.—In those portions of the Atlantic and the Pacific Oceans between the tropics, the wind has, throughout the year, a direction from the east. The following explanation agrees pretty well with the facts. The dense cool air of the polar and temperate regions naturally tends to displace the lighter warm air over intertropical regions, which, heated by the sun, produce vertical movements in the superincumbent atmosphere. Hence the ascending current of warm air between the tropics has its place supplied by the denser air from either side; and thus two currents are set in motion, from the poles towards the tropical regions. The velocity of every point on the earth's surface increases from the poles to the equator. But as the particles of these currents cannot at once acquire a velocity equal to the continually accelerating velocity of the parts of the earth's surface over which they arrive in this transit, the northern and southern currents will gradually seem to acquire a motion in an opposite direction to that of the rotation of the earth, that is, both will gradually decline to the west, assuming in the northern hemisphere the character of a N. E., and on the south side of the line of a S. E. wind; whilst both will become more easterly as they approach the equator. This effect of the augmenting velocity of the earth's surface, in approaching the equinoctial line, is increased by the continual movement of the point over... which the sun is vertical, and consequently his heat the greatest, est, to the west; and the effect of the gravitation of the sun and moon on the atmosphere, as was shown by D'Alembert, must directly tend to increase the force of the easterly wind. The influence of these luminaries on our atmosphere is corroborated by the observations of Bacon, Halley, and Gassendi, on the frequency of storms about the equinoxes, or at full and new moon, and the general occurrence, in calm weather, of light airs of wind at the time of high water. When these causes are not counteracted by the superior rarefaction of air over land heated by the sun's rays, the easterly winds blow with much regularity, as in the great oceans; and, from their important influence on navigation, they have obtained the denomination of trade-winds. They extend to about 30° on each side of the equator; but, from the less density of the atmosphere in the northern hemisphere, arising from the greater quantity of land to be acted on by the sun's rays in that half of the globe, the line which marks the blending of the S.E. and N.E. trade-winds is between two and three degrees north of the equator.

If the surface of the globe had been one expanse of water, the trade-winds would have blown all round the equator, and the only other winds known would have probably been N. and S. winds near the poles, gradually bending from E. to W. as they approached the tropics. But the air over the land being much more heated by the sun's rays than that over the sea, the vicinity of large islands or of continents causes interruptions to the regularity of the trade-winds. Thus, on the coasts of Africa, within the regular limits of the trade-winds, westerly gales are not unknown; and a southerly wind prevails along the western shores of that continent, from Cape Palmas to the Cape of Good Hope; a consequence of the form and climate of Northern Africa, which continues the southern polar current far beyond its limits in the free ocean.

A similar cause appears to give rise to the Calms on that coast, from 10° to 42° N., a tract avoided by seamen, on account of these calms, and sudden alternations, with tremendous storms of thunder and lightning, to which it is subject. The general direction of the trade-winds is affected by their approach to the shores of America; they are expanded, chiefly owing to the resistance they meet with from the land, and also by the additional rarefaction of the air re-acting upon that continent. Precisely similar disturbances from the position of land produce

Monsoons.

The Monsoons of the Indian Ocean. The eastern trade-wind blows regularly in that sea between the parallels of 10° and 30° S.; but from these parallels northwards, or on the sea between Sumatra and Eastern Africa, the course of the trade-winds is reversed for half the year, from April to October, during which period there is a constant wind from the S.W. During the other six months the regular trade-wind, from the N.E., resumes its course. These winds are termed the S.W. and N.E. monsoons; from a Malayan word signifying seasons. The latter is produced by the same causes as the general trade-wind. The other appears to be occasioned by the great rarefaction of the atmosphere over the extensive regions of Eastern Asia when the sun is north of the equator, and the dense air of the Indian Ocean rushes in to occupy the place of the ascending current.

This disturbance of the trade-wind is attended with very remarkable effects on the climate of the country where it prevails. It is the harbinger of the rainy season in India, which is ushered in by violent thunder-storms. A most striking picture of this change of monsoon is given by Mr Elphinstone in his Account of Caubul. "The most remarkable rainy season is that called in India the S.W. monsoon. It extends from Africa to the Malayan peninsula, Georgia, and deluges all the intermediate countries, within certain lines of latitude, for four months in the year. In the south of India this monsoon commences about the beginning of June; but it gets later as we advance towards the north. Its approach is announced by vast masses of clouds, that rise from the Indian Ocean, and advance towards the N.E., gathering and thickening as they approach the land. After some threatening days, the sky assumes a troubled appearance in the evening, and the monsoon in general sets in during the night. It is attended by such a thunderstorm as can hardly be imagined by those who have only seen that phenomenon in a temperate climate. It generally begins with violent blasts of wind, which are succeeded by floods of rain. For some hours lightning is seen almost without intermission; sometimes it only illuminates the sky, and shows the clouds near the horizon; at other times it discovers the distant hills, and again leaves all in darkness; when, in an instant, it re-appears in vivid and successive flashes, and exhibits the nearest objects in the brightness of day. During all this time the distant thunder never ceases to roll, and is only silenced by some nearer peal, which bursts on the ear with such a sudden and tremendous crash as can scarcely fail to strike the most insensible heart with awe." (Malabar is the province most distinguished for the violence of the monsoon.) At length the thunder ceases; and nothing is heard but the continued pouring of the rain, and the rushing of rising streams. The next day presents a gloomy spectacle; the rain still descends in torrents, and scarcely allows a view of the blackened fields; the rivers are swollen and discoloured, and sweep down along with them the hedges, the huts, and the remains of the cultivation which was carried on during the dry season, in their beds.

"This lasts for some days, after which the sky clears, and discovers the face of nature changed, as if by enchantment. Before the storm, the fields were parched up, and except in the beds of the rivers, scarce a blade of vegetation was to be seen. The clearness of the sky was not interrupted by a single cloud, but the atmosphere was loaded with dust; a parching wind blew, like a blast from a furnace, and heated wood, iron, and every solid material, even in the shade; and immediately before the monsoon this wind had been succeeded by still more sultry calms. But when the first violence of the monsoon is over, the whole earth is covered with a sudden but luxuriant verdure: the rivers are full and tranquil; the air pure and delicious; and the sky is varied and embellished with clouds. The effect of this change is visible on all the miniature creation, and can only be imagined in Europe by supposing the depth of a dreary winter to start at once into all the freshness and brilliancy of spring. From that time the rain falls at intervals for about a month, when it comes on again with great violence, and in July the rains are at their height; during the third month they rather diminish, but are still heavy; and in September they gradually abate, and are often suspended till near the end of the month, when they depart amid thunders and tempests, as they came."

Sea and land breezes are occasioned by the same causes as which produce the S.W. monsoon, and are chiefly experienced on tropical sea-coasts.

The sea-breeze commences about 10 A.M., and continues throughout the day, till towards 6 P.M., when it gradually dies away, and about eight in the evening is succeeded by a much fainter breeze from the land, especially in those islands which have considerable mountains. It is well known in Jamaica, but less so in the smaller Antilles, and scarcely at all on the flat coasts of America. It continues during the night, and usually dies away before six or seven in the morning; but it is much less regular in its periods than the sea-breeze. The latter, too, occurs in places where the land-breeze is quite unknown. An admirable description of these is contained in the voyages of Dampier, and given at length in the article Meteorology. The sea-breeze is more or less felt on the coasts of all warm countries. We have had occasion to observe with what regularity the sea-breezes set in from the S.W., during the summer at Gibraltar; and it blows with considerable force throughout the greatest part of the day.

The explanation of the sea-breeze is the rarefaction of the air by the heat of the land, and the rushing in of denser air from the sea to supply its place; and as the influence of the sun decreases, this breeze dies away. The land-breeze is chiefly produced by the descent of air from mountainous regions, flowing by its gravity, towards the sea, over which the atmosphere is less liable to suffer extreme rarefaction and condensation than the air over the land.

In temperate climates the winds are far less regular than in tropical countries. The general causes which produce them act with less force and constancy; and the disturbing causes, such as the vicinity of land, alteration in density from moisture and dryness, act with more uncontrolled energy. The most prevalent winds in Great Britain are from the S.W., especially on the western side of our island. The Royal Society registers give the following average for the winds throughout the year at London:

| Direction | Days | |-----------|------| | S.W. | 112 | | N.E. | 58 | | W. | 53 | | N.W. | 50 |

Or, with Professor Playfair, if we group the S. with the westerly, and the N. with the easterly winds, we shall have the following results, from the Edinburgh register of Playfair, and the Glasgow register of Dr Meek:

| Direction | Days | |-----------|------| | Westerly, Easterly | 365 | | London | 233 + 132 = 365 | | Edinburgh | 242 + 123 = 365 | | Glasgow | 314 + 51 = 365 |

These determinations of the direction of the winds refer to the surface of the earth; but there can be no doubt that the direction of the aerial currents is often different in the upper regions of the air. The permanence of very elevated clouds, their occasional movements in directions opposite to those of the winds below, and the information obtained from aeronauts, all prove this point.

There are several remarkable modifications of the wind, which deserve notice in this place.

The Sirocco of Italy and Sicily is a S.E. or S. wind, which, heated on the sandy wastes of Arabia and Libya, becomes occasionally moist in its passage across the Mediterranean, and oppresses the inhabitants of Italy, Malta, and Sicily with excessive languor, and a sinking of their mental energies. When it sets in, it causes a very sudden and powerful rise of the thermometer, and is accompanied by a haze which obscures the pure sky of those southern countries; the sun appearing dimmed and shorn of his beams.

The Solano of Spain is only a modification of the Sirocco. We have experienced it as most oppressive on the eastern shores of Spain; and it is greatly detested by the natives, who gravely remark, "no animals except a pig and an Englishman are insensible to the Solano." The Italian condemnation of a stupid work, "era scritto in tempo del Sirocco," is not more pointed than the Spanish Physical adage, "no rogar alguna gracia en tiempo de Solano," not Geography, to ask any favour during the Solano; and both proverbs sufficiently indicate the belief of the people of southern Europe in the disagreeable qualities of the S.E. wind. We have observed fine dust deposited from the Solano or Leveater at Gibraltar, and to this we partly attribute the haze with which it is accompanied. If this dust be brought with the Solano from Africa, it is less surprising than the following instance of dust carried by the easterly wind into the Atlantic. On the morning of the 19th January 1826, when the Clyde East Indian was on her voyage to London, in latitude 10° 40' N., longitude 27° 41' W., her rigging was observed to be covered with an impalpable powder of a brownish colour, and on unfurling the sails at two p.m. to catch the breeze, they emitted clouds of dust, which had lodged in them during a strong gale from the E. and N.E. In this case, the nearest land in that direction was about seven hundred miles distant.

The Khamsin of Syria, the Somiel of the Turks, and the Si-Khamsin of the Arabs, is well described by Volney. It is a wind in or Simun, no degree poisonous, except in as far as it is dangerous from its extreme heat and acidity; qualities it derives from blowing over sandy deserts intensely heated by the sun. Denon states, that it is loaded with impalpable sand, which penetrates into the closest packages. These winds derive their Syrian name from their prevalence during the fifty days about the equinoxes. They produce difficult respiration, a shrivelled skin, and a distressing sense of heat. Volney compares the effect of them to the sensation produced by the hot air from a baker's oven. They sometimes blow in squalls, and in Egypt usually continue for about three days at a time. In that period vegetation is withered, the inhabitants shut themselves up in their houses or their tents; and the Simun is dangerous to persons of a full habit, from the sudden entrance of this heated and parched air into the lungs. In the sandy deserts it is greatly dreaded; but fortunately its extent is not considerable, and it seldom lasts long. It has been well described by Chardin, Sir John Malcolm, and Badia.

The Harmattan of the west coast of Africa is of the same kind, and produced by the same causes. The extreme dryness of this wind has been mentioned by all observers. It usually blows from the Great Sahara towards the coast, from the N.E., and, like the Solano, Sirocco, and Simun, is loaded with dust. The dust which fell on the vessel just mentioned was from a violent Harmattan. The only difference between the Sirocco and these other winds appears to be, that the Sirocco obtains some moisture in crossing the Mediterranean; but they all seem to be mere modifications of aerial currents produced by the same causes.

The Tornados, Typhons of the Chinese Seas, the Ox-eye Tornado, of the Cape, and the Hurricanes of the West Indies, are &c., violent and extraordinary agitations of the air, generally accompanied by thunder-storms, and usually of short duration. The destructive violence of the three first has been well described by several navigators; and of the hurricane which desolated Barbadoes in the year 1780 we have a short but good account by Sir B. Blanc.

The name of Whirlwind has been given to eddying currents, seemingly depending on electricity, which sometimes wind rage with surprising fury in particular spots, tearing up trees by the roots, and overthrowing buildings. The extent of the whirlwind is usually confined; and as it passes through a forest it has been observed to form a lane or long track of inconsiderable breadth. The singular columns of moving sand, described by Bruce as occurring in Africa, appear to have been the effect of whirlwinds, or probably analo-

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1 For an account of the Harmattan, see Norris in Philosophical Transactions, lxxi.; and Lind on Hot Climates. 2 Edinburgh Philosophical Transactions, vol. i. gous to the meteor termed a Waterspout, which seems to be decidedly an electrical phenomenon.

A curious account of waterspouts is contained in Michaud's Memoire in the Transactions of the Academy of Turin, which has been translated in Nicholson's Journal (vol. i. 4to series). The conical pillar would appear to consist of condensed vapour, which often has the deep indigo-blue tint of the cloud from which it proceeds. As this column descends to the sea, the spot below it becomes agitated, and, according to some observers, the waters are bodily drawn up towards the cloud. But this opinion seems to be doubtful; for it is stated that the water discharged on the bursting of a waterspout is always fresh, and therefore any water derived from the sea must have passed into the air in the form of vapour. Besides, they generally dissolve merely into mist on touching the land.

On the luminous meteors which appear in the air our space will not permit us to expatiate. The Iris is well known to be produced by the refraction of white light by the spheres of water in a cloud, as has been beautifully and satisfactorily demonstrated by Newton. The explanation of the single and the double iris will be found in every treatise on natural philosophy.

The Aurora Borealis, and the similar luminous Southern Meteor, are still objects of doubtful origin. The brilliancy of the colours is subject to periodical increments and intervals of decline; and, after a considerable period of obscuration, it has, within the last few years, again begun to blaze forth with increasing splendour. We have seen all the prismatic colours present in an aurora; and many years ago, in the Orkney Islands, we were enabled at midnight to distinguish letters of a large print by its light. From the distance to which the same aurora has been seen, the meteor must often be very high above the earth, perhaps in the utmost limits of our atmosphere; but at other times it appears to be nearer to the earth, and we have persuaded ourselves that we have heard a noise accompanying the sudden coruscations of a brilliant aurora.

The Halo, the Parhelion, and Paraselen, are optical phenomena produced by refraction of light by vapour, or minute spicula of ice floating in the air. Parhelia have been described by Scheiner, Gassendi, Delahire, Cassini, Halley, Muschenbroeck, and Epinus.

Lightning has been considered, with other electrical phenomena, under Electricity. The identity of lightning with common electricity, as excited by the machine, was so completely established by the celebrated Franklin, that no doubt has since existed on the subject; and the recent discoveries of Oersted and Seebeck have indicated a close affinity between the phenomena of electricity, heat, and magnetism. As the electrical state of the atmosphere is an important subject of inquiry, we beg leave to recommend to the philosophical traveller some instrument for ascertaining this quality of the atmosphere; and perhaps no better portable electroscope has yet been devised than the pocket instrument of Cavallo.

The Bolis, St Elmo's Fire, Castor and Pollux of the ancients, appear to be electric meteors. The former we have twice seen. The last time was on the 18th of September 1835, at eleven p.m., when at sea. The head appeared an elliptical ball of the most intense, brilliant, bluish-white light; the train had a tint of azure mixed with reddish, which sufficiently distinguished it on the clear sky; and sparks proceeded from its tail. Its motion was nearly horizontal, from S.W. to N.E., at a considerable height. It was visible for five or six seconds, and disappeared in a dense cloud. It was observed by four gentlemen standing together on the deck, and we thought that it emitted a cracking noise as it proceeded, with a majestic uniform movement.

The more common meteor, the Shooting Star, as it is vulgarly termed, seems to be an electrical ball of fire, at a considerable height in the atmosphere. Its general course is from the zenith towards the horizon, which it seldom appears to reach.

The Ignis Fatuus, or Will o' the Wisp, is a meteor which apparently is connected with decaying animal or vegetable matter; and probably is produced by the disengagement of phosphuretted hydrogen. It is frequently seen in boggy grounds, as a flickering, unsteady light, in irregular motion. It sometimes plays over dunghills; and has occasionally, to the extreme terror of the ignorant, appeared as a lambent flame in church-yards. Dr Derham saw it once playing about a dead thistle. The most extraordinary meteor of this kind is that described by Signor Beccari, in the Philosophical Transactions (vol. xxxvi.). In the low grounds to the north and east of Bologna, ignes fatui of remarkable splendour were at that time seen almost every dark night. That in the fields of Bagnara, to the east of the city, was especially brilliant. It appeared and disappeared suddenly; it rose and fell, and often hovered about six feet from the ground; it sometimes contracted its volume, then expanded or divided into two; would again unite, and assume the form of a wave of flame; often it dropped bright scintillations, and occasionally threw as much light upon the road as a large torch.

The curious meteor described by Dr Shaw as running along the ground, and playing on the ears and mane of his horse, has its counterpart, on a smaller scale, in other countries, and has often been witnessed in Britain. It seems in some instances to be an electrical meteor; but possibly it may also arise from the disengagement of phosphuretted hydrogen from living animals, or of a luminous and unctuous matter, like that of the glow-worm, or of phosphorescent marine animals.

A lambent flame has occasionally been observed to play around the heads of children, which has probably a similar origin; it has been finely described by Virgil, in his account of the glory that surrounded the head of the young Ascanius.

Sect. VIII.—Climate.

The temperature of the atmosphere is derived from the heating of that portion of it in contact with the ground, which has received and absorbed the incident solar rays. Whilst the earth revolves on its axis, successive portions of its surface are illuminated and heated by the sun; but the excessive accumulation of heat in any particular place is prevented by the abstracting power of the air, and by the radiation from the earth's surface which takes place during the night. These causes alone, however, could not have secured the diffusion of warmth over a large portion of the earth. Had the axis of the earth been perpendicular to the plane... of its orbit, the same places would have had the sun always vertical; the equatorial regions would thus have been parched with intolerable heat; and much of what is now the fairest portions of the globe, the seats of literature and the arts, would have been doomed to sterility and desolation. The simple yet stupendous contrivance of the inclination of the axis to the plane of the orbit, by bringing successive portions of the northern and southern hemispheres alternately under the vivifying solar influence, has rendered the largest portion of the globe the abodes of animated beings; the fervour of a tropical climate is thus rendered less oppressive, and the limits of the temperate regions are greatly extended. The division into five Zones, and their denominations, give us a general idea of the temperature in each; but these are very inadequate to express the diversities of atmospherical temperature. Ancient geographers distributed the earth's surface into twenty-six parallel bands or Climates, which were supposed to indicate, with sufficient exactness, the differences of the heat of each region, until the invention of the thermometer gave more precise information.

We have already stated how multiplied and careful observations may point out the mean temperature of any place, and the contrivances which have been resorted to for abridging the labour of the meteorologist; and philosophers have endeavoured to supply by calculation what is wanting in observation. The most ingenious of these speculators was Mayer of Göttingen. From an examination of such registers of temperature as he could obtain, he concluded "that the mean temperatures decrease from the equator to the poles, as the squares of the sines of latitude;" and therefore, if the mean temperature at the level of the sea under the equator were ascertained, this formula would enable us to calculate the mean temperature of any given latitude. Mayer applied his formula to the equatorial temperature as determined by Bouguer, = 24° of Réaumur; but this has been found by Humboldt to be too high. It should have been only 22° of that scale, which is equivalent to 27°·5 of the centigrade, and to 81°·5 of Fahrenheit. With these corrections, Mayer's formula for the three thermometers, T signifying the temperature sought, will be

\[ T = \frac{22}{\cos \text{lat.}} \times \text{Réaumur} \]

\[ T = \frac{27}{\cos \text{lat.}} \times \text{Centigrade} \]

\[ T = \frac{81}{\cos \text{lat.}} \times \text{Fahrenheit} \]

Unfortunately, however, later observations have shown that Mayer's formula is worse than useless in high latitudes, and indeed is only found tolerably consistent with observation in latitudes between 40° and 60° N. The thermometrical registers of Scoresby, Parry, Franklin, and Ross, have proved that the mean temperature of the parallel between 70° and 75° is very far below what Mayer's formula would assign, and is even far below what that empirical rule would give to the pole. In low latitudes, too, it is not to be depended on; and it is totally at variance with the recent but well-established fact of meridians of greatest cold. No attempt of this kind has yet been found reconcilable with observation. In the mean time, the celebrated Humboldt has performed an acceptable service to science, by a more critical examination of the accumulated observations of his predecessors and contemporaries. The results he has given in his essay on the Distribution of Heat over the Globe; and the representation of them upon a chart of Isothermal Lines is peculiarly valuable, as deductions from actual observation. An inspection of such a chart shows, that these lines are neither parallel to the equator nor to each other. They are higher in Europe than at 100° E. or W. Thus the isothermal line of temperature 54°·5, in France passes through lat. 45°·46'; in China (long. 116° E.) it is in lat. 39°·54'; and in America, on the western coast (in long. 104° W.), it is in 44°·40'.

The isothermal lines descend lower in America than in Europe. Thus the line of 32°, or the freezing point, in Europe, lies in Lapland between 66° and 68°; but in America it passes on the coast of Labrador in lat. 54°; and this difference, which is small near the tropic, increases much with the latitude. Thus,

| Lat. | Mean Temperature | Difference | |------|-----------------|------------| | 30° | 70°·52 F. | 66°·92 F. | 3°·60 | | 40° | 63°·14 | 54°·50 | 8°·64 | | 50° | 50°·99 | 37°·94 | 12°·96 | | 60° | 40°·64 | 23°·74 | 16°·92 |

In both continents, the most rapid decrease of temperature takes place between lat. 40° and 45°. This circumstance has justly been considered by Humboldt as producing a happy influence upon the civilization and industry of the nations inhabiting these parallels; because there slight changes in latitude produce considerable diversity in the vegetable productions, which may become objects of rural economy; and when contiguous countries differ much in their products, it stimulates the industry of each, and gives vigour to commercial intercourse; circumstances highly favourable to civilization. The range of the thermometer is not the same under the same isothermal lines, as representing the mean temperature of the year. It is greater in the New than in the Old Continent. Thus, Cincinnati in America, and St Malo in France, are both on the isothermal line of 54°; yet the difference between the mean heat of summer and winter is 40° at the former, and but 32° at the latter. Even though the difference between the mean temperature of the seasons increase with the latitude, under the same meridians, yet the difference at Philadelphia in America, in lat. 40°, is much greater than at Copenhagen in Europe, in lat. 56°.

| Lat. | Mean of Summer | Mean of Winter | Difference | |--------|----------------|---------------|------------| | Philadephia... | 40° | 73°·94 | 32°·18 | 41°·76 | | Copenhagen.... | 56° | 60°·60 | 30°·74 | 29°·86 |

The remarkable fact of the influence of longitude on temperature leads to the conclusion, that on each side of the equator there are two meridians, under which the mean temperature is lowest. These have been termed by Sir David Brewster the cold meridians, and their extremities are the poles of greatest cold. The position of these in the northern hemisphere may be approximated from recent investigations; and perhaps we shall not greatly err if we assign the longitude of 95° W. for the American, and of 100° E. for the Asiatic cold meridians. The apparent coincidence of the cold meridians with the general direction of Hantzen's lines of no variation is perhaps more than accidental, when we reflect that there seem to have been, in former ages, migrations of the cold meridians eastward and westward, coincident, as far as we can judge from recorded changes of climate in northern countries, with similar migrations of the magnetic needle.

Elevation above the sea is one of the most important circumstances by which climate is modified. As we ascend elevation in the atmosphere the temperature diminishes, and under above the every latitude the summits of very lofty mountains penetrate into the abodes of perpetual congelation: such sum-

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1 Traité de Mémoires de la Société de Physique et d'Histoire Naturelle de Genève, tom. iv. Physical mists are covered with perpetual snows. The altitude at which this takes place varies with the latitude, diminishing from the equator to the poles. In climates where there is a great difference between the heat of summer and of winter, this point will fluctuate between two limits, one of which is the upper, the other the lower limit of congelation. The distance between them is least at the equator, and increases as we recede from it. This circumstance is important; for it influences the formation of those stupendous glaciers, that excite the astonishment of the traveller, on the elevated regions of the temperate zone, and on the declivities of the arctic circle.

Sir John Leslie concluded, from some experimental investigations, that in moderate elevations we might estimate this decrease of temperature as we ascend, at 1° of Fahrenheit for every 300 feet of elevation; and he has given a formula for computing the point of congelation for every latitude; which, however, does not agree with observation in high parallels. The same remark applies to Kirwan's formula, which presupposes a gradual depression of the point of congelation in receding from the equator. The requisite data for his calculations are the mean temperature at the level of the sea at the equator and at the given place, and the height of the equatorial point of congelation. Thus, if we take the point of perpetual congelation at Humboldt's estimate of 15,744 feet, and the mean temperature at the level of the sea there at 81°-5, the difference between that mean temperature and 32° will bear the same ratio to the equatorial point of congelation, as the excess of the mean temperature at the given place above the freezing point, to the point of congelation at the latter. Thus, if we wish to find the point of congelation in this latitude, 56°, we turn to a table of mean temperatures, and find it = 49° Fahrenheit.

\[ 81°-5 - 32 = 49°-5, \text{and } 49 - 32 = 17, \text{and} \]

\[ 49°-5 : 15744 : : 17 : 5407. \]

On this formula, then, 5407 feet would be the point of perpetual congelation in our climate; and had we any mountain exceeding that elevation, its summit would be covered with perennial snow.

We apprehend, however, that no formula will apply to all latitudes; and we know that Kirwan's does not agree with observation in high latitudes.

Even when the latitude and elevation of two places do not differ greatly, they may suffer very different extremes of temperature. The chief agent in effecting this is the ocean. It has been found that the proximity of the sea, in all climates, affects the extremes of temperature. When cold air sweeps over the surface of the ocean, the caloric of the upper particles is diminished, their density is increased, and they sink, while their place is supplied by fresh particles from below, which becoming cooled in their turn, give rise to internal motions in the ocean. As the law of expansion during cooling from 39° to 32°, which forms so singular an exception in the case of fresh water, does not take place in sea-water, there is nothing to arrest these internal motions in the ocean; and from its enormous mass, it is obvious that no partial application of cold, in this way, can sensibly reduce its mean temperature. Hence we find that the temperature of islands is not in winter so low as that of continents in the same parallels. Thus the severity of winter in Britain bears no proportion to that of continental regions under the same parallel. The Orkney Islands are in the same latitude as St Petersburg; yet, in the former, a frost of a week's duration is a comparatively rare occurrence, whilst at the latter, the wide, deep, and rapid Neva is for several months annually frozen over.

On the other hand, the ocean exerts a great influence in mitigating the excessive heat of tropical climates. It is true, that when the upper particles of its waters become heated, the internal motions cease; but the rarefaction produced sets a current in motion to the west (as becomes apparent in the Gulf Stream), the place of which is supplied by the influx of cooler waters from either pole. This is not all. The increased temperature is slightly diminished by increased evaporation; a process by which as much caloric is absorbed as would be capable of heating an equal weight of water 942° Fahrenheit. Hence tropical islands are less oppressively hot than continents in the same latitude. The summer temperature of the smaller Antilles, for instance, is lower than that of the same parallels in America, Africa, or Arabia. The intersection of continents by arms of the sea, or by vast collections of water, tends to mitigate the fervour of summer. The extended form of the old continent toward the east renders inequalities of temperature much greater in Eastern Asia than under the same parallels in Europe. Thus, at Pekin, in latitude 40° N., longitude 116° 20' E., the mean temperature of summer is 78°-8, and of winter 25°—a difference of not less than 55°-8, which gives rise to a frost of several months' duration in that part of China; yet Pekin is under the same parallel as the southern extremity of Naples, where frost is unknown, and of the central provinces of Spain, in which, though at an elevation of 2000 feet above the sea, ice is an extremely rare occurrence.

The face of a country, though less important than the circumstances already noticed, affects its climate. A region shrouded in primeval forests, or covered with swamps and marshes, will have a different temperature when cleared and drained. Marshes, by evaporation, rob the surface of much of its heat; and forests, by intercepting the sun's rays, the source of terrestrial warmth, and by the increased exhalent surfaces exposed to the air, undoubtedly have some effect on mean temperature. In hot climates forests tend to cool the air, and in frigid regions to prevent the loss of the earth's heat by chilling blasts. The clearing of forests may thus affect the temperature, and the draining of marshes increase the salubrity of particular regions; but the mere effect of cultivation can never be very considerable in changing a climate.

A combination of several of the causes already noticed as affecting climate have conspired to render the North American continent colder than the same parallels in Europe. Thus Nova Scotia and Canada, in latitude 47°, are in the same parallel as the central provinces of France and Upper Italy. In the former, the cold of winter has been observed as low as —40° Fahrenheit, or 52° below the freezing point of water; and the lakes and rivers of that part of America are bound up in impenetrable ice during four or five months of the year; while in the latter, in ordinary seasons, the severity of winter is scarcely felt. Labrador and England, each in latitude 53°, differ widely in mean temperature.

| Mean Temperature | |------------------| | Summer | Winter | General | | Labrador | 51°-8 | 3°-2 | = 27°-5 | | England | 62°-6 | 37°-4 | = 50 |

The climate of America, however, should not be compared with that of Europe, but with the eastern side of the old continent. The eastern sides of both, from causes which perhaps are not easily explained, are certainly colder than the

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1 Essay on Climate. 2 Dr Hope, Transactions of the Royal Society of Edinburgh.