PHYSICALGEOGRAPHY

Physical Geography. THE object of Physical Geography is to describe the external structure, form, and appearance of the globe, and the relations subsisting between it and the various classes of animated and organic beings which inhabit its surface. It ought to comprehend a description of the solid parts of the globe, with their magnitude, position, and the progressive changes they have undergone,—of the fluid parts, consisting of seas, lakes, and rivers, with their extent, motions, and general physical qualities,—of the atmosphere, with the phenomena it presents, including the variety of climates and seasons in different regions. Lastly, it should embrace a general view of the various tribes of animals and vegetables, with the order of their geographical distribution, in the ocean, the atmosphere, or on the land. Physical geography has but recently begun to assume the form of a science, and, like other branches of knowledge depending on extensive and multiplied observations, it can only be improved by slow degrees. To render it perfect, it would be necessary that we should know the true geographical position of every point of the earth's surface, its height above the sea, its mean temperature for every month, the prevailing winds, the annual and monthly depth of rain, the rate of evaporation, the nature of the rocks, the heat of springs, the peculiarities of the soil, with the animals and vegetables which can find nourishment upon it, and are adapted to the climate. We should know also for each part of the ocean, its depth, temperature, general and particular motions, and other physical properties, with its peculiar vegetable productions, and classes of living inhabitants. And to raise this knowledge to the rank of philosophy, effects should be traced to their causes, and the infinite variety of phenomena connected by a small number of general principles, by the help of which we might be able, from the knowledge of a few facts, to form certain conclusions respecting a multitude of others. Though physical geography is still far from this degree of perfection, writers upon the science have already collected, and to some extent generalised, a great number of interesting facts. To exhibit a systematic view of these is our object in the following pages; but we think it necessary to state, that we must necessarily confine ourselves within limits which will preclude extensive details upon any part of the subject.

Magnitude. The earth is one of eleven spherical bodies, denominated planets, which revolve round the sun. Its distance from that luminary is 93,595,000 English miles, and its mean diameter is 7912. It completes its diurnal revolution in twenty-four hours, and its annual revolution in 365^d. 5^h. 48' 51". One satellite, or attendant body, the moon, 2180 miles in diameter, revolves round the earth in 29^d. 12^h. 44', at a mean distance of 475,000 English miles. The relations of these bodies to the sun and the other planets, and

the laws which regulate their motions, belong properly to astronomy. (Playfair's Outlines of Nat. Phil. II. 86, 125, 126, 225.) The earth is not a perfect sphere in its form, but an oblate spheroid, or sphere flattened at the poles. The amount of this compression is such, that the equatorial diameter exceeds the polar diameter, or axis, by twenty-five miles; or the one is to the other as 312 to 311. The centrifugal force arising from the revolution of the earth round its axis, which evidently tends to dilate the equatorial parts, led Newton to infer the oblate or compressed figure of the globe before it was known by experiment. He calculated the ellipticity, from theory, at $\backslash frac\{1\}\{295\}$, which is about one-third greater than the truth. A homogeneous fluid body of the mean density of the earth, and making its diurnal revolution in the same time, would, in fact, have the proportions which Newton supposed; and the ellipticity in this case would be a maximum. But if the revolving body, instead of being homogeneous, increase in density towards the centre, the compression is not so great. Now, the experiments made at Schehallien show that the mean density of the earth is nearly double of that of the rocks at its surface, and, of course, the density of the central parts must be still higher than the mean. Mathematicians have demonstrated, that, were the density to increase so as to be infinitely great at the centre, the ellipticity, in that case a minimum, would be no more than $\backslash frac\{1\}\{297\}$. The structure of the globe as to density, so far as our knowledge extends, is intermediate between these extreme conditions; and since its ellipticity of $\backslash frac\{1\}\{295\}$ is also intermediate between the maximum and minimum, the circumstance affords a strong presumption that its form approaches very nearly to that of a spheroid of equilibrium, and in all probability coincides with it entirely. (Playfair's Outlines, II. 60, 302, 304.)

This conclusion is strikingly confirmed by the phenomena presented by those planets, in which the supposed cause of the ellipticity exists in the greatest degree, Jupiter and Saturn. As each of these bodies has a diameter ten times greater than that of the earth, and revolves round its axis in less than half the time, the centrifugal force ought to be far more powerful, and produce a much greater dilatation of the equatorial parts. Accordingly, in Jupiter, the ellipticity is one-fourteenth of the longer diameter, and, in Saturn, where the effect of the centrifugal force is aided by the attraction of the ring, the ellipticity amounts to one-eleventh. (Playfair's Out. II. 179.)

We have good reason to conclude, that whatever causes gave this oblate figure to the globe, they have acted upon the solid as well as the fluid parts. Though the land and water are unequally distributed over the two hemispheres, the soil, in a general point of view, is as much elevated at the equator as towards the poles. But since the equatorial regions are about

twelve miles farther from the centre than the parts at the pole, it is evident, that, had the sea only been subject to the action of the centrifugal force, the torrid zone must have been completely submerged, and the polar regions left entirely dry, and rising many miles above the level of the waters. The ocean must, in fact, have formed a broad and deep zone round the equator, separating the two continents encircling the poles; of which continents a very small part only would have been habitable in consequence of the elevation of the soil. The figure of the globe affords, therefore, evidence of considerable weight in favour of its original fluidity, and it negatives the idea, that the earth, at the period of its consolidation, had a different axis from what it has at present.

The precession of the equinoxes, and the nutation of the earth's axis, indicate, in the opinion of Laplace, that the density of the globe increases towards the centre. Mr Cavendish, from some experiments with leaden balls, estimated the mean density of the earth at 5.48. (Phil. Trans. Vol. LXXXVIII. 469.) But the calculation most to be depended on is that founded on the experiments at Schehallien, which gives the mean density equal to 4.71 (Playfair's Out. II. 320), that is, about 4$\backslash frac\{1\}\{2\}$ times the weight of an equal bulk of water. As the density of the rocks at the surface, which does not exceed 2.5 or 2.7, is much lower than the mean, it follows, that the density of the central parts must be much higher. If, for instance, the exterior rocks with which we are acquainted should form a shell of 500 miles in thickness, the parts within this shell would require to have an average specific gravity of 5.8 to produce the mean density of 4.7 for the whole mass. Laplace thinks, however, that the change in density is not sudden but progressive, and that it is probably the effect of concentric and elliptical beds, of increasing density, disposed symmetrically round the centre of gravity. (Daubuisson, Traité de Géognosie, Chap. I.) This disposition would naturally take place in a mass simultaneously fluid. And, in fact, the lowest rocks with which we are acquainted have generally a greater specific gravity than those which lie above them. The only considerable class of mineral substances known to us, which have the high density inferred to exist in the central parts of the globe, are the metals and their ores. This circumstance may, perhaps, authorize a conjecture, that these substances occupy the interior of the earth, and that the metallic repositories found in the outer crust consist of minute portions, separated and cast up from the central mass.

It would be foreign to our purpose to enter into the disputes which divide geologists. Conceiving that the principles of geology, at the present day, are to be considered as little better than provisional, we shall recur to them as sparingly as possible. Of the central nucleus, as already stated, we can know nothing but by inference. The outer crust, however, lies exposed, in part, to our view, and the materials of which it is formed, so far as we are acquainted with them, consist of four great classes. 1. Those rocks which neither contain any animal nor vegetable remains themselves, nor are intermix-

ed with rocks which do contain them, and are therefore termed primitive, or primary, as having been formed before the existence of organized beings. These are granite, gneiss, mica slate, and clay slate, which occur abundantly in all regions of the globe, with quartz rock, serpentine, granular limestone, &c. which occur more sparingly. These rocks never contain organic remains, and, till lately, were supposed never to cover rocks containing them; but a larger experience has shown, that this circumstance does not hold true entirely, even of granite itself. (Daubuisson, II. 226.) 2. Rocks containing organic remains, or generally associated with other rocks in which such substances are found, and which, as having been formed posterior to the existence of organized beings, are termed secondary. These are greywacke, sandstone, limestone, and gypsum of various kinds, slate clay, with certain species of trap, and they are found lying above the primary, or older rocks. 3. Above these secondary rocks, beds of gravel, sand, earth, and moss are found, which have been termed alluvial rocks or formations. 4. The name of volcanic formations has been given to beds of lava, scoriae, and other matter thrown out at certain points of the earth's surface by subterranean fire.

A very distinct arrangement can be traced among these rocks, though they present a confused appearance to the eye on a first view. Where those of the primary and secondary classes are exposed together, the granite, which generally exists in unstratified masses, is found almost invariably under all the others; and yet it also occupies the highest points of the earth's surface. The gneiss, which is merely a stratified granite, lies next it, then the mica slate, then the clay slate. The primary limestone, trap, serpentine, &c. lie in occasional beds intermixed with these. And above all, the primary formations are greywacke, sandstone, and other secondary rocks. Farther, the primary rocks are in general highly crystalline in their structure, and at the points where they are exposed to view, are found standing on their edges, or inclining to the horizon, seldom at a less angle than 45°. The secondary rocks again have rather the appearance of mechanical deposits; they occupy a more horizontal position; and their upper edges, or outgoings, are generally found at a lower level than those of the primary class. On these facts, geologists have founded certain general conclusions. From the oblate spheroidal figure of the globe, and the arrangement of its superior mineral masses in beds, or strata, they assume that the whole, or at least the outer crust, was at one time in a fluid state. That the central nucleus, of which we know nothing, is encompassed round and round by a coat or shell of granite, which, crystallising from a fluid state by the force of a strong affinity, has assumed a very irregular figure, shooting up into elevated cones or ridges at some points, which form the naked summits of mountains, and sinking into vast cavities many miles, or dozens of miles, below the present surface at others. That the gneiss crystallizing next in order was deposited above the granite, covering all the lower parts, but leaving the most elevated points of the granite bare. That the mica and clay slate

Physical Geography. were deposited in a less perfect state of crystallization above the gneiss, and in beds rather less inclined; that the older class of secondary rocks of a composition more mechanical than chemical, were then deposited above the mica and clay slate; the newer secondary rocks again above the older, in a position approaching more and more to the horizontal, as their structure became less crystalline, and occupying a lower and lower level; so that the edges, or outgoings, of the older rocks are always found rising above one another, as you advance from low plains to the summits of mountains. It is thus supposed, that the gneiss and other stratified primary rocks are found in beds nearly vertical, because we see them only at those points where, resting on the elevated crests of the fundamental granite, they break through the secondary formations; but that, if we could penetrate through the masses of sandstone, limestone, &c. which cover them in level situations, we should find them there also, and lying in horizontal positions above the granite. Farther, we see how the deposition of the different formations brought the earth nearer and nearer to the state in which we find it. The deposition of the granite, we may suppose, left the surface divided into profound cavities bounded or broken by ridges or pinnacles with steep sides, on which few orders of animated beings would have existed. The stratified primary rocks, deposited on the bottom and sides of these cavities, filled them in part, the older secondary rocks, falling down in less inclined strata, reduced the inequality still farther; and the newer secondary rocks, subsiding mechanically in strata nearly horizontal, made the surface assume a tolerably even and regular appearance. Last of all, the masses of gravel, sand, earth, marl, and other alluvial matter, filling up the smaller crevices and hollows left by the newest rocks, smoothed the surface, and prepared the earth for the nourishment of vegetable substances, and the habitation of animated beings.

We give this outline of the structure of the globe according to the principles of Werner; because, though it cannot be easily reconciled with all the facts now known, it explains the general disposition of the parts of the earth's surface, better than any other that can be given in a small compass. Of the modifications lately suggested, the following is worthy of notice. The great space which the stratified primary rocks occupy, compared with granite, and the fact, that the former sometimes preserve the same direction and dip unaltered on both sides of a granite mountain, have led some to consider the latter not as the fundamental rock, but rather as existing in large kernels or isolated masses, which are stuck in among the strata of the primary schistus, without affecting their dip and direction. The schistus again, instead of being in coats wrapped round the granite, is supposed rather to affect a squamosa structure, or to be disposed in a continued order like the scales of a fish. Gneiss, mica slate, and clay slate, may be conceived, on this hypothesis, to be disposed in strata standing out edgeways, and alternating with one another, all round the globe. The large hollows left in the surface by the irregularity of the crystallization, or produced by the wearing

away of the softer rocks (the mica and clay slate, the more destructible rocks, being generally found in the lowest situations, and the more enduring gneiss, with its kernels of granite, in the highest), are filled up by newer formations, sometimes in conformable positions, but more frequently lying over the ends of the older strata. In this place, it is enough to have mentioned these opinions, without attempting to discuss their merits.

Physical Geography. Changes on the Surface. The phenomena of geology show, that the original formation of the rocks has been accompanied, in nearly all its stages, by a process of waste, decay, and recomposition. The rocks, as they were successively deposited, were acted upon by air and water, heat, &c. broken into fragments, or worn down into grains, out of which new strata were formed. Thus the disintegrated materials of the older rocks, reunited by a cement, constitute those strata of greywacké, conglomerate, and old red sandstone, which skirt the primary mountains; the debris of the secondary rocks are found aggregated in the same way into beds of breccia, tufa, and newer conglomerate; a large portion of the spoils of the slate and trap rocks, both old and new, in a state of great comminution, are spread over the surface in beds of clay; and a still larger portion of the spoils of the quartz rocks, form those immense beds of sandstone and loose sand which cover so great a portion of the dry land and the bottom of the sea. Even the newer secondary rocks, since their consolidation, have been subject to great changes, of which very distinct monuments remain. Thus, we have single mountains, which, from their structure, can only be considered as remnants of great formations, or of great continents no longer in existence. Mount Meisner in Hesse, six miles long and three broad, rises about 600 yards above its base, and 700 above the sea, overtopping all the neighbouring hills for 40 or 50 miles round. The lowest part of the mountain consists of the same shell limestone and sandstone which exist in the adjacent country. Above these are, first, a bed of sand, then a bed of fossil wood 100 feet thick at some points, and the whole is covered by a mass of basalt 500 feet in height. On considering these facts, it is impossible to avoid concluding, that this mountain, which now overtops the neighbouring country, occupied, at one time, the bottom of a cavity in the midst of higher lands. The vast mass of fossil wood could not all have grown there, but must have been transported by water from a more elevated surface, and lodged in what was then a hollow. The basalt which covers the wood must also have flowed in a current from a higher site, but the soil over which both the wood and the basalt passed has been swept away, leaving this mountain as a solitary memorial to attest its existence. Thus, also, on the side of Mount Jura, next the Alps, where no other mountain interposes, there are found vast blocks of granite (some of 1000 cubic yards) at the height of more than 2000 feet above the Lake of Geneva. These blocks are foreign to the rocks among which they lie, and have evidently come from the opposite chain of the Alps; but the land which constituted the inclined plane over which they were rolled or transported, has

Physical Geography. been worn away, and the valley of lower Switzerland with its lakes now occupies its place. Transported masses of primitive rocks, of the same description, are found scattered over the north of Germany, which Von Buch ascertained, by their characters, to belong to the mountains of Scandinavia; and which, therefore, carry us back to a period when an elevated continent, occupying the basin of the Baltic, connected Saxony with Norway. (Daubuisson, Geog. I. 230.)

At the present day, we see rocks worn away by the action of rivers and the sea, or crumbling down by the agency of air, heat, moisture, and frost; we see sands thrown up by the ocean at other points, and carried by winds over the face of the continent; we find new lands forming at the bottom of lakes and the mouth of rivers; we find beds of solid matter thrown out by volcanoes, loose sand agglutinated into hard rock on the shores of the sea; and new stony deposits forming by calcareous solutions. Can we attribute the great changes of which we see traces, to the same causes which operate these effects under our eyes, or have mightier agents been employed? These are questions which geologists have not yet been able completely to resolve, and into which it is not necessary here to enter.

Organic Remains. But the most interesting memorials of the past history of the globe are supplied by those myriads of remains of organised bodies which exist in its outer crusts. In these, we find traces of innumerable orders of beings, existing under different circumstances, and succeeding one another at distant epochs, varying through multiplied changes of form, yet, even in their most dissimilar appearances, exhibiting a constant analogy to one another, and to the organised bodies which still exist. What is not less interesting, the varieties in the character of these extinct beings are not capriciously distributed, but correspond pretty distinctly with the order of their position. Thus, as we pass from the oldest secondary formations to the newest, we find the forms of the animal and vegetable remains approaching more and more to those of the living bodies which now exist on the surface; so that, while the most ancient are but faintly allied by a few general characters to the present animal and vegetable tribes, the most recent can scarcely be distinguished from them.

If we examine the secondary rocks, beginning with the most ancient, the first organic remains which present themselves are those of aquatic plants and large reeds, but of species different from ours. To these succeed madrepores, encrinites, and other aquatic zoophytes, living beings of the simplest forms, which remain attached to one spot, and partake, in some degree, of the nature of vegetables. Posterior to these are orthoceratites, ammonites, and other mollusci, still very simple in their forms, and entirely different from any animals now known. After these some fishes appear, and plants, consisting of bamboos and ferns, increase, but still different from those which exist. In the next period, along with an increasing number of extinct species of shells and fishes, we first meet with amphibious and oviparous quadrupeds, such as crocodiles and tortoises, and some reptiles, as serpents, which show that dry land

now existed. As we approach the newest of the Physical Geography. solid rock formations, we find lamantins, phocæ, and other cetaceous and mammiferous sea animals, with some birds. And in the newest of these formations, we find the remains of herbivorous land animals of extinct species, the paleotherium, anaplotherium, &c. and of birds, with some fresh water shells. In the lowest beds of loose soil, and in peat bogs, are found the remains of the elephant, rhinoceros, hippopotamus, elk, &c. of different species from those which now exist, but belonging to the same genera. Lastly, the bones of species which are apparently the same with those now existing alive, are never found except in the very latest alluvial depositions, or those which are either formed in the sides of rivers, the bottoms of ancient lakes and marshes now dried up, in peat beds, in the fissures and caverns of certain rocks, or at small depths below the present surface, in places where they may have been overwhelmed by debris, or even buried by man. (Cuvier's Essay, S. 29, Daubuisson, I. 362.) Human bones are never found, except among those of animal species now living, and in situations which show that they have been, comparatively speaking, recently deposited.

From these phenomena we are led to infer, that Geological History. rocks now buried at a great depth, constituted, at one time, the surface of continents, and the seat of organic life; and that many orders of beings have been called into existence, and afterwards destroyed by great revolutions, which introduced new classes of mineral deposits, accompanied with new tribes of organic beings; we see farther, that among the variety of vegetable and animal forms which peopled these successive continents, aquatic plants and animals, which appear the earliest, show that the surface was long covered with water. The appearance of amphibious animals and reptiles at a later period indicates the first existence of dry land; but long posterior to these, and among the newest of the solid rocks, we find herbivorous quadrupeds, which shows, so far as our present knowledge extends, that it was only at a late period, speaking geologically, that the earth's surface was clothed with herbage, and rendered in other respects analogous to what it is at present. We find also, that, since the surface assumed this form, it has been subjected to at least two great catastrophes, apparently by irruptions of the waters. Thus the fossil remains of the herbivorous animals found in the Paris gypsum (the paleotherium, &c.) are covered by deposits containing sea-shells, and are not found again in any beds nearer the surface. Hence the waters, after the irruption which destroyed these animals, had again retired; and upon the new continent thus left dry, we find the remains of a different race of herbivorous quadrupeds (the elephant, rhinoceros, &c.), of species which are also extinct, but allied to the existing races in their generic characters. These animals are again covered by loose soil, intermixed with marine substances, which shows that a new irruption of the sea had occasioned their total or partial destruction; and when the waters retired once more, the land had been left dry, as it now exists; for the only class of animal remains above those in question belong to the species now

Physical Geography. living on the surface, and are found in situations where their appearance is easily accounted for. It is among these last that the remains of the human species are found; and no vestiges of any being analogous to man in structure have ever been discovered among older deposits.*

From an attentive consideration of the phenomena now referred to, we are led to conclude, that the appearance of man upon the face of the globe is, geologically speaking, a very recent event; before which the earth had been inhabited for thousands of years, by various families of plants, and tribes of animals, which had been destroyed and renewed in a long series of successions. Farther, the researches hitherto made favour the supposition that the tribes of living beings existing at each epoch were not a remnant of those destroyed by the preceding revolution, retaining the same forms, or changed and modified by external causes, but rather a new creation, adapted, we may presume, to the altered situation of the surface. It is possible, but rather improbable, that the present races of animals may have existed along with the mastodon of America and the mammoth of Russia; but we are almost certain that they did not exist, at least in this part of the globe, with the preceding order of beings, the paleotherium, anaplotherium, &c. The continual approaches which the newer orders of fossil animals make in their forms to those now living can, therefore, only be regarded as a proof that the climate, soil, and circumstances to which the nature of the extinct species was adapted, approached more and more to those under which animal life exists at the present day.

As new orders of beings have been introduced at each change, and the most perfect have appeared the last, have we any reason to believe, from the consideration of these phenomena, that some future convulsion of nature may bury the present races of men and animals, and usher in a new creation, of a still more perfect kind, over which some intelligent being, of a higher class than man, may preside? We think not. The great revolutions, which have so often overwhelmed and new-peopled the face of the globe, seem to have been continually decreasing in the magnitude of their effects, from those which deposited the vast masses of the primitive rocks, to those which left behind them the Paris gypsum and other superficial beds, like a slight sediment in the cavities of preceding formations. They may be compared, in fact, to the motions of a pendulum, which describes a smaller arc at each vibration, till it ultimately settles in a state of rest. The progressively diminishing extent of these successive changes give us reason to believe, that the system of our globe has nearly reached this state of repose. Perhaps, without an improper licence of conjecture, we may explain the phenomena which the past history of the globe presents, by supposing that the Deity, having deemed that state of the earth's surface which we

now witness the best adapted for man, and having considered a long series of changes, such as geology reveals to us, the fittest means of bringing it to this state, did, from the impulse of that benevolence which has peopled every leaf with sentient beings, create tribes of living creatures at each successive epoch, adapted to the existing order of things, and terminating with it. Nor is there any thing in this conjecture inconsistent with revelation. It has been allowed, that the word "day," in the Mosaic account of the creation, may mean a period of indefinite length; and nothing more is necessary to remove every essential difficulty. Cuvier deduces, from certain progressive changes on the earth's surface, as well as from the concurrent traditions of many nations, that the first appearance of man on the face of the globe, or at least the renewal of the human race after some great catastrophe, cannot be referred to a period farther back than about 5000 or 6000 years from the present time. (Essay, Sect. 34.) The researches of science in this respect afford a striking confirmation to the testimony of the sacred historian.

The earth's surface may be divided into dry land and submarine land. The dry land consists of high country, that is, of mountains and hills, with the elevated valleys between them; and of low country, or the extensive plains which lie at the foot of the mountains. The sea has also its plains, mountains, and valleys. High or Alpine country sometimes descends to the sea by a succession of valleys below one another, sometimes by one large plain. Elevated and extensive plains, however, with opposite declivities, do not terminate in one another, but are almost always divided by land of a mountainous character.

Single detached mountains are rare, except in volcanic districts, and in those where trap rocks abound. Mountains are generally found in elevated bands, consisting either of one central chain, with branches running off at right angles, or of several chains or ridges running parallel to each other; and in both cases often accompanied by subordinate or dependent chains, of a smaller elevation. Thus the Cévennes, Jura, Vosges, with the mountains beyond the Danube, are considered as dependencies of the Alps. Mountain chains have generally a great length, compared with their breadth. The ends or extremities of a chain and the outer ends of its branches seldom decline gradually into the level country, but rather terminate abruptly. In principal chains, the highest point is generally near the middle; but in dependent chains, the highest part is that which is nearest the principal. Peaks or elevated summits, rising far above the other parts of the mountain, such as Mount Blanc, in the Alps, are generally placed on the principal ridge, or that which parts the waters, and at the point where two opposite branches join. To this, however, there are exceptions. Cols or necks, which are the lowest parts of ridges, and serve as passages from one country to another, are al-

* Some human bones are said to have been found in loam in gypsum caves, associated with the remains of the rhinoceros and fossil horse. Jameson's Manual of Mineralogy, p. 445.

most always placed at the joining of two opposite valleys. The Col de Brenner, the lowest of those in the great Alpine chain, is 4680 feet above the sea. The other four principal passages through the great chain are from 6500 to 8000 feet high. The lowest passage in the chain of the Pyrenees is about 3400, and in that of the Andes of Quito 12,200, feet above the sea. The summits of great chains sometimes present a sharp ridge, like the Swiss Alps; sometimes a level track, like Mount Langfield in Norway, which is ten or twelve leagues broad; sometimes a vast plain, like the table land of Mexico in North America, 150 leagues broad, and bounded by valleys with steep sides. Where many chains run parallel to each other, the steepest sides of the outer chains are generally turned towards the central chain; and in the dependent chains, the steepest sides are turned towards the principal. In general, also, mountains that surround lakes or basins present their steepest sides to the water. Thus the scarps of the hills which surround the Basin of Bohemia are turned inward; those of the hills which surround the Lakes of Geneva and Constance face these lakes; those of the mountains round the Mediterranean also generally face that sea; and the high lands which extend along the coasts of Africa, Asia, and America, from the Cape of Good Hope to Cape Horn, have their steepest sides to the great basin which forms the Indian and Pacific Ocean. (Daubuisson, I. 60—99.)

The form and arrangement of valleys correspond to those of mountains. A great mountain chain sends out branches at right angles to itself, which are called lateral chains; these lateral chains send out other smaller branches at right angles to themselves, and parallel to the great chain, and which are called subordinate chains; and sometimes these last send out others still smaller. The valleys which separate one principal chain from another, such as the valley of the Rhone above Geneva, are called "principal valleys;" those which separate one lateral chain from another are called "lateral valleys;" and those which divide the subordinate chains from one another are called "subordinate valleys." The subordinate valleys open into the lateral, the lateral into the principal; the whole forming a connected system, in the order of trunk and branches. Valleys often contract and enlarge their breadth, as if they formed a series of basins rising above each other towards their sources. Lakes are most abundant at the upper extremities of valleys. A vast number of small lakes exist along the ridge of the Pyrenees, some of them at an elevation of 8000 feet; and along that of the Andes, where they are also numerous, there are some as high as 13,000. In the Alps there are also many; but the largest, such as those of Constance, Geneva, Como, lie at the foot of these mountains, and at elevations from 650 to 1600 feet above the sea. Some of the basins, formed by the contraction of the valleys, have obviously been lakes at one period, though now dry. But more frequently the sides of valleys, instead of alternately contracting and receding, preserve a very striking parallelism, salient angles on one side being opposed to re-entering angles on the other. Nor is this the consequence of inflections in

the dip and direction of the strata; for these rarely correspond with the declivities of the surface. On the contrary, we find the lateral and subordinate valleys in the sides of mountains cutting distinctly through the strata; so that, when the width is not very great, we can generally trace the same beds in the same order at the opposite sides. In short, the aspect of a mountainous region, on the great scale, is very similar to that of a hillock of soft clay or earth which has been cut and worn away by heavy rains. The furrows made by the water, ramifying into others smaller and smaller, with the little ridges of earth between them, present a very accurate image of an Alpine track, with its system of valleys, its crests and pinnacles, its long chains, and its lateral and subordinate ridges. Had the mountainous district, with some slight original inequalities of surface, been subjected, for a long series of ages, to the agents which act upon it at present, or what is more probable, to agents of the same kind, but of greater power, we have reason to believe that the configuration of the surface would have ultimately been very similar to what we see. It does not follow, however, that mountains have really received their form from such causes. (Daubuisson, I. 85—94, 245.) On the other hand, a debacle, or great current, such as some geologists have supposed, sweeping over the globe, or a particular district, could not scoop out valleys which are almost universally shut at one extremity.

Hills are less regular in their direction and form than mountains. They exist more frequently in groups than chains, and often appear as the last undulations of the mountainous surface. (Daubuisson, I. 108.)

Volcanoes occupy but a small portion of the earth's surface; but, in consequence of the grandeur of the phenomena they present, they have always arrested the attention of mankind. About 205 are known, including only those that have been active within a period to which history or tradition reaches. Europe contains thirteen or fourteen, and of the whole number, it is computed that 107 are in islands, and 98 on the great continents. Very few of these are in constant activity; the greater number sleep for long periods, and some of them for ages. In general their activity is in the inverse ratio of their size, and of those that are active, very few throw out lava, the greater number only emit smoke, or smoke and ashes.

In a volcano that has been in a state of repose, smoke appears before an eruption; subterranean noises are heard; the earth shakes; then ashes, sand, stones, are thrown out; at last, the melted lava runs over the crater, or bursts through the sides, after which the earthquake ceases. The columns of smoke consist chiefly of aqueous vapour, with carbonic sulphuric or muriatic gas. The ashes, which seem to be lava in a state of very minute division, have been known to be carried by the wind fifty or an hundred leagues. When deposited in great quantities, and impregnated with moisture, they form a volcanic tufa, which is sometimes solid enough to serve as building stone. The sands which form the major part of the ejected matter, and the

Physical Geography. principal mass of some volcanic mountains, consist of minute portions of lava which congeal in the air, or of the finer scoriae. They are mixed with crystals of augite and felspar. The scoriae consists of larger portions of the same matter, and has the appearance of the slag of forges. Along with these and the lava are often thrown out unfused fragments of the rock, which forms the walls of the volcano. The velocity of projection is thought to equal that of a cannon-ball, and the force is prodigiously great. Vesuvius throws up large stones 1200 yards above the crater, and Cotopaxi has launched a fragment of an hundred cubic yards (weighing not less than 200 tons) to a distance of three leagues.

The lava which flows out of volcanoes is a dark stony substance, resembling basalt, compact or porous, or with imbedded crystals. It is seen lying in the bottom of the crater in a state of incandescence, like melted metal in a furnace. It has a motion like boiling; and is seen to swell up from time to time twenty or thirty feet; as it rises, bubbles, some feet in diameter, form on its surface, and explode with a noise like a clap of thunder, bursting into a thousand fragments, which are accompanied with smoke and sparks. It then sinks in silence, to rise again in a little time. These alternate swellings and subsidings are evidently produced by the disengagement of elastic fluids. The force of the fluids, however, is rarely sufficient in the large volcanoes to raise the lava to the surface; and hence, in such cases, it generally happens that the flank of the mountain is burst through by the pressure of so vast a column, and the lava escapes laterally. Of ten eruptions of Mount Etna, nine have been by the flank, but Vesuvius, and other small volcanoes, habitually throw out their lava by the crater. On the other hand, the gigantic volcanoes in the Andes, having their sides compressed by the immense mass of the Cordilleras, are never known to throw out lava now in either way, though lava is found in the country. Scoriae and ashes, however, are still ejected. The current of lava descends in general slowly, but sometimes with a velocity of five or six miles an hour. The surface cooling first, forms a sort of tube or tunnel, under which the liquid mass is sometimes seen to flow upwards, in consequence of the pressure of the column behind. These tubes, and inclosed portions of gaseous fluids, often form caverns. When the lava first issues out, it is very fluid, but becomes more viscous as it cools. By means of its solid crust, however, it preserves its heat an astonishing length of time. Spallanzani saw a piece of wood take fire in lava three years and a half after it was thrown out, and at a distance of two leagues from the crater. It has been seen flowing ten years, and smoking twenty-six years, after its ejection. The currents are of various magnitudes. At Vesuvius one was observed eight or ten yards thick, from 100 to 400 broad, and four-fifths of a mile long; but there have been others much larger. In Iceland, in 1783, there was a current twenty leagues in length, by four leagues in breadth. So vast is the quantity of ejected matter in some instances, that the mountain of Jorullo, in Mexico, 1600 or 1700 feet high, standing

on ground which was formerly a plain, was thrown up by an eruption in 1759.

Physical Geography. Currents of water, accompanied with mud, sometimes descend from volcanoes. The water is believed to proceed generally from heavy rains falling on the exterior of the mountain, and occasioned by the sudden change of temperature round the crater; sometimes, however, from reservoirs in the interior; sometimes also, as indicated by the fish thrown out, from the water of brooks finding its way into the bowels of the mountain. Spallanzani thinks that part of the tufas of Italy owe their origin to eruptions of mud. In Cotopaxi, which rises above the line of perpetual frost, the melting of the vast masses of snow, when ignition takes place, produces most destructive torrents, one of which swept away a village thirty leagues from the crater.

The name of Air or Mud Volcanoes has been given to certain spots where gaseous fluids, or mud, or water (generally salt), issue out of the earth, but these have no connection with subterranean fire. At Macalouba, in Sicily, large bubbles of gas, consisting chiefly of hydrogen, formed at some depth, rise to the surface, where they explode, and scatter the soft mud in which they are generated round the opening. Near Modena, there are a number of cones from which, at times, hydrogen gas issues, with explosions which resemble small earthquakes, while stones of several hundred weight are launched to the distance of some yards, and currents of mud are vomited forth 1000 yards in length. In some cases, the gas issues pure from chinks or caverns, and, taking fire, exhibits columns of flame 100 yards in height. The fire-worshippers in Persia built their temples over these fountains of flame; and the Chimera, with many other fables of the Greeks, had their origin in the same natural phenomena. Eruptions of mud and gaseous exhalations have been observed in the Crimea, in Java, in the American province of Carthage, and many other places.

In the district of the Cévennes, in the south of France, there exist above 100 conical hills, which are known to be volcanic by their form and composition, consisting of mixed masses of lava, scoriae, and volcanic stones, and exhibiting in many places very distinctly the appearance of a crater. But they have never been in a state of ignition within the periods to which history reaches, and are therefore considered as extinct volcanoes. Many mountains in other countries present the same volcanic character, though they have never been known to produce eruptions.

Recent observations strongly favour the opinion that many of the trap rocks are the productions of subterranean fire. But if we confine our view to those mineral masses which are recognized as volcanic at present, they form but a very trifling part of the outer crust of the globe. Their formation is obviously posterior to that of the other solid rocks, for they are always found at the surface, and never enter into the frame-work of the earth. Vesuvius has, perhaps, covered with its ejected matter twenty square leagues, and Etna one hundred. The 200 existing volcanoes, estimated according to this scale,

besides elevating their respective cones, may have spread a coat of lava and scoriae, probably an hundred yards thick, over 20,000 square leagues, that is, over the twelve hundredth part of the surface of the globe. It may, therefore, be safely held, that volcanoes, as they now exist, have exercised no general influence on the state of the globe.

A satisfactory theory of volcanoes is yet wanting; but some progress has been made in generalizing the phenomena and circumstances connected with them which, when fully known, will certainly conduct us to the knowledge of their causes. First, water seems to be a necessary agent in the production of volcanic fire. Columns of aqueous vapour ascend from volcanoes; currents of salt water sometimes flow out of them, and heavy rains increase their action. It is only extinct volcanoes, like those of Auvergne, that are found far inland; the others are always at no great distance from the sea; the most active are in its immediate vicinity, and some are actually submarine. The matter which feeds them does not seem to be universally diffused, but rather collected in particular spots. Hence, they almost always exist in groups. Thus, Sicily and the south of Italy present us with one group, Iceland with another, the Archipelago with a third, the Canary Islands a fourth, and the great cones of Cotopaxi, Tungurahua, and Pinchincha, in Quito, a fifth. This proximity of situation in the members of the same group seems to arise, not from any subterranean communication, but from the matter which feeds them being distributed over the space they occupy, and accompanied with the presence of the agents that excite combustion. Thus, the action of one of the volcanoes belonging to the same group, such as Etna, Vesuvius, Stromboli, is found to be entirely independent of that of the others, one being asleep while another is active, and eruptions of one producing no change in the others. The arrangement of some of these groups is perhaps most consistent with the supposition of the matter being disposed in beds; but that of others, such as the volcanoes of Mexico, and the extinct volcanoes of Auvergne, which are ranged in strait lines, rather suggest the idea of the matter being lodged in veins. The distance of some volcanoes from the sea or from large lakes, with which analogy leads us to conclude that they have a necessary connection, the length of time they have burned, and their being the focus of earthquakes, which are felt over a great extent of surface, show that the fire is seated at some distance under the surface; on the other hand, we have every reason to believe, that wherever the source of the heat may be, the erupted matter does not come from a very great depth in the bowels of the earth. The quantity of lava is too trifling to be thrown up by a central fire, or to restore the equilibrium of nature, after it has been disturbed by convulsions so dreadful as those which precede an eruption must be, if their seat is really at a very great depth in the solid mass of the globe. Neither the supposition of beds of coal, nor of pyrites, in the interior of the volcano, explain the origin of the carbonic generated so abundantly and maintained so long. Perhaps the violent heat produced in some chemical processes,

as when the pure metallic bases of potass and soda, are brought into contact with oxygen, chlorine, or water, may give us an idea, though only in the way of analogy, of the source whence subterranean fire is derived. (Daubuisson, I. 160, 192, 208, 218.)

In an article like this, we cannot attempt a detailed description of the physical features of each country, and must therefore restrict ourselves to a very general survey of the two great continents, the old and the new.

If we consider the old continent attentively, we shall find that its general form, the declivity of its surface, and the course of its rivers, are chiefly determined by one great zone of mountains which traverses it from one extremity to the other, at the mean latitude of 40° north. This Alpine girdle has its origin on the shores of the Atlantic between the parallels of 30° and 42°, from which, in several chains, under the names of Atlas, on the south, and the Pyrenees, Alps, Mount Hemus, on the north, it passes into Asia; and there, under the names of Taurus, Caucasus, and Elbourz, it is continued eastward to the 70th degree of longitude. At this meridian it divides into two branches, one of which, the Himalayah range, takes a direction south-eastward, and terminates within 400 or 500 miles of the Gulf of Bengal; the other, Mount Altai and Yablonoun, passes north-eastward till it strikes the Pacific Ocean at the latitude of 55°. Its entire length to east longitude 140 is 8000 miles. Its breadth varies from 500 miles to 2000. The Pyrenees, Alps, and Mount Hemus, ought evidently to be considered as members of one great group with Mount Atlas. They rise under the same meridian, and have a corresponding direction; they are not separated by open plains, but by a long and narrow inland sea, which, from its steep shores, and great but irregular depth, has exactly the character of one of those profound lakes that occupy the bottom of valleys between the parallel ridges of the same mountain chain. The Sierras of Spain, the high ridge of Corsica and Sardinia, the Italian Appenines continued through Sicily, and the mountains of Southern Greece, form so many transverse branches connecting the great southern and northern chains of the band. Perhaps most of these transverse branches, where they meet the waters, are continued by submarine ridges. At least this is the case with the connecting branch of the Appenines and mountains of Sicily; for, though the Mediterranean generally is too deep for soundings, there is no more than 100 fathoms anywhere between Sicily and Cape Bon. Through the mountains of the Morea, and Crete, and Mount Rhodope, the African and European chains are connected with Mount Olympus and Taurus. The Carpathians and Erzgebirge are dependant chains of the Alps. Mount Taurus, reinforced by the chain of Lebanon, turns north-eastward round the sources of the Euphrates, where it unites with the lofty group of Caucasus. From this the principal chain passes round the bottom of the Caspian Sea, under the name of Elbourz, including the high peak of Demawend. A collateral branch passes south-eastward, along the basin of the Tigris,

Physical Geography. the north side of the Persian Gulf and Indian Sea, till it sinks into the sandy plains at the mouth of the Indus. Between these two run various ridges in different directions, but generally with a certain degree of parallelism to the outer chains, the whole of which give the character of table land to the surface of Persia. About 70° of east longitude the great band parts into two branches, one of which, in several chains, runs south-eastward, forming a lofty barrier to Hindostan on the north, and giving birth to the Indus, the Ganges, and the Burhampooter. The other, the Altai branch, proceeds north-eastward to the sea of Otchosk, sending off several small branches on both sides. From Uda, in latitude 55°, it skirts the sea to Behring's Straits. These two great branches support the high plateau of Thibet, within or near which all the largest rivers of Asia have their source.

Height. The height of this great Alpine zone is various. Mount Atlas, according to Chenier, is covered with eternal snow, and must therefore exceed 10,000 feet in elevation. The Sierra Nevada, in the south of Spain, is 11,660 feet high; the loftiest summit of the Pyrenees is 11,283 feet, of the Alps, 15,646 feet, of the Carpathians, 8640 feet, Mount Etna, 10,963 feet, Mounts Orbelus, Olympus, and Parnassus in Greece, from 6000 to 9000 feet.* With regard to the Asiatic mountains, very few of their heights have been measured. We know generally that Mount Taurus, in the south of Asia Minor, Mounts Caucasus, Ararat, and Demawend, reach the limits of perpetual snow, and must therefore have an altitude exceeding 9000 feet. But the most elevated part of this mountain band is the Himalayah, the loftiest summits of which, according to the estimate of Captain Blake, rise to the height of 25,000 and 28,000 feet above the level of the sea, and are therefore the most elevated land on the globe. (Edinburgh Philosophical Journal, No. X. 408.) The Altai chain is but little known. Various circumstances show that it does not generally rise to a great height above the surrounding country, but it stands upon a very elevated base, and the extreme rigour of the climate proves that its absolute height is very great. Taking the whole Alpine band together, we shall probably not err far, if we reckon its mean height in Europe from 4000 to 9000 feet, and in Asia from 5000 to 14,000.

Apart from this principal system of mountains, we have two small and separate chains in the northern part of the old continent. These are the Dofrines, 1000 miles long, with a mean height from 4000 to 6000 feet, and of which the mountains of Scotland and England may be considered a detached portion; and the Urals, 1400 miles long, and of a moderate but unknown height. Both of these chains run south and north, or nearly at right angles to the grand central band. The Dofrines have a system of declivities and rivers dependent on them, embracing the whole peninsula of Scandinavia. The Urals seem to produce almost no effect on the general direction of the surface and of the rivers in the great northern plains.

Physical Geography. This great Mediterranean band of mountains may be considered as the spine of the ancient continent. It determines the direction and elevation of the surface, over nine-tenths of Europe and Asia, and one-fifth of Africa, the course of all the great rivers in the old world, except the Nile and the Niger, and in some measure the climate of the different regions. It incloses within its extreme branches Spain, Barbary, Italy, Switzerland, Southern Germany, Hungary, the Mediterranean Isles, Turkey in Europe and Asia, Persia, Bucharia, Thibet, and Chinese Tartary, all of which countries consist either of table land, or of valleys placed between the different chains. The surface of this mountainous zone occupies a space of 5,000,000 square miles, and embraces Persia, Phœnicia, Assyria, Asia Minor, Greece, and Italy, all the early seats of civilization. Spain consists of a succession of valleys from 500 to 2500 feet high, divided by ridges, and in the central parts, especially in the basin of the Douro, approaching to table land. Southern Switzerland and the Tyrol present us with profound valleys from 1000 to 3000 feet above the level of the sea. Northern Switzerland, Wirtemberg, Bavaria, and Austria, consist chiefly of irregular plateaux from 1000 to 2000 feet high, broken by mountain ridges. Italy embraces merely the declivities of the Appenines, Barbary those of Mount Atlas. Greece consists of narrow littoral declivities, and basin-shaped valleys; and the Mediterranean Isles are merely the summits and sides of marine mountains. Bohemia is a circular valley, the bottom of which is probably 500 or 600 feet high; and Hungary a still larger and more level plain, whose bottom, according to Humboldt, is not more than 200 or 250 feet above the Black Sea. Asia Minor is an irregular elevated plateau, fenced on the south by Mount Taurus, and declining generally to the north. Persia, according to Olivier, is also a high plateau, supported on all sides by mountains, and depressed in the middle, so that the waters never penetrate the external barrier, but flow inwards, and stagnate, or disappear by evaporation. Thibet is a plateau of the same kind, but still more extensive and more elevated, forming, in truth, the summit of the Asiatic plains, and the most extensive and lofty range of table land on the globe. From a single fact respecting its climate, Humboldt has inferred that the height of this great plain is from 9000 to 10,000 feet above the sea. Thibet, with the desert of Shamo, which is merely its continuation to the north-east, are supposed to comprise a surface of 1,500,000 square miles. From the little that is known respecting these regions, they are believed to consist of level sandy or stony plains, diversified by mountains of moderate height, and by pastures with inconsiderable streams, which generally lose themselves in salt lakes or marshes. In the eastern part, the country of the Kalkas, though it rises perceptibly from China, the elevation cannot be nearly so great as Mr Barrow supposes, for some of the

* Notes to Leslie's Geometry. Articles EUROPE and GREECE, in this Supplement.

Physical Geography. hills in latitude 45° and 50° are covered with wood, which does not grow on Caucasus (in a corresponding latitude) at more than 6400 feet of absolute height.*

Great Northern Plain. The region on the north, exterior to this great mountain zone, but subordinate to it, is remarkably simple and uniform in its character. Commencing from the eastern shores of the North Sea and the Baltic, it extends in one vast plain, unbroken by a single chain of mountains, except the Urals, to the North Pacific Ocean. This plain, the largest on the globe, including generally the whole space between the 50th and 70th parallels, has an average breadth of 1400 miles, and a length of about 6000, and comprehends an area of 6,500,000 square miles, or rather more than one-fourth of Europe and Asia. It embraces the western part of France, all Holland, Northern Germany, Prussia, and the whole of Russia. Of its western division, that part which comprises France has considerable inequalities of surface, coasts generally rocky, and a good soil, which declines to the west. From Holland to Jutland it is chiefly a heathy or marshy steppe, very low and level, and with flat shores. From Mecklenburg to the Gulf of Finland, sands are more abundant than heath, the shores are flat, the surface, which is low and abounding in shallow lakes, declines to the Baltic for a breadth of 250 miles. From the longitude of 30° to the eastern extremity of Asia, a vast plain extends, one-fifth of which declines to the Black Sea and Caspian, the other four-fifths to the Frozen Ocean. Between the parallels of 50° and 60° the soil is generally capable of culture, and in many places rich, but it is intermixed with extensive sandy deserts impregnated with salt, and abounding in salt lakes. There are large forests, but the surface is, for the greater part, little wooded, presenting extensive open pastures, which are denominated steppes. Beyond the 60th parallel the ground is generally frozen during the whole year, and incapable of culture, but produces some low and stunted wood as far as the parallel of 65° or 67°, and grass or moss to the borders of the Frozen Ocean. The rivers eastward of longitude 55° have all their courses northward; they have little declivity, and are navigable almost to their sources, for the few weeks they are open.†

Zone of Sands. South of the great central band of mountains, we have an immense zone of sandy deserts, 900 miles broad, and 4500 long, extending between the parallels of 18° and 31° north, and between the west coast of Africa, and the mouth of the Persian Gulf. But, in reality, the sandy zone includes also the eastern part of the great Alpine girdle. It is, therefore, more accurate to consider it as extending across the African continent in a band of 13 degrees in breadth. From the Red Sea it turns a little to the northward, and in the form of a truncated triangle, resting upon this sea as a basis, it reaches obliquely across the continent of Asia to the 50th degree of latitude, and the

120th of longitude; including northern Africa, Arabia, Persia, Cabul, Bucharia, Sind, Thibet, and the western part of Chinese Tartary; and embracing an area of 6,500,000 of square miles, or nearly one-fourth of the two continents through which it passes. This tract contains many mountains, and some fertile valleys, but it is characterized by vast desert plains, formed of very light moveable sands, which assume the form of waves,—by burning and pestilential winds,—by an extraordinary aridity and want of rivers,—and by an abundant formation of salt, sometimes deposited like a crust on the surface, sometimes mixed with the inferior soil. Except the Indus and the Oxus, there is not a river of any size within this immense region, which is twice as large as Europe. The Sahara, or western desert of Africa, the peninsula of Arabia, and the great Plateau of Thibet, present the most continued surface of sand. Oases, or fertile spots, are found wherever the springs break through the upper sandy stratum. These oases are comparatively numerous, and extensive, in the eastern part of Africa; while in Persia, Cabul, and Bucharia, the surface is diversified with green mountains, which give birth to fertilizing streams. Of the whole sandy zone, however, it is probable that three-fourths consist of irreclaimable deserts.

South from the Sahara in Africa lies a comparatively fertile tract, the basin of the Niger, the Senegal, the Gambia, and the upper basin of the Nile. This is bounded on the south by the ridge of Jebel Kumra, a mountain of moderate height behind the coast of Guinea, but rising to the region of the perpetual snow in Abyssinia, if the two are really parts of one chain. Little is known of the great southern extremity of Africa, extending from these mountains to the Cape of Good Hope. From the elevation of the coast, and the smallness of the rivers, it may be conjectured to consist chiefly of table land, and to be distinguished by its aridity.

From the head waters of the Ganges, a low and level tract extends to its mouth, and from this to Cambodia, a series of long valleys opening into marshy plains at the shore. The triangular peninsula of India, and the rich country of China on the opposite sides of this tract, are covered by considerable mountains, and have the character of table land. The group of islands which extend from China and Malacca to New Holland are in general full of elevated mountains, many of which are volcanic. New Holland has some of its shores low, and some elevated, but it is not known to possess any very high mountains, or any large rivers. Of the innumerable small isles scattered in the Pacific, the greater part are merely the summits of coral reefs.

The New World forms two great continents, united by a neck of high land. South America consists of one vast expanse of surface of small elevation, everywhere protected on the west by the great

* See Barrow's Travels. Gerbillon's Journies in Histoire Generale de Voyages, Tom. IX.
† Walckenaer, Cosmol. 408, 412. Sauers's Narrative. Pallas's Travels.

Physical Geography. rampart of the Andes. These mountains, which pass along the western coast, at the distance of from 50 to 150 miles from the sea, rise, in one of their summits, to the height of 21,440 feet, and in Peru have a mean elevation of about 12,000 feet. There are three small transverse ridges, one in Caraccas, at the latitude of 8° or 9° north; another, which divides Guiana from the basin of the Amazon; and a third, which parts from the Cordilleras in south latitude 18°, proceeding eastward, and spreading out into a range of table land as it approaches the eastern coast. All these transverse chains are of small elevation. The low region of this continent is divided into three great plains, which form the basins of the three principal rivers, the Orinoco, the Amazon, and the Plata. In the basin of the Orinoco, the eye is fatigued by the unvaried aspect of a boundless level, uniform as the surface of the ocean, without a plant, or any object a foot in height, to break its monotony. Except on the borders of the rivers, these plains are destitute of trees. After the annual rains, they are clothed with a luxuriant herbage, which, during the heats of the dry season, is reduced to dust, and disappears. The soil then presents the aspect of a parched desert, in the chinks and fissures of which, the alligator and the great serpent remain buried amidst the dry mud, till they are awakened by the first showers. The Pampas of La Plata, which extend from 18° to 40° of south latitude, are plains of the same description. But the zone which divides these open plains, and forms the basin of the Amazon, extending from 6° or 7° of north, to 18° of south latitude, is one vast and continued forest. On the eastern side of the continent, this forest extends as far south as latitude 25°, and altogether embraces a surface of 20,000 square leagues. This middle region is also the highest, but so low are all the three, that, if the sea were to rise 50 fathoms at the mouth of the Orinoco and Plata, and 200 at the mouth of the Amazons, it would wash the eastern foot of the Andes, and submerge more than one-half of South America. The Llanos and Pampas afford pasturage to millions of cattle, and are, in truth, steppes like those of Southern Russia. There are no real deserts in South America, except a narrow tract of rock and quicksands on the coast of Peru between Coquimba and Lima, on which no rain ever falls. (Humboldt, Pers. Nar. IV. 292, 311.)

North America. The North American continent, like the South, is distinguished by one great chain of mountains, which traverses it from south to north nearly through its whole extent, leaving a large open level region to the east, and presenting a steeper and narrower declivity to the west. The chain of the Rocky Mountains, which ascend considerably above the inferior limit of perpetual snow, at latitude 46°, has probably an elevation of 8000 or 9000 feet. In a general point of view, these mountains determine the declivities of the soil, and the course of the rivers over nearly the whole continent. On the west side of the chain the slope is rapid, and the rivers, so far as they are known, flow directly to the Pacific Ocean, passing through a high, broken, interrupted chain,

which skirts the coast. On the east side, they bend their course to the nearest sea, over a surface little inclined, flowing to the north-east, and north in the northern parts, and to the south-east and south in the southern parts. On looking attentively at the rivers in the map, it will be perceived that the chain of lakes above Lake Erie, the upper Mississippi, the Missouri, the Arkansas, and Red River, all point in one direction, to the south-east, that is, at right angles to the general line of the coast, reckoning from Nova Scotia to the north-west corner of the Mexican Gulf. But, as if the barrier of the Alleghanies had been thrown up subsequently to the general level of the surface being settled, we find the St. Lawrence and the Mississippi, after running nearly parallel till they were within 500 miles of the coast, suddenly deflected from their south-east course, and proceeding to the sea in directions almost exactly opposite. The mean height of the Alleghanies is from 2000 to 3000 feet.

The woods are thickest on the eastern side of the Alleghanies. In the basin of the Ohio, which occupies the western declivity of these mountains, extensive tracts, called prairies, destitute of trees, but covered with herbage, occur. The declivities of the rocky mountains from the Mississippi to the Pacific Ocean are generally bare of wood, except along the rivers, and within twenty leagues of the sea-coast. In truth, the whole of this vast region consists of open steppes or savannahs, of a rich soil, and affording excellent pastures, but mixed with stony and sandy tracts, especially towards the Mexican frontiers. Salt springs are numerous, and in some districts the surface is incrustated with a saline efflorescence. The whole of the region east of the Rocky Mountains, from the 50th parallel to the Arctic Sea, is generally low, abounds in lakes, and is scantily wooded as far as the 60th degree of latitude, beyond which trees cease to grow. From the Gulf of Mexico to the mouth of Coppermine River, in latitude 67°, the country may be considered as one great plain, the summit of which, about the 50th parallel, is probably not 1000 feet high. The mean height of the great basin of the Mississippi and Missouri, Humboldt thinks, does not exceed from 500 to 800 feet. The interior of the country between Mackenzie's River and Behring's Straits, and between Hudson's Bay and the coast of Labrador, is entirely unknown; but the former is probably fertile and tolerably wooded, as the whole of the region west of the Rocky Mountains has a mild and humid climate. The late discoveries of Captain Parry and Captain Franklin have rendered it probable that the northern limits of the American Continent run generally along the 67th or 68th parallel, and that the space between this and the latitude of 78° or 80° is occupied by a numerous group of islands, of which Greenland may be considered a part. The isthmus which connects the two American continents consists of a prolongation of the chain of the Andes, of moderate height, and which, about the latitude of 18° north, spreads out into a vast plateau of table land, about 7000 feet above the sea. From this elevated base some detached volcanic mountains ascend

Physical Geography. 10,000, reaching an absolute elevation of 17,000 feet. (Warden's America, Chap. 31, 32, 33. Walckenaer, Cosmol. Chap. 14.)

New and Old Continents Compared. A comparison of the great continents of the Old and New World shows that the advantages are greatly on the side of the latter. All the interior parts of both the American continents have the advantage of being nearer the sea than the interior parts of Asia and Africa. There are in America no great deserts like those of the Old World, which not only withdraw a vast portion of the soil from the use of man, but, by the burning winds they generate, render the neighbouring regions scarcely habitable, and, by the wild beasts and robbers to which they afford shelter, oppose a more formidable barrier to the intercourse of nations than stormy seas or snow-clad mountains. But the grand advantage of the New World is its immense facilities for inland navigation, in consequence of the little elevation of its surface and its multitude of noble rivers. The St Lawrence, the Mississippi, the Amazons, and the Plata, penetrate the heart of the two American continents in a manner to which there is nothing analogous in the Old World. By means of these great streams, and their branches, there is scarcely a district of any extent, however remote from the sea, which has not a more easy and rapid communication with the rest of the world than the countries within 200 or 300 miles of the coast in the greater part of Asia and Africa. Mexico, the only considerable region in the New World without navigable rivers, has the advantage, by the narrowness of its territory, of being everywhere near the sea. We may add, as an accidental advantage of the New World, that its aboriginal inhabitants, from the smallness of their numbers, present almost no obstacle to the establishment and growth of a civilized population; while the Old World is too thickly peopled in most of its parts to admit of being easily colonized, and by tribes who discover no aptitude for civilization themselves.

The extent of the four great divisions of the world is as follows:

Rivers are natural drains which convey to the sea that portion of the waters falling upon the earth, which does not pass off by evaporation, or go to nourish organic bodies. They invariably occupy the lowest parts of the surface of the districts from which their waters are derived, and these districts are called their basins. The basin is bounded by high lands, which are sometimes mountainous. The water descending from these collects into brooks, the brooks unite into rivulets, the rivulets united form

the main trunk or river, which conveys the waters of the whole to the sea. All these descend over inclined planes, so that the lowest point of each brook is that where it joins the rivulet—the lowest point of the rivulet that where it unites with the main stream—and the lowest point in the whole system of inclined planes, is that where the river falls into the sea. These basins form natural divisions in physical geography. Thus the basins of the Rhone, Garonne, Loire, Seine, and part of the basin of the Rhine, comprehend nearly all France; Northern Italy consists chiefly of the basin of the Po, and Bohemia entirely of the basin of the Upper Elbe.

Physical Geography. The form and appearance of river courses lead to the conclusion that their channels are generally the work of their own currents. We never find them flowing in cavities which retain their natural shape, but always in beds cut below the adjoining surface, and corresponding to the quantity of water; the bed of the main stream being larger than those of the great branches, and the beds of the great branches larger than those of the small. They do not accommodate themselves to the inequalities of the country, but flow near the surface in low plains, and cut through a high ridge when it comes in the way; preserving a pretty uniform rate of descent, however great may be the undulations of the superior soil. The deep ravines that have been cut through hard rocks in this way, by an agent that operates so slowly, impresses us with a conviction that immense periods of time must have elapsed since the operations commenced. Before this took place, the courses of rivers must have consisted chiefly of a series of lakes communicating by cataracts. The waters flowing down from the high grounds in one district would collect and form a lake, which would increase till the flood found a passage over the lowest part of the bounding ridge, and fell into a second cavity, in which they would again accumulate, till they were discharged by another cataract into the next natural hollow. By a twofold chain of operations these lakes and cataracts disappear. The bottom of the lake is constantly rising by the earth and gravel carried down by the torrents and spread over it, and the level of the waters is constantly sinking by the action of the stream in deepening the outlet. At length the lake becomes a dry level plain, with a gravelly bottom, and the cataract a deep cleft or ravine, through which and the plain above, the river flows with a uniform and gradual descent. Many of the haughs or holms seen on river courses have undoubtedly been lakes of this kind.

The surface of the globe everywhere presents traces of these changes. The celebrated passage of River Ecluse has exactly the dimensions and appearance of a channel cut by the Rhone itself, and exhibits marks of the action of the water far above the present surface. Three distinct basins are observed in the course of the Rhine. The lake of Constance occupies the first, the second extends from Basle to Bingen below Mentz, and the third from this to the sea; and each of these basins is separated from the others by a narrow rocky strait. In the Danube

Physical Geography. may be distinguished the basins of Bavaria, Austria, and Hungary, from each of which the river escapes by a mountainous defile. The celebrated defile of Tempe in Thessaly, the deep and rugged clefts by which the Potomac, Susquehannah, and Delaware, penetrate the barrier of the Alleghanies, all bear decisive marks of the action of the stream. In many cases, the subsidence of the water can be traced from one level to another by terraces of gravel left on the sides of the mountains. In the valley of the Rhine these are seen to the number of four or five, at the distance of twenty, thirty, or forty feet below one another, and from them Professor Playfair ascertained that this river had once flowed at the height of 360 feet above its present bed.*

In the greater number of river courses we see those changes completed, but in the chain of North American lakes on the St Lawrence, we see them still in progress. On the rocky sides of some of these lakes Mackenzie observed marks of the action of the water considerably above its present level. The Niagara falls have been observed to recede about eighteen feet in thirty years, by the water's wearing away the limestone rock. This rock forms a large level platform, from the outer edge of which, six miles below the falls, the cataract has receded, and worn out a deep channel of that length by the constant attrition of its current. Had the surface of this rock dipped rapidly to the southward, the lake would have been emptied long ago. The fact is interesting in another point of view. Supposing the rate at which the rock is wasted away by the stream (eighteen feet in thirty years) to be well ascertained, and to have continued uniform, since the fall has gone backwards six miles from the position it must have originally occupied, it may be inferred, from calculation, that 50,000 years have elapsed since the waters of the St Lawrence began to flow. If we could depend upon the smaller unit—the annual waste of the rock, we should consider this as a tolerable criterion for estimating the antiquity of the present physical arrangement of the earth's surface. (Hall's Travels, p. 235.)

Alluvial Deposits of Rivers. Where rivers which pass through low and level tracts are subject to annual inundation, the earth, sand, and gravel they bring down, deposited most abundantly on their banks, raise them gradually above the surrounding country, while a part of the matter carried to the sea enlarges the coast, or forms sand or mud banks which rise, by degrees, above the water. It is thus that the Ganges, Po, Nile, Mississippi, and many other rivers, flow on the top of ridges. During floods, the elevated sides are sometimes burst through, and the waters which escape either stagnate in lakes, or return into the main stream lower down, forming islands, or travel to the sea by a separate mouth, and form a delta. Hence the Amazons, Orinoco, Mississippi, Nile, Danube, Wolga, Ganges, abound in islands, and have deltas from 50 to 200 miles in breadth, at their

Physical Geography. mouths. In the tracts of new ground which are thus gained from the sea, the soil may be seen of every degree of solidity, from soft mud to firm land. In the broad plain at the mouth of the Ganges, between the Tipperah hills and Bardwan, wherever the ground is penetrated, no virgin soil is found, but an alluvial deposit of sand and mud arranged in strata. The finest soil transported to the sea renders the water muddy to the distance of twenty miles from the coast. The Mississippi is supposed to add about two leagues to the coast in a century, or 300 feet per annum, at the point where its main stream falls in. (Warden's America, II. 492.) But the increase, if equally distributed over the whole delta, would not exceed two or three feet per annum. Volney calculated, but on very loose data, that the Nile had advanced four miles at Rosetta, and ten at Damietta, since the time of Herodotus; a quantity far too great, amounting to sixteen feet per annum over the whole delta. But Girard ascertained, that the annual floods of the Nile had raised the surface of Upper Egypt about six feet four inches (English) since the commencement of the Christian era, or four inches in a century. Now, as the sea deepens on the coast, at the rate of a fathom in the mile, supposing the deposit to have been as great along the whole shore as in the Thebais, the addition would amount to no more than one mile and a quarter since the time of Herodotus. The great error in all estimates of this kind has arisen from confounding the changes that belong to geological with those that belong to historical periods. The Po, which sweeps rapidly through a rich valley, transports a vast quantity of alluvion, and is found to encroach upon the Adriatic, at the astonishing rate of 228 feet per annum, at its two principal mouths. Adria was once upon the sea, and a circular space of twenty miles in diameter must have been filled up with solid soil since it was built. Assuming that 2500 years have elapsed since the building of Adria, and allowing the new ground a depth of twelve feet, it follows, that the river must have deposited 45,000,000 of cubic feet annually at its mouth, exclusive, perhaps, of as much more floated away by the sea, or spread over the banks higher up. Even this quantity of alluvion is a trifle compared with that of the Yellow River of China, which is 2,000,000 of cubic feet per hour, if Mr Barrow's estimate may be credited. Great as these effects seem to be, they sink into insignificance when compared with the whole extent of the ocean. Assuming that the mean depth of the sea is two miles, and that each district of land, of the same extent as the basin of the Po, furnishes an equal quantity of alluvion, it may be shown that it would require 400,000,000 of years to fill up the present bed of the ocean.†

All rivers are subject to occasional or periodical Periodical floods. Those of the Nile, observed in a country Inundations. where no rain falls, have been a subject of speculation since the time of Herodotus. Within the torrid

* Daubuisson, Geog. I. 105. Hall's Travels in America, p. 268, 346. Playfair's Works, I. lvi.

† Volney's Trav. I. 28. Edinburgh Phil. Jour. No. V. p. 53. Cuvier's Geol. Essay, Suppl. Barrow's Trav. 491.

Physical Geography. zone, these floods are produced by the annual rains, and occur during the summer months; but, beyond the tropics, they occur at various seasons, and in high latitudes, chiefly in spring, when the snow and ice melts. As the season of rains at each parallel within the tropics commences generally when the sun passes the zenith of the place towards the pole, and continues till he pass it again on his return, or a little later, the floods, of course, last longer as we advance farther within the torrid zone, and at the equator are perpetual. Hence, the Amazons, which derives its waters from both sides of the equator, may be said to be always in flood; but as its largest branches belong to the southern hemisphere, its greatest inundations are during our winter months. Those of the Plata take place during the same season, but are of shorter duration. These floods cover a vast but unknown range of country. The Orinoco begins to rise in May, overflows its banks in June, and returns into its bed in September, but, like the other tropical rivers, continues to fall till March or April next year. The Nile, which is really a tropical river, begins to rise in June, reaches its maximum height of 24 or 28 feet about the middle of August, and continues to fall till May next year. Its increase is irregular, two or three inches a day at first, and sometimes, when near its height, as much as three or four feet. The long valley of Egypt, from three to four leagues broad, with the delta at its extremity, is now covered by the water, except some elevated spots, and the higher parts of the river's banks, which rise above the adjoining surface. In Northern India, where the seasons depend chiefly on the monsoons, the Ganges begins to swell in April, when the rains fall on the mountains, and increase at the rate of one, two, or three inches a day, till it reaches the height of 15$\backslash frac\{1\}\{2\}$ feet before the showers have commenced in the plains. When the latter begin in July, the river rises at the rate of five inches a day, till it reaches its maximum of 31 or 32 feet, about the same period as the Nile. In the end of July, the low country, for a breadth of 100 miles, presents a wide expanse of water, whose surface is diversified by the trees that everywhere rise above it, and by the multitude of villages standing like islands, between which, the peasants are seen passing in boats. The rains cover the low tracts before the bed of the river is filled, and its banks, which rise above the rest of the country, appear like long mounds which separate the waters of the inundation from the stream of the river. The floods are computed to advance over the low region at the rate of half a mile per hour, and when at their height, cover a large tract of country to the depth of twelve feet. The floods of the Indus fertilize a district reaching thirty or forty miles on each side of its course. They attain their full height, like those of the Ganges, in August, but they begin rather later, and sooner, and are probably smaller. The celebrated Euphrates, which has its

sources in the high mountains of Armenia, begins to swell in March, reaches its extreme height in April, and, from this period till June, during which heavy showers fall every day at noon in Armenia, it remains nearly stationary. After June it falls rapidly. Its extreme rise is twelve feet in Mesopotamia, and so level is the country, that, with this small elevation, it covers nearly the whole plain between it and the Tigris, in the latitude of Bagdad. The Mississippi is lowest in October; in some of its remote branches the floods commence in February, with the melting of the snows, but they are not much felt in the low country till March, from which time they mount rapidly till June, and then fall again till October. From the junction of the Missouri to that of Red River, the great volumes of water thrown by the Mississippi over its western banks, spread out into shallow lakes, which are dried up by the autumnal heats, and become parched and solid plains. Below Red River, where the delta commences, the ground is a wide swamp, rising very little above the level of the tide. The only soil capable of cultivation is the long stripe of elevated land which forms the bank of the river, and is dry during eleven months. Consisting of fine travelled earth, it is luxuriantly fertile. Its breadth is small, sloping downwards as it recedes from the stream, and at a distance, varying from half a mile to a mile and a half, terminating in the swampy level. In the rivers that water the northern parts of the new and old continents, the floods, which almost invariably proceed from the melting of the snow and ice, are violent but transitory; and occur in March, April, May, or June, according as the sources of the river are farther from, or nearer to the arctic circle. In some of these rivers there are three successive floods,—the first in March or April, when the thaw takes place in the low country,—the second in May, when the thaw has reached the middle regions, and heavy rains fall,—and the third near the end of summer, when the glaciers begin to dissolve. Thus in the frigid, as well as the torrid zone, the season of floods is generally the four months nearest the summer solstice.*

As water cannot be heaped up where it is not confined on all sides, it follows that the floods in rivers will gradually sink as they approach the sea, and assume the form of an inclined plane at their upper surface. Thus the rise in the Mississippi in the height of the inundation, at the distance of 1000, 300, and 100 miles from its mouth, is 50, 25, and 12 feet respectively. The Ganges, which rises 31 feet at Cuscoe, rises no more than 14 at Dacca, and 6 at Luckipoor. The extreme height of the Nile, which is 24 or 28 feet at Cairo, is only 4 at Rosetta. From the great additional rapidity thus communicated, when the depth is only doubled, the discharge is often quadrupled. In the case of the Nile, where the depth is trebled, the discharge is probably augmented tenfold during the height of the flood. The

* Humboldt, Pers. Nar. V. Shaw's Trav. II. 221. Rennell, Phil. Trans. 1781. Kinneir's Geog. Mem. of Pers. 228. Morier's Journey in Persia, I. 342. Kerr Porter's Trav. in Babylonia, II. 404. Warden's America, II. 506. Darby's Geog. Descript. of Louisiana, 126, 236. Tuckey's Marit. Geog. I. 251.

Physical Geography. quantity of water delivered into the sea by the rivers between the equator and the parallel of 35° north, is perhaps as great in the single month of July, as during the eight months from September to April.

The following table, which we have prepared with considerable labour, exhibits some interesting calculations with regard to the most considerable rivers of the globe. The first column of figures gives their proportional lengths, measured always to their remotest branches, the length of the Thames being reckoned unity. The second column gives the area of their basins in square English miles; the third the proportional magnitude of those basins; and the fourth the quantity of water which each discharges, that of the Thames being unity. The last column, which is necessarily hypothetical, is constructed on the following basis. The fall of rain is found to bear a certain relation to the latitude, and is assumed to be equal (though it is not strictly so) along all the parts of the same parallel. It is further assumed, that of the rain which falls, a certain fixed propor-

tion is received by the rivers. It is evident, then, that, having the extent of the river basin, its mean latitude, and the depth of rain at that latitude, we have the elements for computing the proportional discharge of the rivers. In the ingenious article HYGROMETRY, in the Edinburgh Encyclopedia, the author deduces, from data partly theoretical, the amount of rain for each 5° of latitude. But, from Cotte's Tables, Captain Parry's observations, and various facts stated by Humboldt, we have been led to conclude, that the mean depth of rain at the equator is at least 10 inches more, and at the pole considerably less than given in the article referred to. On these and other grounds we have endeavoured to connect the results of observation by an empirical rule. Assuming the annual deposit of rain and dew at the equator to be 83 inches, and at the pole 8, we find the intermediate terms by the following formula:

75. (Rad.—Sine of Lat.) + 8 = depth of water in inches.

To deduce the approximate lengths of the rivers in miles from the proportional lengths, we may multiply the latter by 180, this being pretty nearly the distance between the remotest source of the Thames, and its mouth at the Nore, following the sinuities of the river. To convert the proportional discharge into known measures, we would multiply by 1800 to obtain the number of cubic feet per second, or by 4 (or 16), to find the annual discharge in cubic miles.

It is a question of some difficulty to ascertain

what proportion of the rain, snow, and dew, which falls on the ground, passes off by the rivers. Mr Rivers, Dalton, computing the annual deposit of moisture in England to be 31 inches of rain, and 5 of dew, concludes that 13 inches, or more than one-third, is drained off by the rivers. (Manchester Transactions, V. 359.) But this is founded entirely on a conjectural estimate, not an actual measurement, of the depth and velocity of the Thames, in the basin of which the fall of rain is only 22 or 23 inches at London and

Oxford. Since it is shown, however, by the observations of Dr Dobson, Dr Watson, and Mr Dalton himself, that the evaporation from water is about equal to the fall of rain,—that from a grassy plot in a dry season, it is sometimes 0.7 inches in a day, or $\backslash frac\{2\}\{3\}$ of what pure water yields,—and that from ordinary soils, 30 inches out of 38 escape by evaporation, leaving only $\backslash frac\{1\}\{3\}$ for the streams, there is surely more than a presumption that the rivers cannot carry off 13 out of 36 inches for England, and still less 13 out of 26 or 28 inches in the basin of the Thames. (Phil. Trans. Abrd., XIV. 137. Manchr. Trans. V. 359.) The Mississippi, which drains one of the largest valleys on the globe, was found by Mr Darby to have a breadth of 2400 yards not far above New Orleans, with a depth of 153 at high, and 130 at low water. (Geog. Descrip. of Louisiana, p. 126.) The section considered as a semiellipse is 280,000 square feet, and the discharge, with the mean velocity of one mile an hour, which he assigns to it, is 410,000 cubic feet per second, equal to 88 cubic miles per annum. As we have taken the high water level with the mean annual velocity, the difference of 23 feet may stand as equivalent to the loss of water by the bayous, or outlets, of Iberville and Atchafalaya, and the temporary lakes farther up. Now this discharge, distributed over the basin of the Mississippi, amounts to no more than four inches of water in depth. If we add one-half to it, on the supposition that his estimate of the velocity is rather low, even this will only raise the rain water to six inches upon the area of the basin. But the depth of rain corresponding to the mean latitude of the basin of the Mississippi ($41^\{\backslash circ\}$) is by the rule 34 inches, and at Cincinnati, in latitude $39^\{\backslash circ\}$, it has been found, by observation, to be 36 inches. This is exclusive of dew, and from what we know of the climate of St Louis, and of Columbia river, we have no reason to think that the average for the whole basin can be much less. (Warden's America, II. 237, III. 121.) We would be entitled, from these facts, to estimate the depth of the watery deposit at 40 inches; but suppose we reduce it to 30, the proportion drained off by the rivers amounts only to one-fifth. From the nature of the basin of this great river, which avoids all extremes, having a moderate elevation, and consisting neither of a continued forest, nor of a sterile desert, but of a mixture of the most common varieties of soil, it affords probably as fair a criterion as could be obtained for deciding a question of this kind. We think, therefore, that both theory and observation warrant the conclusion, that, in the case of large rivers flowing over extensive and tolerably level surfaces, the water carried to the sea does not exceed one-fifth of what falls in rain, dew, and snow. Where the basins are small, the surfaces steep and rocky, and where heavy rains continued for months, keep the soil in a state of saturation with moisture, the discharge may be greater. In other cases, where the basin is long and flat, the watercourse broad and shallow, the temperature high, and the soil sand or gravel, the loss of the river by direct evaporation, and by infiltration, through its bed and sides, may be very great, and does in some cases (as in that of the Platte) absorb the whole wa-

ter. The rule, of course, is not meant for extreme, but for ordinary cases. Founding then on the rate of discharge of the Mississippi, that of the Thames should be about 1800 cubic feet per second, or $\backslash frac\{1\}\{10\}$ of a cubic mile per annum, equal to nearly five inches on the surface of its basin. This may serve when it is considered as the unit of the scale, but, in point of fact, the discharge is probably larger, as the basin is small, and the depth of rain greater than belongs to the parallel in general.

If we estimate the mean fall of rain, not for the whole globe, but for the dry land, at $2\backslash frac\{1\}\{3\}$ feet, or 31 inches, it will amount to 25,500 cubic miles per annum, of which, according to our calculation, the sea receives, by the rivers, about 5100. Of this quantity, the Amazon discharges $\backslash frac\{1\}\{10\}$, the Plata $\backslash frac\{1\}\{25\}$, the Mississippi $\backslash frac\{1\}\{25\}$, the Ganges $\backslash frac\{1\}\{25\}$, the Danube $\backslash frac\{1\}\{100\}$, and the Thames $\backslash frac\{1\}\{100\}$ part. The water thus delivered into the ocean, if not counterbalanced by evaporation, would raise the level of the sea two inches in a year. It amounts to $\backslash frac\{1\}\{100\}$ part of the mass of the ocean, supposing it to have a mean depth of two miles. In other words, the water discharged by the rivers would fill the present bed of the ocean in 56,000 years. We may infer farther, that the greater part of the supply of water belonging to each region circulates between the atmosphere and the earth, suffering a loss of one-fifth every time it falls upon the latter, and receiving an equivalent addition from the vapours of the sea. The smallness of this supplementary portion, compared with the whole quantity, is a circumstance which consists well with the general regularity of the annual supply of moisture, and the stability of the order of the seasons.

As very large rivers, with numerous tributary streams, necessarily occupy the lowest situations in all countries, it follows that their courses have a very small declivity. The surface of the Amazons, at Jaen, 3000 miles from the sea, has only an elevation of 194 toises, which gives five inches per mile for the mean fall; and over the whole of this space, the navigation of that majestic river is not interrupted by a single cataract. In the last 200 leagues of its course, the inclination is believed not to exceed 11 feet, or $\backslash frac\{1\}\{10\}$ of an inch per mile. A series of tides, felt to the distance of 600 miles from its mouth, exists in its bed contemporaneously, following one another up the channel, and giving its surface the form of a wavy line. The Ganges, reckoning its sinuities, has only a fall of four inches per mile, from Hurdwar, where it leaves the Himalayah chain to the sea. Humboldt thinks the declivity in the lower course of the Mississippi is still smaller. The Wolga, from its source to the Caspian Sea, falls 957 French feet, or about five English inches per mile. The Nile, though it falls from a height of 10,400 feet at its head (barometer 22 inches), if Bruce may be believed, has a very small inclination in the lower part of its course. The celebrated cataracts of Syene, which were said by the ancients to produce endemic deafness, consist merely of ten successive steps of six inches each, dispersed over two-thirds of a mile, and which are capable of being passed by boats. There is still one singular phenomenon connected with large rivers, which has been little attended to.

Physical Geography. Their beds, at some places, like the lakes of mountainous regions, have a greater absolute depth than the adjoining seas. In the Amazon, above the Rio Negro, about 1000 miles from its mouth, Condamine found no bottom with a line of 103 fathoms. Now the height of its surface here could not be above 70 fathoms, and its bottom must, therefore, have been at least 100 feet below that of the sea on the coast. At a small distance above New Orleans, where the height of the surface of the Mississippi could not exceed 40 or 50 feet, Mr Darby found the depth to be 153 feet; and hence the bottom must have reached 50 or 60 feet below that of the shallow sea at its mouth.*

Lakes. Lakes are among those natural objects which contribute, in the highest degree, to the picturesque beauty of the earth's surface. Like the sea, they exercise a beneficial influence on the climate and soil, by moderating the extremes of heat and cold, and by diffusing humid vapours over the land. When they occur in the courses of rivers or at their fountains, they serve as back reservoirs, to equalize the discharge, and to prevent those sudden inundations which are often so destructive. Of all the great natural features of the globe which are subject to mutation, lakes are probably the least permanent. The waste of mountains, and the raising of the bed of the great ocean, are operations of which the effects can scarcely become sensible in millions of years, but in the filling up of lakes, striking changes are often produced within the lifetime of an individual; and within the periods to which history reaches, several have entirely disappeared.

Lakes are chiefly of two kinds; those which are formed in deep hollows between the ridges, or at the foot of mountains, and which are fed by springs or torrents; and those which are formed in low and level countries by the surplus water of rivers, or in consequence of the want of a general declivity in the ground. We have thus two grand systems of lakes in the old continent. The one accompanies the great Alpine girdle, and includes the lakes of the Pyrenees, Alps, Appenines, those of Asia Minor, Syria, and Persia, with the Caspian Sea, the Aral, Balkash, Baikal, and all the series of lakes found at the foot of the Altaic chain. The other begins at the low shores of Holland, and extends along the south-east coast of the Baltic and Gulf of Bothnia, and thence in smaller numbers along the Frozen Ocean to Behring's Straits. Except in central Africa, the regions south of the great mountain band, so far as they are known, contrast remarkably with those of the north, by the fewness of their lakes. A chain of lakes, but generally smaller than the mountain lakes of the old world, accompanies the Andes from their southern extremity, through the isthmus and Mexico, to their northern termination. In the level part of South America, as in the southern regions of the old world, lakes are comparatively rare; and the coincidence is no less striking in the north, where

the regions round Hudson's Bay present a multitude of lakes, corresponding in numbers, character, and geographical situation, with those which skirt the shores of the Baltic and Frozen Ocean.

Physical Geography. The Caspian, which is the largest lake in the world, and has really the character of an inland sea, ^plan. merits particular notice. Its length is 750 miles, breadth about 200; it embraces an area of 170,000 square miles, and receives the waters of a surface about five times as large. Its general depth is 60 or 70 fathoms, but, near the south end, no bottom has been found with a line of 380. It is salt, and, like the ocean, is subject to storms; and, either from the effect of winds, or the unequal discharge of the rivers, it is subject to irregular elevations, varying from four to eight feet. The Caspian is distinguished from all other lakes and seas in the world, by the remarkable lowness of its surface, which was found by Engelhardt and Parrot to be 334 feet beneath that of the Black Sea. The inhabitants, therefore, of Astracan, and other places on its shores, live at a lower level by 200 or 300 feet than any other people on the globe. (Tuckey, Marit. Geog. I. 457. Edin. Phil. Jour. No. VI. 408.)

Lakes are salt only when they are placed in districts where the soil contains saline matter, and their saltiness is invariably greater when they have no efflux. Most of the lakes of Europe are either fresh or slightly saline; but the Caspian Sea, the lakes Aral, Baikal, and others that accompany the Altaic chain, as well as those of Persia, being situated in the midst of plains full of salt, or in regions where salt springs abound, are almost universally impregnated with this mineral. The most highly saline waters known are found in lakes which receive streams but give out none. Thus the lake Ourmiah, in Persia, and the Dead Sea in Judea, receiving continual supplies by the streams from the neighbouring soil, which is every where full of salt, while nothing but fresh water is carried off by evaporation, the mineral accumulates, and their waters are found to contain, in the one case six, and in the other eight times as much salt as those of the ocean. (Edin. Phil. Jour. No. IV. 356.) When the quantity carried down by the streams exceeds what the water will hold in solution, the surplus is deposited in beds at the bottom; and hence the numerous layers of salt which are found mixed with thin strata of mud at the bottom of dried up lakes. Large shallow lakes, like that near Teheran, in Persia, which is 150 miles long, often lose their waters in the heat of summer, and become salt marshes, or plains, covered with a salt crust. (Kinneir's Geog. of Persia, 117.)

The depth of lakes in mountainous districts is often remarkably great. That of Loch Ness, on the line of the Caledonian Canal, is 130 fathoms in some parts, which is four times the mean depth of the German Sea, and its bottom is actually 30 fathoms below the deepest part of that sea between the latitudes of Dover and Inverness. The bottom of the

* Humboldt's Pers. Nar. IV. 310, 455, V. 57. Edin. Phil. Jour. No. V. 409. Rennell, Phil. Trans. 1781. Histoire Gen. des Voyages, Abr. XII. p. 330—380.

Lake of Geneva, at the depth of 161 fathoms, reaches from the high plateau which surrounds it to within 200 feet of the level of the Mediterranean. It is difficult to account for the existence of such deep lakes among mountains whose debris are spread abundantly over countries much lower; but perhaps the profound cavities discovered in the beds of great rivers, like the Amazons, may lead us to suspect that they owe their existence to the agency of debacles, or vast currents of water, at some recent geological period, but anterior to the existence of man.

The temperature of deep lakes presents what we should at first view consider as an anomaly. The heat of the waters at the bottom is never that of the containing solid strata, as we should expect, but in temperate climates always much lower. In Loch Katrine and Loch Lomond, Mr Jardine found the temperature about $41^\{\backslash circ\}$ at all depths below 40 fathoms, while that of the surface was $57^\{\backslash circ\}$ or $59^\{\backslash circ\}$, and the mean heat of the climate is $47^\{\backslash circ\}$. The deep waters in the Lakes of Thun and Zug have a temperature of $41^\{\backslash circ\}$ and $41\backslash frac\{1\}\{2\}^\{\backslash circ\}$, and those of the Lake of Geneva $42^\{\backslash circ\}$, though the mean heat of the climate is $49^\{\backslash circ\}$. In the Lake Sabatino, near Rome, where the annual temperature is about $60^\{\backslash circ\}$, the thermometer indicates $44\backslash frac\{1\}\{2\}^\{\backslash circ\}$ at the depth of 80 fathoms. This apparent anomaly in lakes is accounted for by the physical constitution of water. As this fluid attains its maximum density at $40^\{\backslash circ\}$, the upper strata, if cooled down to this degree, descend to the bottom; but if heated up ever so high, remain at the top. The cold impressions received at the surface are thus carried below by the internal movement of the water, while the hot impressions can travel down only by the conducting power of the fluid, which is remarkably small. The variable influence of the seasons is little felt below 15 or 20 fathoms, and disappears entirely at 40 fathoms. In all fresh water lakes, exceeding this depth, there must, therefore, be a constant tendency in the inferior waters to approach the temperature of maximum density. To a certain degree this must be controlled by the heat of the strata which form the basin, and this may perhaps account for the slight difference in the temperature of the deep waters in the Italian lakes and those of Switzerland and Scotland. (See the Art. CLIMATE.)

The ocean, though it presents to the eye only the image of a watery waste, sustains a most important part in the physical economy of the globe. It is the great fountain of those vapours which replenish our lakes and streams, which dispense fertility to the soil, and clothe the surface with all the pomp and pride of luxuriant vegetation. By its salutary action on the atmosphere, it tempers the extremes of opposite seasons and of opposite climates. It affords an inexhaustible supply of animal food, and of salt, a substance of the utmost value to human life. As the great highway of commerce, it connects the most distant parts of the globe, and affords the ad-

vantages of free and abundant communication to nations which mountains and deserts seem to have separated from each other. Its shores have been in every age the great seats of civilization; in all the great continents, rudeness and barbarism grow upon us as we advance into the interior; and it requires no great sagacity to discover that the central regions of Asia and Africa, from their want of inland seas like the Mediterranean or Baltic, or navigable rivers like the Amazons, will be the last portions of the habitable globe over which the arts will extend their empire.

The ocean, with all its inland bays and seas, covers an area of 145,600,000 square English miles, or nearly three-fourths of the surface of the globe. About $\backslash frac\{7\}\{8\}$ of the great body of waters lie in the southern hemisphere, and $\backslash frac\{1\}\{8\}$ in the northern. In the one, the ocean is to the land nearly as 7 to 5, and in the other as 13 to 2. Laplace has calculated that its mean depth is but a small fraction of the difference between the axes of the earth,* which is 25 miles. If, therefore, we suppose the mean depth to be two miles, the cubic contents will be 290,000,000 of cubic miles.

Geographers divide the entire mass of the ocean into five great basins. The Pacific, the largest, separates America from Asia; the Atlantic separates Europe from America; the Indian Ocean separates Asia and its isles from Africa; the Arctic or North Polar basin encompasses the North Pole; and the Antarctic the South.

The Pacific Ocean, 11,000 miles in length from Pacific east to west, and 8000 broad, occupies a superficial space rather larger than the whole mass of the dry land. From Cape Horn to the head of the Bay of Bengal a rampart of mountains, containing the highest chains in the world, is arranged round this sea, at a less or greater distance from its shores. But an inner and broken chain extends from Alyaska through the Aleutian Isles and Kamtschatka, Japan, the Philippines, Borneo, Celebes, and New Guinea, to New Holland; and this chain, with the Rocky Mountains and Andes, seem placed on one continued vein of igneous matter, for they include in their wide circle the most numerous and active volcanoes in the world. From the Sea of Otchosc to Cape Horn its coasts are closely girt by the great American and Asiatic chains, which leave only a stripe of low shore, too narrow to admit of deep gulfs or large rivers. But from the Sea of Otchosc southward, the great Asiatic chain recedes from the shores, and opens an extensive country, which inclines to the Pacific. Yet though this basin forms more than one-third of the whole ocean, it certainly does not receive more than one-eighth of the whole river water. On the western side and between the tropics, its surface is studded with innumerable groups of islands, all remarkably small, and consisting generally of coral reefs, rising up like a wall from unknown depths, and emerging but a very little above the

* A determination in more precise terms would have been desirable, but we take the statement as given by Daubuisson (Geognosie, I. 35), not having seen the Memoires of the Institute for 1818, to which he refers.

Physical Geography. These islands are the work of myriads of minute insects, whose incessant labours are thus gradually creating new lands in the bosom of the ocean. On the western side, it communicates with the inland seas of Otchoshk and Japan, the Yellow Sea, and the Chinese Sea. These and the inlets of California and Queen Charlotte's Sound we shall not stop to describe. The Pacific Ocean, in consequence of the wide expanse of its surface, is remarkably exempt from storms, except near its mountainous shores, and hence the name. Its small isles, in which the fervid heat of the torrid zone is mitigated by the presence of so vast a body of water, enjoy perhaps the most delicious climate in the world.

within this polar zone consists rather of clusters of islands than of a continent. What proportion of the space the sea and land respectively occupy, it is impossible to calculate. Of the many deep gulfs which open into this basin on the north coasts of Europe and Asia, the White Sea is the only one of any importance to navigation. The rest are shut almost constantly by the ice.

A detailed description of all the great inland seas Physical Geography. would be out of place in this brief sketch; but the mean.

Baltic and the Mediterranean, on account of their superior importance, merit a special notice. The Mediterranean, the finest inland sea in the world, is 2350 miles long, from 100 to 650 broad, and with the Adriatic, but exclusive of the Black Sea, embraces an area very nearly of 1,000,000 of square miles. Its depth, which is generally too great to afford soundings, diminishes to 100 fathoms between Sicily and Malta, and to 30 between Malta and Cape Bon in Africa. Placed in the midst of high mountains, its shores are steep and narrow; and, if we exclude the waters of the Nile and the Black Sea, the districts whose rivers it receives do not quite equal its own surface in extent. Adding 500,000 miles for the basin of the Nile, the space which may be called the river domain of the Mediterranean, amounts to 1,400,000 square miles. But its inland position gives this sea a proximity to a vast range of country; and, since vapour diffuses itself by its own elasticity, independently of the winds, were each portion of the adjoining land to draw its moisture from the nearest sea, so far as the natural limits of evaporation permit, we may calculate that the vapours of the Mediterranean would be diffused over a space five or six times larger than the basins of its rivers; and that it expends probably three times as much water as it receives. Its surface, depressed by this constant drain, is said to be 34 feet lower than the Red Sea; and hence, powerful currents rush in from the Black Sea by the Dardanelles, and from the Atlantic by the Straits of Gibraltar to restore the level. Its superior saltness is also accounted for by the constant influx of salt water from the ocean, while fresh water only is carried off by evaporation. Like all inland seas, which open to the west, it has no general tides; but local tides are felt, which rise three feet at Venice and at Marsala in Sicily, one foot at Naples, one to two at Toulon, and six inches on the Syrian coast. A current circulates round the line of its coasts, entering the Straits on the south side, setting along the African shore to Syria, where it turns north-westward, and, joined by the current from the Dardanelles, it makes the circuit of the Adriatic, then of the coasts of Tuscany, France, and Spain, and ultimately returns into itself at the Straits. (Tuckey, II. 122, 355.)

The Black Sea, and Sea of Azoph, are properly Black Sea. inland lakes (like Ontario and Erie), which discharge their surplus waters into the Mediterranean. These two seas occupy a superficial space of 170,000 square miles, and receive the waters of a surface more than five times as large as their own, or about 950,000 square miles. Hence they have a constant efflux: their waters are turbid with floating soil, and so fresh, that ice appears in the bays of the

Atlantic. The Atlantic basin extends from 70° of north to 35° and 50° of south latitude. Its length is about 8500 miles; its breadth, which in the latitude of 52° north is 1800 miles, and near the equator 2100, at the northern tropic spreads out to 5400, including the Mexican Gulf. It covers about 25,000,000 square miles, exclusive of inland seas, which is nearly one-half of the extent of the Pacific Ocean. Its southern division does not contain one single deep inlet, nor one island of any magnitude; while its northern division abounds in large islands, and in deep and numerous inland seas on each side, which penetrate far into both continents, and have rendered it the seat of the most extensive commerce in the world. Few large rivers fall into this sea on the east side, but on the west it receives the three largest rivers on the globe, the Plata, Amazon, and Mississippi.

Indian Ocean. The Indian Ocean extends between 40° south and 25° of north latitude, and from the Cape of Good Hope to Van Dieman's Land. Its length is about 4500 miles, its mean breadth is nearly the same, and it covers a surface of about 17,000,000 of square miles. Its shores are generally mountainous; it contains many islands, two large open bays, those of Bengal and Oman, and two deep inlets, the Persian Gulf, and Red Sea. A particular system of winds, called Monsoons, prevail in the northern part of this basin.

Antarctic. The Antarctic basin, which surrounds the south pole, joins the Pacific in the latitude of 50°, and the Indian Ocean in the latitude of 40°. It embraces an area of about 30,000,000 of square miles. This sea is generally covered with floating ice as far north as latitude 60°. The appearance of a fixed barrier of ice, filling nearly all the space within the Antarctic circle, has led geographers to infer the existence of a mass of land near the pole, but no land was seen by navigators in a higher latitude than 60°, till the discovery of New South Shetland in 1819. This country lies between 55° and 65° of west longitude, and reaches north to the 62d parallel, but how far it extends southward, and whether it is one large region, or a cluster of islands, is unknown. (See Edin. Phil. Jour. No. VI.)

Arctic. The Arctic basin, or Frozen Ocean, comprises a great part of the space within the 70th parallel. The discoveries of Captain Parry, and the very recent observations of Mr Scoresby in East Greenland have gone far to prove that the tracts of land

Black Sea, and covers the Sea of Azoph for four months every winter, though they are placed under the same parallels with Italy and the south of France. The channel of the Thracian Bosphorus, from 600 to 2000 yards wide, has undoubtedly been excavated by the current, which flows at present with a velocity of from three to five miles an hour; and traces of a connection between the Sea of Azoph and the Caspian exist in the low sandy tract along the course of the Manitch, which abounds in salt lakes, and marine exuviae.*

The Baltic, 1200 miles long, embraces an area of 175,000 square miles, including the Cattegat. It occupies the bottom of a natural basin, which extends from the Dolrines to the Erzegebirge, and the heights of Valdai, and it receives the waters of a surface nearly five times as large as its own. Hence, like the Black Sea, it has an efflux current, and its waters, which are remarkably fresh, are partially or entirely frozen over, for three or four months in its southern parts, and for five or six in the Gulfs of Bothnia and Finland. The tides advance no farther than the three passages of the Sound, and the Great and Little Belt, which have a breadth of $1\backslash frac\{1\}\{2\}$, of 8 and of $\backslash frac\{1\}\{2\}$ miles respectively, with a depth varying from 19 to 27 fathoms. The mean depth of the Baltic is 60 fathoms. Its currents, which vary with the state of the winds, and the freshes of the rivers, begin in the Gulfs of Bothnia and Finland, and set outwards on both sides, through the Sound and Belts. At the mouth of the Baltic, as well as that of the Mediterranean, an under current has been suspected to exist in an opposite direction to the upper, and probably smaller in magnitude. The difference of specific gravity between the Atlantic waters and those of these two seas render the existence of the under current credible. (Tuckey, I. 211.)

The other most remarkable inland seas are, in Europe, the North Sea, 160,000 square miles in extent, reaching from Calais to Orkney. Its mean depth is 31 fathoms, its greatest 190. It is deeper at the north than the south end, and at the sides than in the middle, where vast sand-banks cover its bottom. In America, there are, on the west side of the Atlantic, the Gulf of Mexico, 580,000 square miles in extent, whose western shores are remarkably destitute of ports, and are covered by annually increasing sand-banks. Farther north are the Chesapeake Bay, 200 miles long; the Gulf of St Lawrence, shut in by Newfoundland; Hudson's Bay, in the same latitude with the Baltic, but twice as large, and loaded with ice even in summer; Baffin's Bay, almost constantly shut by the ice, communicating, we have reason to think, with the Polar Sea and Hudson's Bay, and surrounded chiefly by large islands.

In the Indian Ocean the only deep inland bays are the Persian Gulf, 600 miles long, with parched sandy shores; and the Red Sea, 1500 miles long, which abounds in coral reefs, and, unlike every other sea in the world, does not receive a single river. It has considerable tides, and, as might be expected, a

current inwards from the ocean. It has a particular system of winds, which blow always either up or down its channel. (Tuckey, III. 67, 88.) The inland seas in the Pacific Ocean have already been briefly noticed.

The temperature of the ocean, like that of the land, varies with the latitude; but the variations, whether depending on the situation or the season, are less great, less sudden, and less frequent. The diurnal heating power of the sun is spent in the one case on a sheet of water ten or twelve feet in thickness, while, in the other, it does not penetrate more than one or two inches into the land. The mean temperature of the sea at the equator, according to Humboldt, is from $78^\{\backslash circ\}\; 8\text{'}$ to $81^\{\backslash circ\}$; it is tolerably uniform for every meridian as far as $28^\{\backslash circ\}$ of latitude; and diminishes at a corresponding rate in both hemispheres as far as the 48th parallel. Beyond this the cold is greater in the southern hemisphere. From the effect of currents, however, and the want of continued registers, it is difficult to state numerically the temperature of each latitude. The recorded observations of navigators exhibit striking anomalies. In general, the temperature of the ocean at the surface does not adapt itself to temporary changes, or to the diurnal variation, but follows the mean monthly temperature of the parallel, without, however, reaching the extreme points to which this temperature goes in the opposite seasons. This may be inferred from the phenomena of insular climates. (Humboldt, Pers. Var. II. p. 50, 65, 87.)

In the ocean, as in lakes, we find the water colder as we descend farther below the surface. This decrease of heat is neither uniform for the same parallel; nor does it bear any constant relation to the depth; nor are its variations capable of being connected with the distance from the equator or the pole. At the depth of 100 fathoms, the difference is sometimes no more than $1^\{\backslash circ\}$, and sometimes so great as $20^\{\backslash circ\}$; and this, too, when the heat at the surface was about the mean annual heat for the parallel. Sometimes the coldness attains its maximum at 100 fathoms, and sometimes it increases to 400 or 500. Humboldt thinks that, on a mean, the change is about six times more rapid than in the atmosphere, or about $1^\{\backslash circ\}$ Fahrenheit in 50 feet; but the facts are too anomalous to be easily brought under any general rule. Perhaps they agree best with the supposition that strata of different temperatures and densities, preserving themselves pretty distinct from each other, move slowly in different directions in the inferior waters; while the lowest and coldest, gliding over the unequal bottom, descend to greater depths, or mount nearer the surface, according as the absolute depth of the sea is greater or less. Hence the coldness of the waters on sand-banks and shoals; and hence also the changes produced in the temperature of the sea by storms, which mingle the water of different strata. The law that connects these changes, when rightly understood, may perhaps afford a measure of the depth for parts that cannot be sounded. To the coldness of the inferior strata, however, there is one

Physical Geography. remarkable exception. It was ascertained by Mr Scoresby, and is confirmed by the observations made in the late polar expedition, that in the Greenland sea the temperature increases with the depth. The increase of heat does not follow any determinate ratio, being from $1^\{\backslash circ\}$ to $5^\{\backslash circ\}$ in 100 fathoms; but it is constantly progressive, and at the depth of 730 fathoms (lat. $79^\{\backslash circ\}$, long. $5\backslash frac\{1\}\{2\}^\{\backslash circ\}$ E.), it was found to be $8^\{\backslash circ\}$. In fact, the bottom waters at Spitzbergen, as Mr Scoresby observes, are from $16^\{\backslash circ\}$ to $20^\{\backslash circ\}$ above the mean temperature of the climate. The most plausible explanation of this anomaly is, that the westerly current from Nova Zembla, rendered specifically light by the fresh water of the great rivers of Northern Asia, may rise above the branch of the Gulf Stream which penetrates to these latitudes; and the warm waters of the latter will thus be found at the bottom. The specific gravities observed by Mr Scoresby, however, do not tend much to confirm this hypothesis.*

Ice. A permanent zone of ice surrounds each pole, the breadth of which varies with the seasons. On the west side of the Atlantic, the extreme point of open sea, in winter, is found on the coast of Greenland at latitude $66^\{\backslash circ\}$ or $67^\{\backslash circ\}$; but in the longitude of $5^\{\backslash circ\}$ or $6^\{\backslash circ\}$ east, the sea remains open to lat. $76^\{\backslash circ\}$ or $79^\{\backslash circ\}$. In summer navigators have penetrated along the margin of Baffin's Bay as high as $77^\{\backslash circ\}$, on the coast of Spitzbergen to $81^\{\backslash circ\}$, and at Behring's Straits to $71^\{\backslash circ\}$. In the southern hemisphere the utmost point to which Cook was able to penetrate was $71^\{\backslash circ\}$ in summer; but navigation was very difficult beyond the parallel of $60^\{\backslash circ\}$; floating ice overspread the sea nearly as far as $50^\{\backslash circ\}$, and detached icebergs are carried by currents in both hemispheres as low as $40^\{\backslash circ\}$. (Cook's Voyages. Scoresby, I. 262.) In a general point of view, the ice-bound seas and lands are nearly conterminous with the arctic circle in the north hemisphere, and with the parallel of $60^\{\backslash circ\}$ in the south. In the one case they occupy about one-twelfth, and in the other about one-seventh of the hemisphere.

Saltness. The saltness of the sea is affected by currents, storms, heavy rains, the discharge of rivers, and by evaporation. Hence there is often little consistency in detached observations, but the results upon the great scale agree with theory. According to the experiments of Dr Marcet, the mean specific gravity of the main ocean is 1.02777. The Southern Ocean, which receives a less proportion of river water, is saltier than the northern. Inland seas, like the Baltic, Black Sea, and White Sea, which receive more rain water than they expend in vapour, and have an efflux current, are fresher than the ocean. On the other hand, the Mediterranean, which is a sort of natural saltern, from which the waters of the Atlantic, poured in with their salt, are drawn off fresh by evaporation, contains more saline matter than the main ocean. We may infer from theory, that the same thing holds true of the Red Sea. There is no proof that the sea is saltier under certain meridians,

or at certain depths, than at others; but in general, the sea is saltiest where it is deepest and most remote from land. Salt water parts with its salt on freezing, and hence the dissolution of ice freshens the sea. The following are some of Dr Marcet's results:

(Ed. Phil. Jour. No. IV. 356.)

The tides are periodical oscillations in the waters of the ocean, which take place twice every twenty-four hours, and are produced by the attraction of the moon and the sun. In the open sea, they are at their height three hours after the moon has passed the meridian of the place, and the meridian opposite. They are smallest towards the poles, and greatest at the equator, where, however, they do not exceed two or three feet in the great ocean. Their greatest elevation takes place in narrow seas, where the action of the moon is aided by winds, currents, the position of the coast, &c. The highest tides known are in the Gulf of St Malo, where the flood, driven back by the coast of England, rises to the height of seven or eight fathoms. Inland seas, whose entrances face the west, have rarely any tides, because the moon acts on all their parts at once, and there are no lateral waters to flow in and produce a local elevation. (Tuckey, I. 34.)

Independently of the motion of the tides, the waters of the ocean are found to be scarcely anywhere stationary. Local or temporary currents are produced by winds, the discharge of rivers, the fusion of fields of ice, the evaporation of inland seas, and other causes. But, exclusive of these, which we shall not attempt to describe, there are certain permanent and general currents, which are supposed all to originate in two great movements,—that of the tropical waters westward round the globe, and that of the polar waters towards the equator. The movement of the tropical waters westward is ascribed to the agency of the trade winds, which, blowing constantly in one direction, must impress their own motion on the sea to a certain extent. But the resulting current is necessarily modified by the position of the great continents. The motion of the polar waters towards the equator is not so well accounted for, nor perhaps is its general existence so well ascertained. It is supposed, that the copious evaporation at the equator, reducing the level of the sea there, the more elevated waters of the polar regions flow in to supply the loss; and, farther, that the fusion of ice in these seas must produce an accumulation of water, which naturally seeks a level by moving to the equator. But it is reasonable to believe, that the loss of the equatorial seas by evaporation is balanced by the greater fall of rain; and if the melting of ice

* Forster's Observations, p. 60. Scoresby, Account of Arctic Regions, I. 187-210. Edin. Phil. Jour. No. XI. 165. Humboldt, Pers. Nar. II. 56. Dr J. Davy, Phil. Trans. 1817.

produce a current from the pole during one season, the congelation of the water should produce an opposite current during another.

The great tropical current which flows round the globe prevails generally between $30^\{\backslash circ\}$ north and south of the equator, and has a mean velocity, according to Humboldt, of nine or ten miles a day. In the Atlantic, it separates into two branches, one of which constitutes the well known Gulf Stream. This branch flows northward, through the middle of the Atlantic, till it reaches the Cape Verd Islands; it then turns west, passes through the Caribbean Sea and the strait between Cuba and Yucatan, winds round the Mexican Gulf, and rushes out by the Bahama Channel; then, spreading out to a greater breadth, it sweeps along the shores of the United States to Newfoundland. At this point it is deflected south-eastward by a southerly current from Baffin's Bay, and, passing the Azores and Canary Isles, returns into itself, and repeats its circumgyration. The waters of the North Atlantic between $11^\{\backslash circ\}$ and $43^\{\backslash circ\}$ thus form a continued whirlpool, performing a circuit of 3800 leagues in about thirty-four months. Its velocity is inversely as its breadth. In the Bahama Channel, its breadth is fifteen leagues, and velocity from three to five miles an hour. In its retrograde course, from longitude $50^\{\backslash circ\}$ to the Azores, its breadth is 160 leagues, and velocity seven or eight miles a day. A zone of motionless water, 140 leagues broad, separates the direct and retrograde courses. This great current sends off one branch near Newfoundland, which proceeds north-eastward, and sometimes deposits tropical fruits on the shores of the British Isles and Norway. A second branch, escaping at the Azores, enters the Straits of Gibraltar. The Gulf Stream preserves a temperature of $71^\{\backslash circ\}$ in the latitude of Boston, while that of the neighbouring waters is $63.5$. It is not improbable that the westerly part of this current, involving by accident in its course the canoes of the early inhabitants of Western Africa or the Canary Isles, has hurried them to the coast of Caraccas, and thus been one means of transplanting the human race from the Old World to the New. (Humboldt, Pers. Nar. I. 46—60.)

The other branch of the great tropical current is supposed to set along the coast of Brazil and double Cape Horn. In the Pacific Ocean, a general current, westward, is said to carry the waters away from the coast of Peru. It is less perceptible on the west side, till it enters the Indian Ocean, when strengthened by the northerly currents there, it flows along the eastern coast of Africa, and round the Cape in a rapid stream, 130 miles broad, and $7^\{\backslash circ\}$ or $8^\{\backslash circ\}$ warmer than the contiguous waters. A current from the South Pole sets along the west side of New Holland into the Bay of Bengal. It is supposed that other portions of the general polar current deflect the great westerly current northward, after it has passed the southern promontories of Africa and America.*

In the North Atlantic, in the space comprised be-

tween Greenland and the coasts of Britain and Norway, and between Labrador and Spitzbergen, a great body of waters, acted upon by three or four lateral currents, is supposed to perform a perpetual revolution. These waters receive their impulse eastward from a branch of the Gulf Stream, which passes from Newfoundland along the north-west coasts of Scotland and Norway. At the North Cape, a great westerly current from Nova Zembla turns the waters north-westward, along both sides of Spitzbergen. Beyond this island, being met by a current from the Pole, they turn south-westward, and pass along the coast of Greenland to Davis' Straits, where they are deflected southward by a fourth current from Baffin's Bay, and at the Bank of Newfoundland recommence their revolution. Thus two great whirlpools, connected with one another, touching at the Bank of Newfoundland (which seems to be a bar cast up by their conflicting waters), and revolving in opposite directions, occupy four-fifths of the North Atlantic. The small current which sets from the Bay of Biscay across the mouth of the English Channel, and through the Channel of St George, is most probably a branch of the Gulf Stream which parts from the revolving current about the Azores. It is reasonable to believe, that great vortices of the same description will be found in the other parts of the ocean, when they have been as minutely examined as the North Atlantic. (Scoresby, I. 206.)

The value of any part of the earth's surface to climate, depends not merely on its extent, position, and soil, but also on its climate, which regulates the nature and variety of its productions, the degree of its fertility, and, to a certain extent, its salubrity, and its capacities of improvement. With no essential superiority but that of climate, the small island of Ireland supports, and probably will always support, a greater number of inhabitants than the whole northern section of the plain of Siberia beyond the 60th parallel.

The climate of a country is chiefly influenced by its distance from the equator, and its elevation above the level of the sea. But it is affected in a smaller degree by the nature of the surface, the abundance or scarcity of humidity, the proximity or remoteness of the sea, of lakes, of mountains, of arid or frozen plains, and perhaps also by the internal heat of the earth.

The mean temperature of the earth at the equator, which is pretty uniform under every meridian, is estimated, by Humboldt, at $81^\{\backslash circ\}.5$ of Fahrenheit's scale. The decrease of heat, as we recede from the equator, on either side, follows different laws in the two hemispheres, and in the same hemisphere under different meridians. On the west coast of Europe, about the meridians of London and Paris, the cold increases much more slowly upon us as we go northward from the equator, than in any other part of the world. In North America, at the meridian of $80^\{\backslash circ\}$ or $90^\{\backslash circ\}$ west from London, and in Asia at the same distance east, the increase of cold is much more ra-

* Varenus's Gen. Geog. Chap. XIV. Tuckey, I. 43. Dr J. Davy, Phil. Trans. 1817.

pid as we approach the pole. In the northern hemisphere, generally the cold is greatest on the east side of both the old and new continents, and least on the west. Humboldt has generalised this fact, and inferred, that all continents and large islands are warmer on the west side than the east. The following table, extracted from Humboldt's admirable Essay, exhibits the different gradations of the mean annual temperature in Western Europe and North America, continuing the scale to the equator.

We have few observations for the east coast of Asia; but the following shows that the climate of this region approaches to that of Eastern America rather than Western Europe.

Thus the mean annual temperature of North America is 9° lower than that of Western Europe at the latitude of 40°, 16° lower at lat. 60°, and 33° lower at lat. 70°; a similar difference obtains between the climates of Western Europe and Eastern Asia. By comparing places under the same parallel, we find that this change is not sudden, but progressive. Petersburg, on the same parallel with Upsal, is 3° colder, and Moscow is 5° colder than Copenhagen. The annual temperature of West Greenland in lat. 70°, which is 17° or 18°, according to Sir C. Giesecke, is very nearly the mean between that of the North Cape and Melville Island, as its intermediate situation would lead us to expect.—(Edin. Phil. Jour. No. IX. 201.) These differences are rendered more sensible when we connect the places having the same annual temperature by lines, which Humboldt has named isothermal lines. Thus the isothermal line of 59° (Fahrenheit) passes along the latitude of 43° in Europe, but descends to lat. 36° in America. The isothermal line of 41° passes from the latitude of 60° in Europe, to that of 48° in America, showing that the same annual temperature which is found at the 60th parallel on the eastern side of the Atlantic, is only found at 12° farther south on its western shores. The western coast of

North America again is warmer than the east; and hence if we were to trace the isothermal lines round the northern hemisphere, they would all have concave summits at the east sides of Asia and America, and convex summits at the west sides of America and Europe.

The difference of mean temperature between summer and winter (reckoning each to consist of three months) is nothing at the equator, and constantly increases as we approach the pole, as shown in the following table:

The extreme difference of the seasons is smaller under the warm meridians of Western Europe than any where else, and seems to be greatest where the mean annual temperature is lowest,—near the east coasts of Asia and America.

If we draw a line on the map in a north-east direction from Bordeaux to Warsaw, and continue it till it strike the Wolga in latitude 55°, all the places situated under this line, at the same elevation, will have nearly the same summer temperature of 69° or 70° of Fahrenheit's scale. The lines of equal winter temperature decline in an opposite direction, and deviate much farther from the plane of the parallels. Thus a straight line drawn from Edinburgh to Milan, almost exactly at right angles to the line of equal summer temperatures, would pass over places, all of which, if equally elevated, would have nearly the same winter temperature of 37° or 38°. The other lines of equal summer and winter temperatures have a direction corresponding to these, but not exactly parallel.

* Deduced by interpolation from the temperature of Natchez, Cincinnati, Churchill Fort, and Melville Island, reckoning that of Melville Island—2 Fahrenheit.

On comparing the climate of the two hemispheres, we find that the southern is rather colder than the northern, but is more remarkably distinguished by the greater equality of its seasons. This last effect may obviously be referred to the influence of a greater surface of sea upon a smaller extent of land. In the southern hemisphere also, the temperature seems more uniform under different meridians. At Port Jackson, Buenos Ayres, and the Cape, all nearly under one parallel, the annual mean temperature is almost exactly the same.

The second general cause that affects the temperature of places is their elevation above the level of the sea. The following table, by Humboldt, shows that the ratio of decreasing temperature, as we ascend in the atmosphere, is not the same at different latitudes, nor is it uniform at the same place for equal successive altitudes.

This table shows, that, at the equator, the thermometer sinks 10° in the first thousand yards of ascent, or 1° for 310 feet. In the next thousand yards it sinks no more than 6°, or 1° for 524 feet. In the third and fourth stages, there is a remarkable acceleration. In the whole column of air to the limit of perpetual snow, at 15,965 feet of elevation, the decrease is 1° for 341 feet. Humboldt ascribes the smaller rate of decrease in the second and third stages to the large clouds chiefly suspended in this region, which intercept the radiant heat of the earth. In the temperate zone the atmospheric cold increases more rapidly. The decrease of heat for the first thousand yards is at the rate of 1° for 253 feet, but to the limit of perpetual snow (at a temperature not of 32°, but of 23.3) it is 1° for 317 feet, or, in round number, 1° for 100 yards. Observations made, however, in the free regions of the air, may give different results from those which were made on the sides or summits of mountains. But generally in the temperate zone, of two adjacent places, if the one is 1000 yards higher than the other, it will have a climate 12° colder. For smaller heights the decrease

is proportional. In the upper regions of the atmosphere, the difference between the heat of night and day, summer and winter, is smaller than at the surface of the earth. This law of decrease explains to us the extreme cold felt in the elevated plains of Siberia, and the mild temperature enjoyed in the torrid zone, on the table land of Mexico, the plateau of Pastos, and other high lands. Humboldt calculates, that in the temperate zone, every hundred metres of ascent (110 yards) diminishes the heat as much as a change of 1° of latitude.

The temperature of countries is affected by the Local Temperature of the sea, and the nature of the adjacent land. The extremes of temperature are always comparatively little felt in small islands, remote from continents. In the United States intense cold is experienced as often as the wind blows from the frozen plains round Hudson's Bay. (Warden's America, I. 154.) From high mountains, gusts of cold wind, called snow winds by mariners, rush down and cool the circumjacent plains. (Volney's Trav. in Syria, I. 340.) At Calabozzo, in Venezuela, the temperature, which was from 87° to 90° in March, rose to 104° or 105°, whenever the wind blew from the parched and dusty surface of the Llanos, or great plains. (Humboldt, Pers. Nar. IV. 325.) The heat accumulates to an astonishing degree, when the wind passes over extensive deserts of fine and almost impalpable sand, which rises in the air, and hangs over the surface like a fog, or mounts in whirling columns to a great height, mixing its burning particles with the mass of the atmosphere, and communicating to it an intolerable heat. (Kinneir's Geog. Mem. of Persia, 216, 222.) In Europe, where the proximity of the sea cools every part of the surface by the agency of the winds, the accumulation of heat never proceeds so far as in Asia and Africa. Even in low plains, sheltered on the north, the temperature scarcely ever exceeds 100°; but at Bagdad and at Bushire, where the south wind arrives, heated by the burning sands of Arabia, the thermometer sometimes stands at 120° or 125°; and on the west coast of Africa, where similar causes operate, it is said to rise to 130°. * Though the mean annual temperature is highest at the equator, the extreme summer heat, so far as it proceeds directly from the solar action, is greatest at the tropic, and is even rather greater at the latitude of 45° than at the equator. (Art. CLIMATE, in this Supplement, p. 182, Table.) We may therefore conjecture that the hottest summer climate in the world is to be found in the western parts of the Sahara of Africa, under the northern tropic, where the winds blow over a zone of sandy plains 4000 miles in breadth, unbroken by any considerable mountains, or by any surface of water

* Morier's Journey, II. 97. Edin. Phil. Jour. No. V. 197. Tuckey, II. 442. Humboldt repeatedly observed the temperature of the soil in Equinoctial America at the hottest time of the day. It varied with the nature of the mineral substance. The highest he has recorded, we think, was that of a coarse granitic sand at Magpures, whose temperature at two P. M. was 140°.5. That of a fine granitic sand at the same place was 126°.5; of a granitic rock, 117°.6. We may hence conclude, that the air, though perfectly stagnant, could in no possible circumstances be heated above 140°, and this only within two or three feet of the ground. Humboldt's Pers. Nar. V. 165.

Physical Geography. except the narrow inlet of the Red Sea. At the same time, it is possible that a much narrower surface than this may suffice to communicate the maximum effect of the solar heat to the atmosphere, and the actual heat generated may depend more upon the nature of the surface and its low elevation, than upon its extent.

It appears that within the tropics seas have very little effect in tempering the accumulating heat of the land, when situated to the westward of a continent, but a great effect when situated to the eastward. In the steppes of Caraccas, which are open to the sea on the east, the heat rarely exceeds 99°, but even in the wooded regions of Senegambia and Guinea, much nearer the coast, it rises to 130°. * In the temperate zones, on the other hand, it is clear that the sea exerts its influence in a direction precisely contrary. When the land lies to the eastward, the sea mitigates the extremes of heat and cold very much; when to the westward very little. This is abundantly proved by the difference of temperature already noticed between the east and west coasts of the two continents. Now, the sea can exert very little influence over the temperature of the land, except through the agency of the atmospheric currents; and the phenomena in both cases seem to be accounted for by the prevailing direction of the winds. Within the tropics, these are almost constantly from the east; but from the tropics to the latitude of 60° north (and probably much farther), the prevailing winds are from the west. † Within the torrid zone, therefore, we should expect to find the extreme summer heat constantly accumulating from the east side of continents and islands to the west; but in the temperate zones, the extremes both of heat and cold will as regularly increase from the west to the east. This is found to be the case, and whatever other causes may be conjoined with those now assigned, there can be no doubt that the latter have really a great influence.

Internal Heat of the Globe. The temperature of each zone has such a correspondence with the amount of the solar impressions, as to lead to the inference, that the heat of the globe is entirely derived from the sun; and Professor Leslie has calculated, that, upon this hypothesis, the mean heat of the interior ought to be 66°.8. Humboldt has ascertained, however, that, in latitudes above 45°, the mean heat of springs and caves generally exceeds that of the atmosphere. The difference, which amounts to 6° or 7° at the parallel of 70°, he ascribes to the covering of snow in the higher latitudes, which prevents the loss of heat during the winter months by radiation, or the contact of cold winds. But there are facts which indicate that this heat may be derived from another source. To say nothing of volcanoes, we have hot springs in all parts of the world, at all temperatures below boiling

water; and evidence still less equivocal is afforded by the high temperature of deep mines. The following are examples: Physical Geography.

At Giromagny, in the Vosges, annual temperature at surface, 49°; at 110 yards depth, 53°.6; at 336 yards, 65°.8; at 472 yards, 74°.6.

In Saxony, in four of the deepest mines, annual temperature at surface, 46°.4; at 170 to 200 yards depth, 54°.5; at 280 yards, 58°; at 360 yards, 62°.6. (Daubuisson, Geog. I. 444.)

In the coal mine of Killingworth, the deepest in Britain, annual temperature at surface, 48°; at 300 yards, 70°; at 400 yards, 77°. In seven others of the deepest coal mines in Britain, a corresponding gradation was observed. (Mr Bald, Edinb. Phil. Jour. No. I. 134.)

In these British mines, the increment of temperature is about 1° for 15 yards of descent. In the Vosges, it is about 1° for 20 yards, and in Saxony, 1° for 22 yards. Taking 20 yards as a mean, if the increase follows the same arithmetical ratio to a considerable depth, we should find the temperature of the Bath waters (116°) at 1320 yards below the surface, and that of boiling water at 3300 yards, or nearly two miles. The frequency of hot springs from 80° to 120°, the rarity of those that approach the boiling point, and the constancy of temperature in them all, are circumstances remarkably consistent with this hypothesis. The facts, if they do not establish, at least strongly support three conclusions; 1st, That the heat of the interior of the earth is always greater than at the surface; 2d, That this heat augments progressively as we descend, in a ratio bearing some relation to the depth; 3d, That, even at moderate depths, this heat is greater than the mean heat of the globe ought to be, if entirely derived from the sun. Such an interior heat, if it exists, must be constantly diffusing itself towards the surface; and at the surface it may be kept down, so as to affect the temperature derived from the solar action very feebly, by the greater or less rapidity of its dissipation. But as it is very improbable that it should be diffused with perfect equality round the whole exterior shell of the globe, it may be the true source of some of those anomalies of climate (such as the discrepancy in the annual heat under the same parallel) which cannot be easily referred to other known causes. ‡

The atmosphere is an elastic fluid which surrounds the globe on all sides, and serves the most important purposes in the economy of nature. It sustains the life of an immense variety of organic beings, spreads the moisture of the ocean over the land in fertilizing showers, receives, dissipates, or decomposes, the innumerable mephitic vapours which are continually exhaled from the surface; and is the grand agent

* Humboldt, Pers. Nar. IV. 315. Tuckey, II. 442, 444.

† Cotte, Recherches sur les Vents Dominans, Journal de Physique, Tom. XXXIX. p. 267.

‡ As nearly the whole of the preceding view of climate has been taken from Humboldt's excellent Essay on Isothermal Lines, and the Distribution of Heat over the Globe, particular references were not thought necessary. The English translation, given in the Edinburgh Philosophical Journal, No. 5—9, has been consulted.

which, by its incessant motion, tempers the extremes of heat and cold, and enables man to support himself in every climate from the equator to the vicinity of the pole.

The fluid which forms the atmosphere consists of two gaseous substances, oxygen and nitrogen, united by a weak affinity, in the proportion of 21 parts of the former to 79 of the latter. The vital functions of animals and plants depend chiefly on the presence of oxygen, though nitrogen also enters into their composition. With these two are intermixed a very small quantity of carbonic acid gas; a variable portion of aqueous vapour, and in the highest regions of the atmosphere, where meteors are common, hydrogen has been supposed to exist. Air from the tropics and the polar regions, from the most healthy and the most insalubrious countries, exhibits no sensible difference in its constituent parts. A hundred cubic inches of air at the surface of the sea, when the thermometer is at $60^\{\backslash circ\}$, weigh 30$\backslash frac\{1\}\{2\}$ grains. The whole atmosphere is equal in weight to a sheet of mercury, 30 inches, or to a sheet of water, 3$\backslash frac\{1\}\{2\}$ feet deep, and were its density every where the same as at the surface, would reach no higher than 27,000 feet, or five miles. But its extreme elasticity causes the upper strata to expand indefinitely, and the phenomenon of meteors show that it exists in a state of extreme attenuation, at a height a hundred times greater than this. Considering the incessant currents which agitate its whole mass, there is a surprising uniformity in its pressure. The annual range of the barometer, which indicates the variation of the pressure, does not exceed from $\backslash frac\{1\}\{2\}$ to $\backslash frac\{1\}\{4\}$ inch in the torrid zone; is about two inches at Liverpool, the same at Petersburg, and was found by Captain Parry to be 1$\backslash frac\{1\}\{2\}$ inches at Melville Island in one year.* The extreme variation, which nowhere exceeds 3$\backslash frac\{1\}\{2\}$ inches, is not greater than one-ninth part of the total pressure. It is greater in the temperate zone than at the equator or the pole, and in winter than in summer. The density of the air decreases in a geometrical ratio as we ascend, and in temperate climates is diminished to one-half at an elevation of 3$\backslash frac\{1\}\{2\}$ miles. We are hence enabled to measure heights by the variations of the barometrical column. Changes in the atmospheric pressure, when they are great, have been observed to take place simultaneously at Edinburgh, Paris, and Geneva, and must, therefore, affect a great range of country at the same time.

The unequal distribution of heat over the surface of the land and water necessarily disturbs the equilibrium of the atmosphere, and produces currents of air, or winds. These currents, however various, have been supposed to result from two general movements, pervading the whole mass of the atmosphere. The heavy and cold air of the polar and temperate regions having a tendency to displace the warm and rarified air of the torrid zone, generates a current in each hemisphere towards the equator. To replace the air abstracted from the higher latitudes, an up-

per and counter current flows back from the equator to the pole; and thus the atmosphere, while it performs a constant revolution, tempers the extremes of climate, by transporting the cold of the frigid zone to the equator, and carrying back the heat of the equator to the frigid zone. These great south and north currents, which are to be considered as the primary winds, receive various modifications. The under current, which proceeds from the polar regions, having impressed upon it the slow rotary motion of these regions, does not acquire in its journey the velocity of the parts it passes over. Instead, therefore, of proceeding directly along the meridian, it is deflected to the westward. It pursues this lateral course more and more as it approaches the torrid zone, and the impulse south and north being destroyed when the currents from the opposite hemisphere meet at the equator, the motion westward alone remains. This constitutes the well known trade wind, which blows from the east at the equator, from the north-east at the northern tropic, and the south-east at the southern. The upper and counter current again, carrying with it the rapid velocity of the equatorial regions, does not travel right along the meridian, but deviates more and more to the east as it advances; and when its progress towards the pole is stopped by the accumulation of air from the opposite meridians, the motion eastward alone remains, and it settles into a west wind. Thus a constant east wind should prevail at the equator, and a constant west wind near the poles; but the latter, being primarily an upper current, may not be invariably felt as a west wind at the surface. It does not follow, however, that the upper and under currents preserve their relative situation over all the temperate zone. From the greater accumulation of air in the higher latitudes, and from variations of temperature produced by local causes, the upper current will often be bent down to the surface, and the lower current ascend. This interchange will take place occasionally at all parts of the temperate zone. Hence, in high latitudes, storms of wind, which mingle the warm upper current with the cold air below, always produce an increase of heat, as Captain Parry found.—(Voyage, p. 118.) In the northern hemisphere, then, when the cold current from the pole sweeps the surface rapidly, we have a north wind; it becomes a north-east wind when its motion southward is retarded; an east wind when it is checked, and a south-east when it is deflected back, by mingling with a current from the south; all of which, except the last, are generally found to be cold winds. When the warm current from the south descends and sweeps the surface, we have a south wind if its motion northward is rapid; a south-west when its motion northward is retarded; a west wind when it is checked; and a north-west when it meets and mingles with a current from the north. All these, except the last, are generally warm winds, as experience proves. The line of division between the upper and lower cur-

* Manch. Mem. IV. 534. Edinb. Trans. II. 213. Parry's Voyage, 269.

Physical Geography. rents should be where the mercury stands at 15 inches, which is at the height of 3½ miles. This should be the region of clouds; and clouds do generally approach to this elevation.

Trade Winds. This theory, upon the whole, agrees tolerably well with the facts. But the variable surface and temperature of the land greatly affect the course and velocity of the winds. In the torrid zone, within the parallel of 25° or 30°, on each side of the equator, the trade winds blow constantly from the east. From the superior warmth of the northern hemisphere, the line that separates the opposite trade winds is not the equator, but the 2d or 3d parallel north. To a certain extent, also, they follow the course of the sun, reaching a little further into the south hemisphere, and contracting their limits in the north when the sun is on the south side of the equator, and making a reverse change when he declines to the north. In a zone of variable breadth in the middle of this tract, calms and rains prevail, caused, probably, by the mingling and ascending of the opposite aerial currents. High lands change or interrupt the course of the trade winds. Thus, under the lee of the African shore, calms and variable winds prevail near the Cape Verd Islands, while an eddy or counter current of air from the south-west is generated under the coast of Guinea. The lofty barrier of the Andes shelters the sea on the Peruvian shores from the trade winds, which are not felt till a ship has sailed 80 leagues to the westward; but the intervening space is occupied by a wind from the south. In the Indian Ocean the trade wind is curiously modified by the lands which surround it on the north, east, and west. The southern trade wind blows regularly from the east and south-east, from 10° of south latitude to the tropic, but in the space from 10° south latitude to the equator, north-west winds blow during our winter (from October to April), and south-west in the other six months; while, in the whole space north of the equator, south-west winds blow during summer, and north-east during winter. The cause of these remarkable winds, which are called monsoons, has not been very satisfactorily ascertained. (Varenus's Gen. Geog. Chap. xxi.)

Variable Winds. The region of constant winds is confined within the 30th parallel on each side of the equator. On the outer side of these limits, calms prevail pretty generally over a narrow space, beyond which the region of variable winds extends to the pole. In the open sea, where the true direction of the winds is best known, navigators have repeatedly observed that throughout this region the prevailing wind is from the west. Mr Forster observes, that beyond the tropics, the west winds are the most universal. That east winds have an ascendancy within the Antarctic circle, as he thinks, may be questioned; few observations having been made there, and those few not to be depended on, in consequence of the local influence of the ice. (Observations, p. 127, et seq.) According to the author of Lord Anson's voyage, "a westerly wind almost perpetually prevails in the southern parts of the Pacific Ocean" (he speaks of latitude 60°), and a similar wind in the northern parts carried the Spanish galeons from Japan to Ca-

lifornia, along the parallel of 35° or 40°. (Book I. Chap. ix. II. Chap. x.) West winds prevail in Kamchatka; they blow three-fourths of the year in Hudson's Bay; the west and north-west winds predominate greatly at Melville Island; and it is a familiar fact among mariners, that, by means of these winds, the voyage eastward across the North Atlantic is generally accomplished in about half the time of the voyage westward. (Scoresby, I. 411. Parry, Voyage, 299.) On the west coast of Europe, too, the west and south-west winds are the most general. It is remarked also, that in our climate, south and south-east winds are the most rare; that winds between north and east are almost invariably cold; those between south and west warm; and those between north and west of a mixed character. So far the facts correspond generally with the theory, though many anomalous circumstances may be found.

Local Winds. The change of the seasons which affects the local temperature so much, necessarily influences the atmospheric currents. Accordingly, the most violent tempests are about the equinoxes, and in every country there are prevailing winds peculiar to certain seasons. It may be suspected that most of the winds observed on land or in confined seas are merely local eddies, or reflex currents, produced by the irregularities of the surface. Thus, while the south-west wind prevails almost one-half of the year at Dover, London, and in the west of England generally, it is scarcely felt at Liverpool, which lies in the gorge of a valley, where the western chain of hills is interrupted; and on the other hand, the south-east, so uncommon in the rest of England, is the predominating wind here. (Manch. Trans. IV. 601, 615.) In the long basin of the Red Sea, the wind blows in no direction but right up or down. In Lancaster Sound, Captain Parry found the wind to blow always either east or west. For ten months of the year a wind (which is probably a smaller branch of the north trade wind) blows constantly up the valleys of the Mississippi and Ohio, preserving a breadth in the latter of twelve or fifteen miles. (Tuckey, III. 70. Warden's Amer. I. 155.) In the list of Vents Dominans for the interior of Europe, collected by Cotte, nothing like a system can be discovered—nothing but local currents running in every possible direction, according to the position of mountains and valleys. Winds, indeed, even when strong, are often confined to a space surprisingly small. In the temperate, but still more in the frigid zone, two or three winds are often seen blowing from, or to, different points within a few leagues; nay, of two ships within sight of one another, one is sometimes becalmed, while the other is seen struggling with a storm. In the northern seas even strong gales, when they have carried a ship into frozen water, invariably desert her, or give place to a wind which blows from the ice. The effect can only be attributed to the comparative coldness of the ice, and warmth of the water, generating a local descending current over the one, and an ascending current over the other. (Scoresby, I. 403—409.)

The Mediterranean has a system of winds peculiar to itself, which we have not room to explain. All warm countries, where sandy deserts abound, give birth to pernicious hot winds. Such is the Si-

moor or hot wind of the desert, and the Sirocco, so well known in the east, and along the shores of the Mediterranean. For the description of these, as well as the sea and land breezes, and other peculiar winds, we refer to the Article METEOROLOGY.

Rain, so essential to the fertility of the soil, is very unequally distributed to the different regions of the globe. Nature has arranged it so, that the supply of the atmospheric moisture is most abundant in those latitudes where evaporation is most rapid. Or rather the processes of precipitation and evaporation, depending partly on the same principles, and exerting a reciprocal influence, tend to adjust themselves to each other, though this tendency is perpetually disturbed by local causes. Rain is produced by the mixture of atmospheric currents of different temperatures. The power of air to hold water in solution does not increase in the same ratio with the increase of its temperature, but in a much higher ratio. Hence, when two masses of air, saturated with moisture, and of different temperatures, are mixed, the resulting compound is not capable of holding the whole water in solution, and a part of it is precipitated in rain. As the whole atmosphere when saturated is calculated not to hold in solution more water than would form a sheet of 5 inches in depth, while the mean annual deposit of rain and dew is probably 35 or 40 inches, it is obvious that the supply of atmospheric humidity must be many times renewed in the course of a year. The quantity of rain at any place is affected chiefly by three circumstances. It is greater at the equator than towards the poles,—at the sea coast than in the interior,—and on mountains or high grounds than on plains. The winds, too, exert a certain influence, which, at any particular place, depends on their temperature, and on their travelling to that place over a dry or a humid surface. Where so many causes interfere, whose effects are scarcely capable of estimation, it would be difficult to determine the depth of rain proper to each parallel, even were our observations more numerous and accurate than they are. In a former part of this article we gave as an approximation 83 inches for the equator, and 8 inches for the pole. We have since found the following estimate by Humboldt, to whose authority we readily defer in every question of this kind. Except within the tropics, it corresponds generally with that which we gave.

Since the supply of humidity is greatest in the vicinity of the sea, the rains must be generally more abundant there than in the interior. On the other hand, we know that currents of air fraught with vapour move with great rapidity, and that the atmosphere is far from needing an entirely new supply from the ocean after each precipitation, because three-fourths probably of the rain that falls soon rises again into the air, and continues thus to circulate between the clouds and the earth. These circumstances lead us to conclude, that the difference in the annual

depth of rain, between inland places and those on the coast, is not so great as might be imagined. We find, accordingly, that a small increase of elevation compensates for a great distance from the sea. Thus 29½ inches of rain fall at Puy at the sources of the Loire, while 24½ only fall at the sea-port of Rochelle near its mouth, and at Geneva the depth is 40 French inches, while at Paris, 300 miles nearer the sea, it is no more than 19½. If we compare Toulouse, Paris, and Gottingen, inland places not far from the coast, with Buda and Prague, places in the heart of Europe, and, like the others, not much elevated, the mean fall of rain at the three former is 20.4 French inches, at the two latter 14.1. (Cotte, Jour. de Phys. T. XXXIX.) These facts are cited to support the idea of a decreasing scale of rain, rather than to mark the ratio of decrease. The latter is a problem for which we are yet unprovided with data. We may infer from theory, that the decrements, so far as they are affected by mere distance, will form a geometrical series. If the atmosphere, for instance, loses one-third of its moisture in the first 400 miles from the sea, it will lose one-third of what remains in the next 400; and so on.

The elevation of the land has a much more marked effect on the quantity of rain than its distance from the sea. Mountains precipitate the humidity of the atmosphere, not so much by attracting it to their summits, as in consequence of their rocky or grassy sides, when acted upon by the sun, heating large masses of air in the cold upper regions of the atmosphere. These warm masses of air, which abstract moisture from the aerial columns around them, stream upwards and mingle with the cold strata above, or come into contact with cold currents moving laterally, or suffer sudden and partial changes of temperature from the vicissitudes of night and day, and thus incessantly generate the circumstances which cause precipitation. It is evident that snow-clad mountains, from the constancy of their temperature, will serve much less perfectly than those whose sides are bare as nuclei for precipitating the atmospheric humidity; and that, in temperate climates, the rains will be more abundant on all mountains in summer than in winter. Along the shores of the Adriatic, and in the valley of Lombardy, the annual fall of rain is from 26 to 35 French inches; but in the Carnic and Julian Alps, it amounts to 100 inches. From the effect of this great barrier in intercepting the aqueous vapour, and producing desiccation, Feltre, on the east side of the first chain, has one-half more rain than Trent, which is higher, but on the west side; and Coire in the Grisons, 1900 feet above the sea, has less rain than the champaign cities of Vicenza and Verona. (Cotte's Tables.) In England it is found that Keswick and Kendal, situated among the mountains, have 67 and 59 inches of rain respectively, while places in the level interior country and on the sea-coast have only about 24 inches. But though more rain falls in mountainous than level countries, the depth is greater at the bottom of a mountain than at the top, and close to the surface of the ground than at any distance above it, in all cases.

In the torrid zone, a small thick rain falls every day on that side of the equator on which the sun is,

Physical Geography. but generally intermits during the night. When it ceases in the one hemisphere, it commences in the other, and is nearly perpetual in a zone of two or three degrees in breadth, which separates the two systems. When the rains begin, the heat increases, and the trade wind subsides. Yet, even in the torrid zone, there are tracts where rain seldom or never falls, such as the Sahara of Africa, the low coasts of Caraccas, and the desert shore of Peru, between 15° and 30° of south latitude. In many parts there are two wet and two dry seasons in the year; and in some regions, from the effect of mountain ranges and peculiar winds, places under the same parallel have their wet and dry seasons at opposite periods.* Though the annual depth of rain is greatest within the tropics, the number of rainy days follows an inverse order, and diminishes as we advance from high latitudes to the torrid zone. According to Cotte, the mean number of rainy days from 12° to 43° of latitude is 78, while it is 161 from latitude 51° to 60°. More rain falls in summer than winter in all latitudes, but in the temperate zones the rains of winter are more frequent than those of summer, though less in quantity. (Cotte, ubi supra.)

Mr Dalton reckons the annual deposit of water in England to be 31 inches of rain, and five of dew. His estimate is perhaps rather high. The distribution of the rain through the different seasons is so much affected by local circumstances, that it is difficult to ascertain the mean result. In general, the fall seems to be least in spring, and greatest in autumn, and less in the six winter months than in the summer. The following table affords an interesting view of the connection between the phenomena of wind and rain in this country. It gives, for one year (1775), the number of days each wind prevailed at London; the quantity of rain that fell during the prevalence of that wind; and the humidity of the winds, or the relative quantity of moisture which each would deposit in the same space of time.

The whole quantity of rain for that year was 27.11 inches; of which it will be observed that two-thirds fell with the south-west wind. The prevailing winds are the south-west and north-east, the primary winds of our hemisphere. The least frequent are the east and west. The south and north winds are the driest, and the south-west and west the most humid, as might be expected. †

The geographical distribution of plants is a subject

of such extent, that our narrow limits compel us to dismiss it with a few general remarks, extracted from Humboldt's Prolegomena de Distributione Geographica Plantarum. Each plant has generally a determinate climate, to which it is best adapted; there are other climates, however, in which it can be raised, though less advantageously; but beyond certain limits it either ceases to grow altogether, or is supplanted by other plants better adapted to the situation. Since vertical elevation has the same effect on climate as distance from the equator, there are properly no plants which are peculiar to the frigid zone, because the mountains of the torrid zone, embracing between their base and their summits every variety of climate, either produce, or are capable of producing, all the vegetables of the temperate and polar regions. Plants are absolutely most numerous, and exhibit the greatest variety of species, and the most luxuriant growth, within the tropics, where nature has supplied most abundantly the heat and humidity which contribute chiefly to the development of their vital functions. They diminish in number, variety, and magnitude, as we recede from the tropics; and in Greenland, Spitzbergen, and the plains of Northern Russia, the wealth of the vegetable kingdom has shrunk to a very small number of mosses and stunted shrubs. The lines that limit the growth of certain plants depend on the average summer temperature, for plants which require a long and moderate heat; on the temperature of the warmest month for those which require a short but great heat; and on the temperature of the coldest month, for those which are unable to resist a considerable cold. Their growth is affected also by the foggy or serene state of the atmosphere, because the direct stimulus of the solar light is as essential to the success of many plants as a certain determinate temperature. Instead of losing ourselves in the labyrinths of general botany, we shall illustrate these principles by a few facts drawn from the history of cultivated plants.

The plantain (Platano harton), which furnishes a primary article of food in tropical America, requires a mean annual heat from 82° to 73°, and its proper climate is therefore from the equator to the 27th parallel. In the equinoctial zone (lat. 0°—10°) its fruit does not ripen at a greater altitude than 500 toises (534 fathoms). The sugar cane (Saccharum officinarum), which has nearly the same range, thrives where the mean annual temperature is from 82° to 73°, and is cultivated, but with less advantage, in the old world to the latitude of 36½°, where the annual temperature is about 67°. In North America, it is cultivated as far as latitude 31°, beyond which it does not succeed, for, though the summer heat is sufficient to ripen its produce as far as 35° or 40°, the rigour of the winter destroys the plant. It succeeds on the table land of Mexico, at the altitude of 900 toises. The cotton plant (Gossypium) has its favourite climate between latitude 0° and 34°, where

the annual heat is from $82^\{\backslash circ\}$ to $68^\{\backslash circ\}$. But it succeeds wherever, with a mean summer heat of $73^\{\backslash circ\}$ or $75^\{\backslash circ\}$, that of winter is not below $4^\{\backslash circ\}$ or $48^\{\backslash circ\}$. It is cultivated as high as latitude $37^\{\backslash circ\}$ in America, beyond latitude $40^\{\backslash circ\}$ in Europe, and grows at Astrakan in Western Asia, latitude $46^\{\backslash circ\}$. (Storch, Tableau de la Russie, II. 250.) The date palm (phoenix) thrives best between latitude $29^\{\backslash circ\}$ and $35^\{\backslash circ\}$, but on the shores of Italy, in places sheltered from the north winds, it is cultivated as far as latitude $44^\{\backslash circ\}$. The citron (citrus) has nearly the same range, and is cultivated at Nice in spots 150 toises high. This tree, with the sweet orange, grows in Louisiana, under latitude $30^\{\backslash circ\}$, but above that it is injured by the frosts. (Darby's Louisiana, p. 151.) The olive (olca) ranges in Europe between latitude $36^\{\backslash circ\}$ and $44\backslash frac\{1\}\{2\}^\{\backslash circ\}$. It succeeds wherever with a mean annual heat from $66^\{\backslash circ\}$ to $58^\{\backslash circ\}$, that of summer is not below $71^\{\backslash circ\}$, nor that of the coldest month below $42^\{\backslash circ\}$. This last condition excludes all the North American continent beyond latitude $34^\{\backslash circ\}$, though the olive thrives $10\backslash frac\{1\}\{2\}^\{\backslash circ\}$ farther north in Europe. The vine (vitis) thrives where the mean annual heat is from $62^\{\backslash circ\}$ to $50^\{\backslash circ\}$, or even $47^\{\backslash circ\}.5$, providing that of winter is not below $33^\{\backslash circ\}$, nor that of summer below $66^\{\backslash circ\}$ or $68^\{\backslash circ\}$. These conditions are fulfilled on the sea-coast of Europe as high as latitude $47^\{\backslash circ\}$, in the interior as high as latitude $50^\{\backslash circ\}$, and in North America only as high as latitude $40^\{\backslash circ\}$. But the favourite climate of the vine in the Old Continent is between latitude $36^\{\backslash circ\}$ and $48^\{\backslash circ\}$, where it yields a generous and excellent wine. The Cerealia (wheat, rye, barley, and oats), so important to civilized life, yield profitable returns in places where the mean annual temperature descends to $28^\{\backslash circ\}$, providing that of summer rises to $52^\{\backslash circ\}$ or $53^\{\backslash circ\}$. In Lapland, barley comes to maturity whenever the mean temperature of the summer months rises to $47^\{\backslash circ\}$ or $48^\{\backslash circ\}$. Barley and oats, from the rapidity with which they germinate and ripen, adapt themselves to the short summers of the north, and are found along with the potatoe as high as latitude $69\backslash frac\{1\}\{2\}^\{\backslash circ\}$ in Lapland. In Eastern Russia, upon the Lena (longitude $120^\{\backslash circ\}$), grain of any kind refuses to grow beyond latitude $60^\{\backslash circ\}$. (Sauer, p. 24, 137.) Wheat, which is a precarious crop, and little cultivated beyond latitude $58^\{\backslash circ\}$ in Western Europe, yields good returns at this part of the temperate zone, when the mean heat of the seven months from March to October is $55^\{\backslash circ\}$; but if it is no more than $46^\{\backslash circ\}$, neither this grain, nor barley, oats, or rye, come to maturity. The Cerealia generally, which succeed only in low grounds beyond latitude $60^\{\backslash circ\}$ in Europe, are cultivated to the height of 550 toises on the sides of the Alps in latitude $46^\{\backslash circ\}$; to the height of 1020 toises (barley and oats) on Caucasus in latitude $42^\{\backslash circ\}$; and on the sides of the Andes, under the equator, the proper climate of the Cerealia is found between 700 and 1600 toises of elevation (4480 and 10,240 feet). Maize accompanies the vine in the west of Europe,

but extends farther north on the east. It is most extensively cultivated in its native soil, the New World, where it forms the chief nourishment of man and domestic animals from La Plata to the Canadian lakes. Coming to maturity in four or five months from the time it is sown, and requiring a short but hot summer, it is admirably adapted to the climate of America. There are few situations under the latitude of $45^\{\backslash circ\}$ where it will not grow, but the parallel of $35^\{\backslash circ\}$ is that to which it is best adapted. (Darby's Louisiana, p. 138, 144.) The oak ceases to grow beyond latitude $63^\{\backslash circ\}$ in Norway: it terminates at $60^\{\backslash circ\}$ or $61^\{\backslash circ\}$ in Finland; and at $57\backslash frac\{1\}\{2\}^\{\backslash circ\}$ in the government of Perm. The Pinus sylvestris (Scots pine) reaches the height of 60 feet at the latitude of $70^\{\backslash circ\}$ in Lapland, and grows in situations 125 toises above the sea. The Betula alba is found under the same parallel at 250 toises of elevation. In the eastern parts of Asiatic Russia trees are very thin beyond latitude $65^\{\backslash circ\}$; the larch, pine, birch, and mountain ash, disappear about $60^\{\backslash circ\}$ or $68^\{\backslash circ\}$; willow and birch bushes, eight inches high, are found as far as $69^\{\backslash circ\}$. At Hudson's Bay trees of all kinds terminate about latitude $60^\{\backslash circ\}$. (Sauer's Account, &c. 70-91. Mackenzie's Voyage, p. 404.)

Let us now endeavour to make a general summary Range of of these details, confining our view to the great con- each Plant. tinents merely, without regard to the islands. The plantain or banana we find occupies a zone of $54^\{\backslash circ\}$ in breadth on either side of the equator, which embraces about four-fifths of Africa, one-sixth of Asia, and one-third of America, and excludes the whole of Europe. The climate of the sugar-cane, allowing it to extend to $30^\{\backslash circ\}$ on each side of the equator in the New World, and $35^\{\backslash circ\}$ or $36^\{\backslash circ\}$ in the Old, includes the whole of Africa, nearly one-third of Asia, and two-fifths of America, and only touches Europe at its southern extremity. The climate of the olive consists of two zones on the opposite sides of the equator, extending from latitude $35^\{\backslash circ\}$ to $44^\{\backslash circ\}$ in the old continent, and embracing very little of the new; this tree being equally injured by the excessive heat of the tropical, and the excessive cold of the extra tropical regions in America.* The vine grows as far north as $50^\{\backslash circ\}$ or even $52^\{\backslash circ\}$, but its profitable culture in Europe does not extend much beyond $48^\{\backslash circ\}$. In Asia it succeeds at Astrakan latitude $46^\{\backslash circ\}$, in Bucharia, and at Hami in Chinese Tartary, latitude $43^\{\backslash circ\}$.† It, therefore, extends, we may suppose, to lat. $45^\{\backslash circ\}$ in Asia generally. Except in islands, or elevated plains, it is probable there are few palatable wines made at a lower latitude than $30^\{\backslash circ\}$. The vine may hence be considered as occupying a zone of $15^\{\backslash circ\}$ in breadth on each side of the equator (from $30^\{\backslash circ\}$ to $45^\{\backslash circ\}$) in the Old World, and two corresponding zones of $10^\{\backslash circ\}$ in breadth in the New (from $30^\{\backslash circ\}$ to $40^\{\backslash circ\}$). These zones embrace about one-sixth of America, one-third of Europe, two-sevenths of Asia, and one-tenth of

* If Humboldt has accurately defined the temperature for the year, and the coldest month proper for this tree, there ought to be no part of North, and perhaps few parts of South America, where it will thrive. It has not been introduced into the territories of the United States.—Darby's Louisiana, p. 150.

† Topographie des Vignobles, 489, Paris, 1816. Pallas's Travels, II. 430.

Africa. Of the productions which serve for the ordinary sustenance of man, maize and the potatoe have the widest range. The culture of the potatoe, as Humboldt observes, extends from the extremity of Africa to Labrador, Iceland, and Lapland. Maize bears any natural temperature, however high, and the only circumstance essential to its successful culture seems to be a mean summer heat, not below 68° or 70°. In consequence of this advantage, its climate embraces the whole of the torrid zone, and reaches probably to lat. 48° in America, to 50°, or 52°, in Europe, and 48° in Eastern Asia. It hence includes about five-eighths of America, three-sevenths of Europe, the whole of Africa, and three-fifths of Asia. The Cerealia do not succeed in the torrid zone in low situations. The climate adapted to their culture, which begins about lat. 30°, extends to lat. 50° in America, to 70° in Lapland, 62° in Finland, and 60° throughout the north of Russia, generally including five-sixths of Europe, three-fifths of Asia, one-tenth of Africa, and one-fourth of America. The region of forests, or the climates adapted to the growth of wood, embraces the whole of the Old and New World, except a narrow zone on the north border of each, which includes one-tenth of America, one-tenth of Europe, and one-tenth of Asia. As the prolific powers of the soil depend chiefly on the stimulus of heat and moisture, it is reasonable to believe with Humboldt, that the general fertility of the earth increases as we approach the equator. (Pers. Nar. IV. 106.) In warm countries, the produce not only bears a much higher proportion to the seed, but the husbandman has the advantage generally of two harvests in one year. We may probably assume, then, without any great error, that, with regard to plants which grow both in the warm and temperate zones, the productiveness of the soil increases in the ratio of the annual temperature, and the depth of rain; and that for countries under the same parallels, it is generally in proportion to the duration and intensity of the summer heat. The cosine of the latitude, or the distance from the axis, as it expresses pretty nearly the proportions of heat and moisture for each parallel, may therefore be taken as an approximate measure of the relative fertility. But when we take into view the superior productiveness of vegetables which are peculiar to warm regions, such as maize, rice, and the banana, there can be no doubt that the power of the soil to sustain animal life, augments in a much higher ratio than the cosine of the latitude, and that at the equator it is at least ten times as great as at the latitude of 60° in Europe. It is scarcely necessary to qualify those statements by saying, that we speak solely of low and level regions, and of ordinary soils. There are warm valleys in the north, where the productions of southern climes will grow beyond the limits we have assigned; there are elevated tracts in the torrid zone where every plant found in cold countries can be raised; and in all the zones, there are deserts and rocks which bid defiance to the vivifying power of the sun, and the genial influence of the seasons.

The number of human beings upon the globe has been variously estimated. A thousand millions have been mentioned, apparently for no other reason than

the convenience of a round number. M. Malte-Brun, a late and very respectable authority, reduces the amount to 650,000,000. We think his enumeration for Asia, Africa, and America, still rather high, and without detailing the grounds of our judgment, or pretending to accuracy, where any thing better than loose approximations is unattainable, we submit the following estimate as the result of our inquiries.

The diversity of colour, stature, form, physiognomical expression (and we may add language), among different portions of the human race, have at all times arrested attention; and have in many cases created, and in all inflamed, those feelings and prejudices which have made nations the enemies of each other. Philosophers have endeavoured to analyse and class the characters which produce these diversities; and since migrations and conquests have blended the distinctive peculiarities of different races, they have attempted to ascertain those primary types, from the combination of which all the other varieties have arisen. We have no room to discuss the many idle and fantastic hypotheses which have been built upon this subject, but shall merely give a brief outline of the system of the learned Blumenbach, who has prosecuted the inquiry with the aid of a profound knowledge of physiology. This philosopher has resolved the varieties of the human species into five primary classes,—the Caucasian, Mongolian, Ethiopian, American, and Malay. In the Caucasian race the skin is white, the cheeks red (a character scarcely found in any other race), the hair brown, running into yellow and black, soft, long, and undulating; the head symmetrical, and rather globular, the forehead moderately expanded, the cheek-bones narrow, not prominent, the face oval, the nose narrow and slightly aquiline, the lips gently turned out, the chin full and round, the facial angle large. This variety comprehends all the Europeans except the Finns, Samoieds, and Laplanders; the Western Asiatics, as far as the Oby and the Ganges, and the people of North Africa. In the Mongolian variety, the skin is olive, the hair black, straight, and stiff, the head almost square, the cheek-bones prominent outwards, the face broad and flat, and its parts less distinct than in the Caucasian race, the eye-brows little arched, and the space between them flat, the nose small and flat, the aperture of the eye-lids narrow. This includes the remaining Asiatics (except the Malays), the Finns, Samoieds, Laplanders, Greenlanders, and Esquimaux. In the Ethiopian race, the skin is black, the hair black and crisp, the head compressed laterally, the forehead receding, the cheek-bones standing much forward, the nose flat, and blending with the cheeks, the lips thick, the chin rather receding, the jaws lengthened forwards, and the under part of the face prominent. This includes all the African nations, except those of the Caucasian race in the north.

In the American race, the skin is of a copper colour, the hair black, stiff, and straight, the forehead short, the cheek-bones and face broad, the orbits and eyes deep, the nose rather flat, but still prominent. In this are included all the American tribes except the Esquimaux. In the Malay race, the skin is tawny, the hair black, soft, curled, thick, and abundant, the head rather narrow, the forehead slightly arched, the parietal bones prominent, but not the cheek-bones, the nose full, broad, and bottle-shaped at the point, the mouth large, the face projecting forward in the lower part, but the features viewed in profile, more distinct than in the Ethiopian race. This variety comprehends the inhabitants of Malacca, the Marian, Philippine, Molucca, and Sunda Isles, and Polynesia. It is to be observed, that it is only in some particular tribes, or portions of each race, that its characteristics are strongly marked. None of the leading varieties are absolutely pure, and all of them may be found passing into each other by insensible gradations, from the effects of mixture.

The question has been long agitated, whether all the existing varieties have sprung from one parent stock. The arguments in favour of the affirmative appear to us to preponderate. 1. All the different races are capable of uniting with each other, and begetting offspring, which continue themselves by pro-

creation. This is held by naturalists to be one of the most decisive tests of identity of species. 2. There is no essential difference of structure in the various races. None of them have permanently a bone, a blood-vessel, or an organ of any kind, which the others want. There are no differences analogous to those of the stag and the roebuck, or the one and the two humped camel, which clearly mark a diversity of race. 3. The differences which do exist among mankind at large, such as those of complexion, stature, physiognomy, and mental capacity are analogous to those which manifest themselves in one tribe, or nation, and even in one family. We have, therefore, only to suppose that the causes which produce these minor and accidental differences are rendered general and permanent in their operation, by peculiarities of food, climate, or habits, to account for the stronger and more lasting distinctions among nations. This view of the subject is farther confirmed by the changes which climate, food, and different modes of treatment, produce upon domestic animals. 4. The affinities of language, which promise to throw much light on this subject when fully developed, have already afforded presumptive proofs of a common origin in the case of nations whom physiologists class under distinct races.*

(B. B. B.)

PHYSIOLOGY.

PHYSIOLOGY, or the study of the phenomena of life, differs from all the other branches of philosophical inquiry, by its involving the consideration of the final as well as the physical causes of these phenomena. A new principle of arrangement is thus introduced, which is scarcely ever applicable in the sciences relating to the properties of inert and inorganic matter. Those sciences are formed by applying to the subjects they concern, the rules of philosophical induction. By comparing together phenomena, and uniting in one class such as are of the same kind, we arrive at the knowledge of a certain number of general facts, which we regard as laws of nature, and from which, when once established, we deduce the explanation of a multitude of subordinate phenomena, resulting from the simple or the combined operation of these laws. By a series of experiments we succeed in removing all extraneous circumstances, and in reducing each class of phenomena to its simplest conditions; and we are also enabled to vary the combinations, so as to compare the results with the appearances presented to us by nature. But the application of the same methods to the physiology of animal or vegetable life is attended with peculiar difficulty. The immense number of species of living beings, the variety and complexity of the appearances they exhibit, and the extended chains of connection that pervade every part of organic nature, are strikingly contrasted with

the simplicity, the constancy, and the uniformity of operation of those physical forces which regulate the changes occurring in the inorganic world. The extensive generalizations which have been so successfully accomplished in the sciences of natural philosophy and chemistry, can be effected only in a very limited degree among the diversified phenomena which constitute the science of physiology; and we have, accordingly, made but very imperfect approaches to the knowledge of those laws to which they are ultimately reducible. The resources of experimental inquiry are here extremely narrowed, in consequence of the close connections which subsist among the powers concerned, and which preclude us from any opportunity of studying them in a separate or isolated state, and of ascertaining distinctly their respective operations.

Amidst these obstacles which impede our progress in the direct but thorny avenues to science, we naturally turn to the more alluring, though circuitous paths, which open upon us on every side, in the contemplation of the final purposes of the changes we are examining. So strongly is the character of design impressed upon all the phenomena of organic beings, that we are irresistibly led to associate the views suggested by their relative subservience to particular purposes, with the more strictly philosophical relation of cause and effect, by which they may also be connected. The relation of means to an end be-

* See J. F. Blumenbach, De Generis Humani Varietate Nativa Dissertatio, 1795, 8vo, and Prichard's Researches into the Physical History of Man.

Physiology. comes thus a leading principle of association among the phenomena presented to us by living beings: it gives to the science a new aspect, and creates an interest of a different, and even superior kind to that which mere physical relations are calculated to inspire. The study of the functions of life, that is, of the purposes to which the actions constituting life are subservient, is generally regarded, indeed, as the principal object of physiology; and all the facts relating to it are distributed according as they tend to the accomplishment of these purposes. Dazzled by the brilliancy of these objects, physiologists have often lost sight of the essential distinction which subsists between these, and the more sober purposes of philosophical inquiry. In framing theories to explain the phenomena of life, they have most frequently satisfied themselves with pointing out their final causes, that is, the objects which are answered in the economy: and as the detection of this final cause often required the exertion of considerable sagacity, the inquiry has terminated here, and it has not been perceived that the physical theory was left in as great obscurity as before. This proneness to substitute final for physical causes has been the source of frequent delusion, by insensibly leading us to believe that we are really in possession of the physical law on which the phenomenon in question is dependant, when we have merely given it a name with reference to the intelligent agency, by which it was adjusted to its object. In our eagerness to grasp at this kind of knowledge, we have too often mistaken the shadow for the substance.

Final, mistaken for Physical Cause. The writings of the older physiologists exhibit continual instances of this confusion of ideas. Thus the notion of an archæus, or anima, entertained by Van Helmont and Stahl, that is, of a presiding spirit, to the operation of which were referred all the vital actions, although, perhaps, naturally suggesting itself to the mind, was yet evidently an unphilosophical assumption, incompetent to explain the phenomena in question, and occasionally even at variance with those very phenomena. The vis medicatrix naturæ, to which Hoffman and Cullen so frequently appeal in their pathological reasonings, and which supplied them with ready solutions for every obscure morbid change that embarrassed them, was, in fact, nothing more than a branch of the same doctrine. Nor have the more sober theorists of modern times been sufficiently on their guard against this illusion. In the attributes which John Hunter ascribes to his vital principle, we may continually trace the same want of discrimination between that intelligence, by which the conditions of animated nature were originally adjusted to a variety of contingent circumstances, and those physical laws and agents, by the instrumentality of which the intended objects are attained. When it is said, for instance, in the language of this school, that the coagulation of the blood is occasioned by "the stimulus of necessity," it is clearly the final cause only, and not the physical cause of this phenomenon that is assigned: and it is also evident that no advance is thereby made towards the discovery of the latter. In like manner, the principle of life is represented to us as a new power, with which organized beings are endowed; a power which modifies and controls the operation of those

simpler physical laws, to which the same matter, in Physiology. its unorganized state, is subjected; a power which imposes new cohesive forces on the materials of the solid structures of the body, which imparts to the fluids a new property of coagulation, which alters the order of chemical affinities between their elements, retaining them, contrary to their natural tendencies, in a certain state of equilibrium, and resisting the agency of several causes tending to destroy it; and which, lastly, produces, in a degree somewhat corresponding to the wants of the system, either an evolution or an absorption of caloric. All these, it must be acknowledged, are purposes of manifest utility; being directly conducive to the welfare of the individual, and indeed essential to its continuance in the living state. As means conducive to a specific end, the reference of all these phenomena to the same class is unobjectionable. The fallacy lies in regarding it as a philosophical generalization of effects, of a similar kind, indicative of the operation of a simple power in nature. Between many of the effects in question, considered as physical phenomena, there exists not even the remotest analogy. But it is the fundamental principle of the method of induction that similar effects alone are to be ascribed to the agency of the same principle. Judging from the observed effects, therefore, which differ much from each other, as well as from other phenomena in nature, we ought to infer the agency of several distinct principles, the concurrence of which is required to produce all the complex phenomena of life. We are unavoidably led, no doubt, to view these phenomena as conjoined, because we readily perceive that they tend to the same object, the preservation and welfare of the beings to which they relate. But the unity of design is an attribute of intellect alone, and does not necessarily imply the unity of the agent employed in their production. However natural it may be to conceive the existence of a simple principle of life, and however possible it is, that this hypothesis may ultimately be established as the true one by future discoveries, we should recollect, that, in the present state of our knowledge, it is a mere fiction of the mind, not countenanced by the phenomena themselves, in which we see so much diversity, and, therefore, not admissible as the result of a truly philosophical induction.

Bichat, who was impressed with the necessity of drawing certain lines of distinction among the powers of life, has yet perplexed his system, by taking final causes as the basis of his divisions; a principle which is incompatible with a philosophical analysis of those powers. Thus, the distinction which he labours to establish, between the muscular contractility of animal life, and that of organic life, is founded, not upon any real difference in the nature of the power concerned; for, as we shall endeavour to show in the sequel, the power which resides in the muscles of the voluntary and of the involuntary motions is in all cases the same; but upon a difference in the application that is made of this power in the economy. Dumas has been guilty of a still more palpable error, in thinking it necessary to add to his catalogue of principles, consisting of the acknowledged powers of sensibility and contractility, a third power, which he terms force de resistance vitale; thus as-

Physiology. associating a final cause in the same rank with causes that are strictly physical. To multiply examples of this mistake would be endless; for it pervades almost every physiological system that has yet been framed.

Design of this Article. As a full account has already been given, under the head of PHYSIOLOGY, in the Encyclopædia, of the principal facts relating to the functions of the animal economy, which, as we have stated, are commonly regarded as the leading objects of that study, we shall, in this article, present an outline of what may be considered the philosophical department of the science, by attempting an analysis of the principal laws, or ultimate facts to which the vital phenomena are reducible. Setting aside, then, all consideration of functions, we shall examine the changes that occur in the living system, simply as physical phenomena; and we shall endeavour to class them according as they agree or differ among themselves, without reference to the purposes which they may happen to serve in the animal economy. Thus, if we take as an example, the phenomena of the circulation of the blood, which, when viewed with relation to the function, form together so beautiful and harmonious a system, we find that, considered abstractedly, these phenomena are ultimately resolvable into such as result from a few general powers, as muscular contractility, membranous elasticity, the hydraulic properties of the blood, &c. If a similar analysis were made of the phenomena of digestion, we should find them to be the effects of the combined agencies of the muscular action of the stomach and intestines, of the chemical properties of the gastric juice, the bile, &c. of the organic powers of secretion, and so forth; all of which concur in the production of a definite object, namely the conversion of the aliment into chyle. The processes subservient to this object constitute the function of digestion.

These analytical investigations have not been prosecuted by physiologists with the attention their importance deserves. Our knowledge of the principles of the science has, however, been considerably increased by the experimental inquiries that have recently been instituted both in this country and on the continent; and we shall therefore avail ourselves of the present period, as affording peculiar advantages for taking a comprehensive review of the facts which bear upon these questions, and for deducing consequences that may throw considerable light upon the animal economy, and the theory of diseases.

Classification of the Vital Powers. Of the changes which occur in the living system, some are easily referable to particular classes; others, again, having a less distinct character, are generalized with greater difficulty. Some of the powers concerned in the phenomena of life may be recognized as being identical with those which produce changes in inanimate matter, while the rest are powers peculiar to the living state. Thus the cohesion, elasticity, and tenacity of the particles of the solids of the body; the strength, resistance, &c. of the materials which compose them; and the hydrostatic and hydraulic laws of its fluids, are principles upon which we may safely reason in their application to the mechanism of the living system. Many of the laws of chemistry are also applicable to its phenomena. From our knowledge of these principles we may

predict the effects which will, under given circumstances, ensue: and if experience, in any case, teaches us that the result is different, we may thence infer the operation of new causes, peculiar to the living state, and constituting other classes of phenomena.

The powers comprehended under this latter division have usually been described as reducible to two species, namely contractility and sensibility; for it was supposed that all the phenomena might be arranged under these two heads. We shall endeavour to show that their analysis is incomplete, and to point out at least four distinct principles of which the operation may be recognized in the living body: these are Muscular Contractility, Nervous Agency, Sensorial Power, and Organic Affinity. Let us first, however, fix the meaning of these terms.

Muscular contractility is a power too well known by its effects to require any elaborate definition or illustration in this place. It consists in that property by which, in consequence of the impression of certain agents, the extremities of muscular fibres are made to approach each other, with a force greatly superior to the ordinary mechanical sources of motion.

By the term nervous agency we would be understood to mean that power which resides in the nervous system, and by which certain effects, hereafter to be described, and which for the present we shall call impressions, made on one part of that system of organs, are immediately succeeded by certain other effects at a remote part of the same system. The application of an irritating substance, for example, to one extremity of a nerve, will excite, in one case, muscular contraction; in another case, sensation; in a third, secretion; in a fourth, it will raise the temperature; while, in other instances, it will produce changes of vascular action, and lead to an alteration of the organic structure of parts. Since this propagation of impressions is a fact not exactly analogous to any other phenomenon in nature, we must regard it as the effect of a distinct power, concurring with the rest in producing the general result which we term life.

Another power, perfectly distinct from the former, although it also resides in a portion of the nervous system, is that from which the corporeal operations connected with sensation and volition result. Dr Wilson Philip, to whom we are indebted for having clearly pointed out this distinction, has given to this property the name of sensorial power. As the term seems to be sufficiently appropriate, we shall adopt it as the designation of this specific property of the nervous system. Dr Darwin had employed the same term in a more extended sense, as including the power of muscular contraction. It should be remembered that it is here limited to those physiological changes in which the mind is immediately concerned.

By the admission of the powers already enumerated do not yet supply us with the means of explaining a variety of phenomena exhibited by the living system, and which are sufficiently analogous in their nature to warrant our classing them together as depending upon the operation of a common principle. The phenomena in question are those of secretion and

Physiology. nutrition, including those of the growth, extension, and modelling of the various organic structures which compose the animal fabric. The powers by which these effects are produced exist in the vegetable as well as the animal kingdom, and appear to result from that particular arrangement of parts which is termed organization. These powers, when considered abstractedly from the purposes they serve in the economy, we shall denominate the organic affinities, by way of contradistinction from the ordinary chemical affinities to which they are so frequently opposed.

I. We shall first examine the MECHANICAL PROPERTIES of animal structures.

Organization of Animal Membrane. The basis of the organic texture of the different parts of the body is a peculiar substance, which, amidst its various modifications of cellular tissue, membrane, vessels, neurilema, visceral parenchyma, &c. may be recognized as essentially the same. The term membrane, which is applied more especially to a condensed lamina of this substance, may be conveniently extended to the rest, and employed as the generic term for the whole. It has been disputed whether this substance was ultimately resolvable into plates or fibres, and the microscope has been appealed to in support of both opinions; but after all that has been said about the primordial animal fibre, which was stated by Haller to bear the same relation to anatomy which a line does to geometry, the whole may possibly be more the fruit of imagination than the sober account of the real fact. Fontana had viewed it as an assemblage of cylindric fibres, which were twisted and interlaced with each other; but Monro has shown that he was deceived by an optical illusion, to which the incautious use of the microscope frequently gives rise. Bichat describes its intimate structure as composed both of filaments and of laminae, variously intermixed; and hazards a conjecture that the former are exhalant and absorbent vessels. Bordeu appears to have been the first who advanced the opinion of the homogeneous nature of the cellular tissue, which he compares to froth or glue. Quessay considered it more as a fluid than as an organized solid. Wolff rejects entirely the idea of its being cellular, and regards it as a homogeneous and glutinous substance, without organization. Blumenbach, Platner, and Meckel, have adopted these views; a statement of the arguments in favour of which is given by Bérard in his Additions à l'Anatomie Générale de Bichat. The subject has already been noticed under the head of ANATOMY in this Supplement. Whatever weight may be allowed to the arguments in favour of the cellular tissue being a homogeneous substance, it seems, from other considerations, more reasonable to suppose that the mechanical structure of all animal substances is framed, even in the simplest cases, with a greater degree of complication than we shall ever have the means of fully ascertaining.

Its Mechanical Properties. Although ignorant of the arrangement of particles which constitutes the organization of animal membrane, we observe certain mechanical properties to result from it, analogous, it is true, to those that are met with in several inorganic substances, but in ge-

Physiology. neral much superior to them in degree. These are flexibility, extensibility, and elasticity. These properties, in their different degrees, are variously combined and modified in the different forms of animal substance, but exist more or less in every organ. As it is not our object to enter into any consideration of the functions to which these properties are subservient, we shall abstain from any remarks on the utility of these properties, but confine ourselves to their physical relations. In this respect, one very striking circumstance requires to be pointed out, namely, that the force of elasticity among the particles composing these animal structures is rarely found in a state of neutrality, but is kept in equilibrium by the mechanical circumstances of situation. When these circumstances are deranged, elasticity comes into play, and produces a shrinking of the substance. In other words, every part is kept upon the stretch, and retracts when set at liberty by the removal of the extending cause. This will happen when its extremities are brought nearer to one another, when the contents of the hollow parts are withdrawn, and whenever they are divided transversely. This property has long been known, though described under different names; that of tone, or tonicity, has frequently been applied to it. Bichat, who has very well described its effects, has denominated it contractilité de tissu, and contractilité par défaut d'extension, and has distinguished it from tonicity, which he regards as a vital property.

Fibrous Tissues. The mechanical properties we have enumerated are greatly modified by diversities of structure. The same substance, when in the state of greatest condensation, composes what Bichat has denominated the fibrous tissue. Chaussier has considered it as of a peculiar nature, differing from ordinary membrane, and has given it the name of fibre albuginée, ascribing to it the character of having a white colour, and a resplendent satin-like surface, an appearance which it owes to its great density. Of these fibres are the tendons, aponeuroses, and ligaments principally composed. Among these we may again trace a diversity of properties. Thus tendons exhibit the smallest degree of extensibility compatible with membranous texture, although they possess great flexibility. The ligaments belonging to joints are still more flexible, and somewhat more extensible and elastic. Those ligamentous structures, on the other hand, which are employed as an antagonist power to gravitation, or to muscular action, such as the ligamentum nuchæ, which counteracts the weight of the head in grazing quadrupeds, are very extensible, and possess a high degree of elasticity. A layer of the same elastic substance extends over the parietes of the abdomen in these animals, for the support of the abdominal viscera. The elastic ligaments which retract the claws of the cat, and other animals of the same tribe, exhibit the same property. Bérard considers this highly elastic substance as a separate modification of membranous structure, distinguished from the tissu albuginé of Chaussier by its yellow colour and peculiar elasticity. Ligaments, having the same properties, but which are white, instead of yellow, are extensively met with in the anatomy of insects.

Among the physical properties of animal membrane must also be enumerated a peculiar kind of contractility, which is accompanied with a sudden corrugation and curling of its substance. This effect, which was noticed by Haller, and which Bichat designates by the term racornissement, is produced by the application of a certain degree of heat, and also of several chemical agents, more especially the concentrated mineral acids. Alcohol and the neutral salts effect a similar change, but much more slowly, and in a very inferior degree; and the effect continues to increase if the agent continues applied, which is not the case when acids or boiling water are used. The continued applications of these latter agents gradually effect the disorganization and solution of the animal matter. Bichat has taken considerable pains to investigate these phenomena, and we must refer our readers to his Anatomie Générale for the details of his experiments. He has pointed out several circumstances by which this property is distinguished from mere membranous elasticity. Although it has been thought to resemble muscular contractility, it will really be found, when strictly compared, to differ from this last property in all essential particulars; and it probably, therefore, depends on causes that are wholly different.

An effect somewhat analogous to the former, although much less in degree, takes place in animal membranes by the evaporation of the water which is united to it, but which appears to be retained by a weak affinity. This constitutes what may be called the hygrometric property, and is very characteristic of dry membranous structures, all of which are found more or less to contract by the loss of moisture, and again to expand by its reabsorption, according to the varying states of dryness and humidity in the surrounding atmosphere. The organic tissues of vegetables exhibit this property; but in a very inferior degree compared with animal membrane.

The mechanical properties of membrane which we have been examining are totally independent of the vital properties that are next to come under our review. They remain some time after the complete extinction of life in all its functions, and seem to be connected with the peculiar arrangement of particles, and the chemical composition of the substance in which they reside. They appear, indeed, not to be affected until the progress of decomposition has become sensible. Hence this assemblage of powers was denominated by Haller the vis mortua.

II. MUSCULAR CONTRACTILITY is one of the most remarkable of those properties which are peculiar to animal life. It is often distinguished by the name of irritability, which was originally given to it by Glisson. But the merit of having clearly appreciated its importance, as a separate and peculiar power, is due to Haller, who speaks of it sometimes under the title of irritability, and sometimes under that of the vis insita. The phenomena of muscular contraction have already been sufficiently detailed in the Article PHYSIOLOGY in the Encyclopædia.

This property has been established in the system as the great source of mechanical power required for

the operations of the animal machine. As in a manufacturing where the force of steam is employed as the prime mover of the whole of its complicated machinery, so, in the animal system, is the muscular power resorted to on every occasion where mechanical force is required. From its vast intensity, this power appears adequate to every purpose; and though, in some instances, it may seem to have been lavished with profuseness for the sake of slight additional convenience, in others it is carefully economized; and perhaps more accurate examination would show that it is in all cases exactly adjusted to the intended effect. Before the time of Haller it was generally regarded as an extension of the nervous power; and so great was the confusion of ideas on this subject, that even Boerhaave speaks of tendon as merely a modification of muscular structure.

The occasion, or exciting cause, which gives rise to the exertion of this power, is termed a stimulus. Thus all muscular contraction implies two things, the irritability, which constitutes the power, and the stimulus, which determines the action of that power. The irritability, according to Haller, is the same in kind, wherever muscular fibres are met with; it only varies in intensity in the different muscles: but it does not in all of them obey the same stimuli. The nervous power is the natural stimulus of all those muscles which are under the influence of the will: on the other hand, the muscles of involuntary motion are affected by stimuli of different kinds, which are appropriated to their different functions, and altogether different from the nervous power. Thus the blood is the natural stimulus which excites the contraction of the heart; and the alimentary canal, the bladder, uterus, &c. are, in like manner, excited to action by their respective contents.

It is only by examining attentively the circumstances in which the muscular power is exerted, that we can hope to attain a knowledge of its nature. That a particular mechanical structure is required for its production is apparent from the regular arrangement of parallel fibres, connected into fasciculi of larger and larger dimensions by separate investments of membrane; and also from their great vascularity. Muscles are more abundantly supplied with blood-vessels than any other parts, excepting the lungs and the organs appropriated for secretion. The minute or capillary veins are more particularly numerous, forming a vascular net-work, and are provided with numerous valves. Much contrariety of evidence exists as to the intimate structure of muscular fibres. Leeuwenhoek represents them as being exceedingly minute, many thousand uniting to form one visible fibre, but that they differ considerably in diameter in different animals, without any relation to the size of the animal. He states, for example, the fibre of the frog to be larger than that of the ox. He thinks their size also varies according to the age of the animal, being smallest in the earlier periods of life. Muys, who was engaged for many years in the most laborious researches on this subject, concludes, on the other hand, that the real ultimate filaments of muscles are in all cases of the same size, even when compared among the mammalia, birds, and insects. Prochaska, again, says expressly that they are not all of the

Physiology. same diameter, but differ in different animals, and even in different parts of the same animal. Their diameter has in general been stated as less than that of the globules of the blood; but Sprengel speaks of them as being equal to the 500th of an inch in the mammalia, and the 250th in birds and fishes. Some microscopical observers have represented them as hollow tubes; but this is probably an optical deception, like that which has led to the belief that hairs are tubular. Several, such as King and Tauvry, have imagined them to be continuations of arteries; an opinion which was connected with the theory of the indefinite extension of vascularity, formerly prevalent, but since sufficiently refuted by observation as well as reasoning. Prochaska asserts, with confidence, that they are solid, and of a polyhedral, prismatic shape, generally flattened, or thicker on one side than on another, so that a transverse section presented an appearance similar to that of basaltic pillars. Hook and Swammerdam reported the fibres to be composed of a series of globules. Cooper and Stuart supposed them to be cellular, and Borelli that they were formed of a string of rhomboidal vesicles. Fontana's account of them in general agrees with that of Prochaska; but he remarks that they are furnished, at regular intervals, with transverse bands; and that they may always be distinguished by their parallel disposition from the fibres of membrane, which are more or less contorted. Sir Anthony Carlisle states, that a muscular fibre, duly prepared, by washing away the adhering extraneous substances, and exposed to view in a powerful microscope, appears to be a solid cylinder, the covering of which is reticular membrane, and the contained part a dry pulpy substance, irregularly granulated, and of little cohesive power, when dead. Mr Bauer represents them as composed of a row of globules, exactly corresponding in size to those of the blood, when deprived of their colouring matter. By long maceration in water, the cohesion of these globules is loosened, and the fibre is broken down into a mass of globules. The statement of these various opinions is sufficient to show how little satisfactory information has been gained on the subject. Whenever an observer has a favourite theory to support, the microscope is ever ready to assist him in seeing what he expects, or wishes to discover.

Mechanical Theories of Muscular Motion. In the infancy of rational physiology, much labour was bestowed upon devising some mechanical arrangement of particles that might account for the phenomena of muscular contraction. With this view Borelli contrived his rhomboidal vesicles, which he supposed to be empty in the relaxed state of the fibre, but suddenly distended by the introduction of a fluid derived from the nerves, which shortened as well as swelled each vesicle, and, consequently, the whole muscle. In this hypothesis, though ingeniously adapted to the phenomena, no power is assigned for the sudden propulsion of so large a quantity of fluid into the vesicles; the resistance that would be opposed to the entry of such a fluid would be immense, and the force required to overcome it would be much greater than even that exerted by the muscle itself, which it was the object of the hypothesis to explain. The supposition, there-

Physiology. fore, involves a greater difficulty than the simple fact. Some have, in like manner, imagined they could explain the phenomena by the turgescence of the numerous arteries which are seen to cross the fibres at right angles; not recollecting that the very force which distends the arteries is itself derived from the muscular power of the heart and arterial trunks, which could not create a power greater than itself. The hypothesis of the spiral course of the ultimate fibres, a form which admitted of elongation or contraction, according to the degree of convolution, is as gratuitous as the preceding; and equally open to the objection that the original source of motion is left unexplained. Dr Fordyce states a fact which places it in a striking point of view, the circumstance of a new force being generated during muscular contraction. If the interior surface of the ventricle of the heart, detached from the body, be pricked gently by a needle introduced into its cavity, the ventricle will thereby be made to contract with such power as to force the needle deep into it. The force of the contraction of the ventricle must have been incomparably greater than the power with which it was pricked by the needle. Muscular power, indeed, bears not the least analogy to any of the other great principles in nature, which are original sources of mechanical force; and, until such an analogy can be traced, all our endeavours to explain the phenomena by mechanical hypothesis must be as fruitless as the attempts to contrive a machine for perpetual motion. Some physiologists, wishing to avoid all hypothesis, propose to explain the phenomena of muscular contraction, by saying that it arises from an increase of attraction among the particles of the muscular fibre. Dr Fordyce calls this force "the attraction of life," a term, which, if it has any meaning, is merely a statement of the simple fact under a new form of expression.

Chemical Theories. Muscular fibres differ from those of membrane in chemical composition, as well as mechanical structure; and it becomes a question how far their properties depend on a peculiar combination of chemical elements. Physiologists have, accordingly, endeavoured to ascertain whether, while the mechanical texture was unaltered, any changes occurring in the chemical condition of a muscle would be accompanied by a corresponding change in its contractile power; and, whether there was any one element in particular, the presence of which was more essential than the rest to the exertion of that power. It was long the fashion to regard oxygen as the source of this power. This theory was advanced by Girtanner, and found strenuous advocates in Humboldt, Beddoes, and Richerand. Some account of the reasonings on which they founded this opinion has been given in the Encyclopædia. Experiments were made to ascertain the influence which the alternate abstraction and restoration of oxygen had on irritability, of the presence of which the galvanic excitation was used as a test. The general result of the inquiry was, that a certain proportion of uncombined oxygen is essential to the maintenance of irritability. The presence of fibrin has been regarded essential to the constitution of the muscular fibre; but Fourcroy has shown that different modifications of fibrin are compatible with irri-

Physiology. tability. The necessity of a due supply of caloric has, in like manner, been the subject of inquiry; and, as might have been expected, the preservation of a certain temperature has been found requisite to muscular action. All that can be safely concluded, however, from these investigations, is, that a certain state of chemical composition and of temperature, is as essential as a certain mechanical structure to muscular contractility, but that these conditions admit of some degree of variation within certain limits.

In the more perfect animals, muscular contractility remains but for a short time after the circulation of the blood has ceased. A ligature on the arteries which distribute this fluid to a muscle, occasions the speedy loss of its irritability; arterial blood, therefore, supplies some material requisite for the preservation of the proper chemical state of the muscular fibre. Yet different classes of animals are very differently constituted with respect to this circumstance. The muscles of the amphibia will remain irritable long after an entire stop has been put to the circulation; and this takes place even in limbs that are detached from the body.

The operation of the muscular power is of so distinct and specific a character, that it appears surprising how phenomena of any other class could be confounded with it; and yet several distinguished physiologists have ascribed to different portions of the cellular substance a contractile power analogous to that of muscles. The tunica vaginalis testis, and its surrounding cellular tissue, are said to exhibit indications of this peculiar species of contractility, from the irritation of stimuli, or by the application of cold. "The contractions that ensue," says Bichat, "are doubtless not to be compared to that of muscles, but they certainly constitute the first degree of that power; they are the same in kind, or rather they hold a middle rank between muscular contractions and those minute and invisible oscillations which others call tonicity, &c." There does not appear to us to be any foundation in fact for this supposed gradation of the muscular power. The motions in question may partly be accounted for by the known power of elasticity, which undoubtedly varies in degree in different textures, and partly by the real but undetected presence of muscular fibres. The vis cellulosa of Blumenbach appears, in like manner, to be no new power, but simply the modified elasticity of the texture in which it resides. A few cases have been pointed out, which are perhaps of a dubious character, such as the motions of the iris, the muscularity of which has been often called in question, though it now appears to be sufficiently established by the microscopical observations of M. Bauer. So much importance has been attached to these apparent anomalies, that it has been thought necessary, in order to account for them, to suppose the existence of a new power, the vita propria; or rather, as it should be said, to invent a new term void of any definite signification.

It has frequently been supposed, that, besides the power of contraction, muscular fibres had also the opposite power of spontaneous elongation, when the former ceased to be exerted. Bichat countenances this doctrine when he says, that "it appears very

probable that the dilatation of muscles is a phenomenon equally vital with their contraction." But all the facts that have been adduced in favour of this notion may, as John Hunter has shown, be completely explained by the operation of other causes; such as that of antagonist muscles, or of the natural elasticity of neighbouring parts, or of the cellular substance contained in the muscle itself; for it should not be forgotten that a muscle, in addition to its contractility, possesses all the properties belonging to animal membrane, which composes so large a portion of its structure. Many phenomena in the movements of animals, which may, at first sight, have the appearance of arising from a spontaneous power of dilatation, such as the elongation of the trunk of the elephant, of the tentacula of polypi, and the bodies of the leech and other vermes, and the extension of the feet, and other soft parts of mollusca, are, like the varied motions of the tongue, only secondary effects of the contraction of certain muscular fibres so disposed as to produce these effects. It has been said that the heart has been found to exert, during its dilatation, a positive force, but probably if the course of all the fibres composing the muscular parietes of that organ were better known, this apparent anomaly would be as easily explained as the rest.

Still less could it have been supposed possible to confound the nervous with the muscular power; and yet, prior to the time of Haller, no clear ideas were entertained of the distinction between them; and, even in later times, the subject has been involved in much perplexity. Muscular fibres used to be spoken of as only nervous fibres on a large scale; exhibiting distinctly, on account of their greater magnitude, the contractions which were presumed to take place in the ultimate fibres of the nerves during their action, but which were insensible on account of their minuteness. The retraction of divided nerves is clearly an effect of membranous elasticity.

III.—NERVOUS AGENCY, or that property of the nervous system by which it receives impressions of Nervous Agency. made on one part, and transmits them to others, is manifested in several ways. First, by exciting muscular contractions. The usual mode in which this occurs is seen in the muscles of voluntary motion, the actions of which are determined by an effort of the will, which produces an impression on the nerves sent to these muscles at their origin in the sensorium; this impression is propagated along these nerves to the muscles themselves, where it appears to act upon their irritability like any other stimulus, and to produce their contraction. But the very same effect takes place quite independently of the mind, by an irritation of a mechanical or chemical nature applied at the origin of the nerves, or in any part of their course. This conducting power in the nerves may, by these means, be called into action long after the extinction of sensibility, and may be observed even in a limb removed from the body; for muscular contractions are produced in it by mechanical irritation of the ends of the nerves leading to those muscles, and still more readily by galvanic excitation. The contraction of muscles which are not under the dominion of the

Physiology. will, such as the heart, are not so evidently the consequence of irritations applied to the nerves which terminate in them, in consequence of the peculiar mode of arrangement of these nerves; but as we shall hereafter show, the conducting power of these nerves is no less real than in the former. Various stimuli, operating through the medium of the nervous system, and different affections of the mind, affect the motions of the heart and arterial system, and the muscular fibres of the alimentary canal; and produce local determinations of blood, and increased vascular actions of particular parts. Secondly, there are various operations, such as secretion, conducted in the minutest textures of the body by means that entirely escape our cognizance, which are materially influenced by certain irritations propagated along the nerves. But the mode in which this influence is exerted is reserved for future discussion. Thirdly, another, and no less important effect of nervous action is the production of sensation. An impression made upon the sentient extremity of a nerve is propagated to the sensorium, when it produces other changes, immediately followed by that affection of the sentient principle, accompanied by consciousness, which we term sensation. This latter class of effects we shall refer, as already stated, to a power distinct from the nervous, and which we have called the sensorial power. Fourthly, among the effects resulting from the nervous power, those which arise from the communication of irritations to very remote parts, which are but indirectly connected by nerves, must not be omitted. They are usually comprehended under the title of the effects of sympathy. In some cases the nervous communications may be traced; in others, it appears to take place through the medium of the central parts of the nervous system, that is, the brain and spinal marrow. In proportion as the anatomy of the nervous system has been more accurately explored, the former appear to be more numerous; though there are still a great number of cases which can only be explained on the latter supposition.

In what way this propagation of impressions by the nerves is effected, we are wholly ignorant. The celerity with which they are transmitted along the whole line of communication, bears a greater resemblance to the transmission of the electric agency along conducting wires, than to any other fact we are acquainted with in nature: and on the strength of this analogy the nervous influence itself has often been conceived to be of an electrical nature. In those fishes which exhibit powerful electrical phenomena, as the torpedo, gymnotus, and silurus, the organs appropriated to the production of these effects, are supplied with an enormous mass of nerves, showing clearly the important part which the nervous influence plays in these phenomena. The processes of secretion are disturbed when the nervous communications between the secreting organ and the brain are intercepted by the division of the nerves; but in the case of the stomach, the natural process of digestion is resumed when galvanic electricity is transmitted through those portions of the nerves which remain connected with the stomach. As Voltaic electricity is known to produce chemical changes in the substances on which it is made to act, it was con-

ceived that the chemical changes constituting secretion were in like manner effected by electricity, of which the nerves were the conductors. Such are the principal arguments brought forward in support of the identity of the nervous and the electrical agencies; a hypothesis which was first advanced by Valli at the period when the effects of Galvanism on the muscles, or animal electricity, as it was then called, began to engage the attention of the philosophic world. We shall have occasion to revert to this theory in the sequel.

The nervous, like the other animal powers, is dependent on a certain mechanical and chemical constitution of the organs which exercise it. Anatomists are not agreed as to the minute and ultimate structure of nervous matter. Ruysch and Leeuwenhoek considered it as vascular, an opinion to which Haller subscribes; but Albinus denies the vascularity of the medullary substance, as neither apparent by the microscope, nor by the evidence of injections. De la Torre asserts that it consists of a mass of innumerable transparent globules swimming in a diaphanous fluid; and that these globules are larger in the brain than in the spinal marrow. Prochaska describes the same globular structure, which he represents as united by a transparent elastic cellular membrane, disposed in fibres. Monro first thought these fibres to be convoluted, but afterwards suspected some optical deception. Fontana found the nerves to be composed of a number of minute cylinders, seemingly composed of a pellicle, and partly filled with a transparent gelatinous humour, and with small unequal globules. Sir Everard Home describes the optic nerves of a horse as "composed of two parts, one opaque, and the other transparent, forming fibres of a peculiar kind, unlike those of any other part of the body. Their course is curious, for they appear to be constantly passing from one fasciculus to another, so as to connect all the different fasciculi together by a mixture of fibres. This is different from the course of the blood-vessels, lymphatics, or muscular fibres; the only thing similar to it is in the formation of nervous plexuses, which leads to the idea of its answering an essential purpose respecting the functions of the nerves."

Whatever be the peculiar organization from which such astonishing effects result, we may at least be assured that the following conditions are requisite for their appearance; namely, a certain continuity of nervous substance, freedom from pressure, and the continued supply of arterial blood. With respect to the first of these conditions, however, the experiments of Dr Philip and of Mr Brodie, the results of which are stated in the Philosophical Transactions for 1822, would seem to show that the mere division of a nerve, if the cut ends are not above a quarter of an inch asunder, is not sufficient to interrupt the transmission of that portion of nervous influence with which the secretions are concerned. It is to be hoped that the prosecution of this curious and important discovery will throw some light on the mysterious nature of nervous agency.

As the nerves, in warm-blooded animals, lose their power in a very short time after they are isolated from the rest of the system, it has been naturally

Physiology. conceived that they derive a supply of power from the large central masses constituting the brain and spinal marrow, where it may be supposed to be prepared and elaborated, in a manner somewhat analogous to the process of secretion. It has been further imagined, that this power was capable of being accumulated in the brain and spinal marrow, forming a kind of stock or perpetual source, to supply the expenditure that takes place by the several nerves, which conduct it off. Prochaska, on the other hand, thinks that the nervous power is generated throughout the whole extent of the nervous system, so that every part derives from its own nerves, taken alone, the cause of its life and movements. There are facts in favour of each side of the question, and the subject is still involved in considerable obscurity.

IV.—SENSORIAL POWER is manifested in the production of sensation and volition. These effects, when they occur in ourselves, we know by consciousness, but in other animals we can only infer their presence by the voluntary actions to which they give rise; that is, by the well-marked expressions of pain or pleasure, and by the contraction of certain muscles, with the evident intentions of gratifying natural appetites, or of avoiding or removing what occasions pain. The term sensibility has been used by modern writers with great latitude, as expressing generally the capacity of being affected by impressions. Thus Bichat speaks of organic sensibility as contradistinguished from animal sensibility. But the extension of this term to any property that does not involve sensation attended with consciousness, is too indefinite, and tends to introduce a confusion of ideas. The nervous and sensorial powers, though in themselves perfectly distinct, had, in general, been confounded together by physiologists. Le Gallois was the first who pointed out the difference between them; but their distinctive characters have been most clearly marked by Dr Philip in his Experimental Inquiry into the Laws of the Vital Functions, a work which has opened new and important views in this department of physiological science. He has brought evidence to show that these two systems do not differ less from each other than they do from the muscular system. It appears from his observations, that after the destruction of the sensorial power, the nervous power is still capable of performing its other functions, although it can no longer excite sensation, because the power on which sensation depends no longer exists. This happens, as we shall afterwards have occasion to notice, at the instant of death.

The power of sensation is called into action by impressions conveyed along the nerves: and the nerves appear to be the only medium through which these impressions can reach those parts of the nervous system, on the changes of which sensation depends. It has, however, been asserted, that other parts, besides the nerves, are endowed with this power. It is alleged, for instance, that muscular fibres are sensible, because the sensation accompanying muscular contraction is different from that of passive impressions; and because spasms are attend-

ed with peculiar pain. But experience shows, that Physiology. the power of exciting sensation is very variable in the nerves of the same part, according to the affections of that part. Increased determination of blood to any organ generally augments its sensibility. Inflammation exalts it in a still more remarkable degree, so that parts usually insensible, as the bones, tendons, and ligaments, become exquisitely sensible in many states of disease. The marrow of cylindrical bones appears, under ordinary circumstances, to possess very little sensibility, as is seen in amputations: but in other states, it becomes highly sensible; and, if irritation be applied in particular ways, as by pressure over a certain extent of marrow, the most acute pain is immediately felt. These variations will explain the difference and even contrariety of statements that have been made by different physiologists on this subject. The greater number of experimentalists have denied that it possessed any sensibility. Duverney and Bichat, on the other hand, represent it as highly sensible. Béclard remarks, that we cannot draw any correct inference from what happens in amputation, because the intense pain suffered during the section of the soft parts, has rendered the animal scarcely sensible to a lesser degree of pain immediately succeeding: but that, if the operation be suspended until this first impression has in a great measure subsided, any injury done to the marrow will be acutely perceived. It appears, indeed, that different nerves have very different powers of exciting sensation: but the consideration of this subject involves the discussion of a previous question of great interest, and on which some light has been thrown by recent discoveries.

The nervous power is manifested, as we have already stated, by the transmission of impressions. Some of these impressions, arising from the remote extremities of the nerves, are conveyed to the sensorium, and produce sensation: others originate in the sensorium, being consequent on acts of volition, and are transmitted to the muscles. With regard to these processes, which take place in opposite directions, the following questions may be asked:— Are the impressions, as far as regards the nerves, in both cases of the same kind, and modified only by the structure of the organs of sense, of that part of the sensorium with which the nerves communicate, and of the particular muscles in which they terminate: or, are the impressions modified by differences in the structure of the nerves, which are the vehicles of their transmission? Or, in other words, are all nervous filaments alike in function, merely conveying, like electric conductors, the same agent exactly as they receive it? In particular, can the same nerve, or nervous filament, transmit both kinds of impressions, namely, those of sensation and of volition: and, if they can, do these impressions, which pass in opposite directions, ever clash and interfere with one another; or do they cross one another, without collision, like the rays of light through a lens? Or, are there two sets of fibres, the one for sensation, and the other for motion, as there are veins and arteries for the transmission of blood in opposite directions? The observations which seem first to have suggested this latter notion, were taken

Physiology. from cases of partial paralysis of a limb, in which the power of motion remains, although that of sensation is lost. Some experiments of Arne's, in which nerves that had been divided, and had spontaneously reunited, were found to have recovered the power of the voluntary excitement of the muscles, but to have permanently lost the power of producing sensation, showed the possibility of a separation of these two functions.

The doctrine of there being two sets of nerves appropriated to these respective offices, had been taught, in the infancy of the science, by Erasistratus and Herophilus; and Galen was inclined to adopt the same opinion, from observing that both in the tongue and in the eye, the nerves supplying these organs are of two kinds. Galen, however, accounted for the anomaly in cases of paralysis, by saying, that a greater nervous power is necessary for motion than for sensation; so that sufficient might remain for the latter, when it was inadequate to the former function. Haller and Sauvages have both subscribed to this doctrine, and the consequences deduced from it. Dr Philip thinks we must admit that the bundles of nerves going directly from the brain or spinal marrow to any part of the body, contain nerves of two descriptions, one set adapted to convey the dictates of the will, the other to convey impressions from the part to the sensorium. This, he thinks, more probable than that impressions move backwards and forwards in actually the same channels. One of these opinions must be correct. If the former is so, there is no difficulty in accounting for the feeling being lost and the power of motion remaining, and vice versa. Indeed, these phenomena of disease seem to go some way towards proving the former opinion. Mr Charles Bell, also, is of opinion that the nerves which we trace in the body are not single nerves, possessing various powers, but bundles of different nerves, the filaments of which are united for the convenience of distribution, but which are distinct in their office, as well as in their origin. "It is remarkable," he observes, "that an impression made on two different nerves of sense, though with the same instrument, will produce two distinct sensations; and the ideas resulting will only have relation to the organ affected. There are four kinds of papillæ on the tongue, but with two of those only we have to do at present. Of these, the papillæ of one kind form the seat of the sense of taste; the other papillæ, more numerous and smaller, resemble the extremities of the nerves in the common skin, and are the organs of touch in the tongue. When I take a sharp steel point, and touch one of these papillæ, I feel the sharpness. The sense of touch informs me of the shape of the instrument. When I touch a papilla of taste, I have no sensation similar to the former. I do not know that a point touches the tongue, but I am sensible of a metallic taste, and the sensation passes backward on the tongue." Richerand states some experiments which he made on dogs, from which it appears that the functions of the different nerves sent to the tongue are widely different. (Elem. de Physiologie, T. II. p. 66, 8th edition.)

That the functions of the different sets of filaments composing the spinal nerves, and which arise respec-

tively from the anterior and from the posterior fasciculi of the spinal marrow, are exceedingly different, has been proved by an experiment of Mr Bell's, of which he gives the following account. Former researches had led him to suspect that the functions of these two portions of the spinal marrow were different. He found that injury done to the anterior portion convulsed the animal more certainly than injury done to the posterior portion: but it was difficult to make the experiment without injuring both portions. But on considering that the spinal nerves have a double root, and being of opinion that the properties of the nerves are derived from their connexions with the parts of the brain, he thought this an opportunity of putting that opinion to the test of experiment, and of proving at the same time that nerves of different endowments were in the same cord, and inclosed in the same sheath. On laying bare the roots of the spinal nerves, he found that he could cut across the posterior fasciculus of nerves, which took its origin from the posterior portion of the spinal marrow, without convulsing the muscles of the back: but that on touching the anterior fasciculus with the point of the knife, the muscles of the back were immediately convulsed.

In a paper published in the Philosophical Transactions for 1821, Mr Bell considers the nerves as distinguished, from their functions, into two classes; the one composing what he calls the original or symmetrical system, and the other the superadded or irregular system. The former are more expressly provided for the purposes of sensation and locomotion. In animals where these functions are not complicated with those of circulation and respiration by central organs, these nerves are very simple, consisting merely of two cords, running the whole length of the body, and giving off lateral branches to the several divisions of which their annulated frame is composed. This is the case with insects, and with most of the vermes. As we ascend to the higher orders of animals, we find a greater complication of functions and a greater intricacy of nervous connexions, arising from the necessity of establishing extensive links of association between the organs that perform these additional functions. Hence the second class of nerves are provided, which crossing the former in a variety of directions, and occasionally uniting with them, gives rise to the intricacy and apparent confusion, in which the anatomy of the nervous system has hitherto been involved. The nerves belonging to the first class may be distinguished in the human body as forming the original system, if abstraction be made of all the superadded nerves. The nerves of the spine, the tenth or sub-occipital nerve, and the fifth or trigeminal of the system of Willis, constitute this system. All these nerves agree in the following essential circumstances: they have all double origins; they have all ganglia on one of their roots; they go out laterally to certain divisions of the body; they do not interfere to unite the divisions of the frame; they are all muscular nerves, ordering the voluntary motions of the frame; they are all exquisitely sensible, and the source of the common sensibility of the surfaces of the body. When accurately represented on paper, they are seen to pervade every part; no part is

Physiology. without them; and yet they are symmetrical and simple, as the nerves of the lower animals. On the other hand, the nerves which connect the internal organs of respiration with the sensibilities of remote parts, and with the respiratory muscles, are distinguished from the former by their not arising from double roots, and having no ganglia on their origins; they come off from the medulla oblongata and upper part of the spinal marrow; and from this origin, they diverge to those several remote parts of the frame, which are combined in the motions of respiration. If the nerves be exposed in a living animal, those of the former class exhibit the highest degree of sensibility; while, on the contrary, those of the second are comparatively so little sensible, as to be immediately distinguished: in so much that the quiescence of the animal suggests a doubt whether they be sensible in any degree whatever. If the fifth pair, and the portio dura of the seventh, be both exposed on the face of a living animal, there will not remain the slightest doubt in the mind of the experimenter, which of these nerves bestows sensibility. If the nerve of the first class be divided, the skin and cellular substance are deprived of sensibility; but the division of nerves not belonging to this class does not at all deprive the parts of their sensibility to external impression. There is also a wide distinction in their powers of exciting the muscles. The slightest touch on the portio dura convulses the muscles of the face, but the animal gives no sign of pain: while, by means of the branches of the fifth pair, which, if touched, give great pain, it is difficult to produce any degree of action in the muscles.

The brain has, from the earliest times, been regarded as the organ chiefly connected with the affections of the sentient and intelligent principle. Galen taught that the governing spirit resides in the brain, and is especially contained in the ventricles, where it acts upon the nerves at their origin: for, on opening them, he observes, the spirit escapes, and the animal is immediately deprived of sensation and motion. The immediate seat of sensation appears to be confined to a particular portion of the nervous system; and observation and experiment concur in showing that this portion is restricted to much narrower limits than was formerly imagined. It certainly does not extend to the great mass of the hemispheres of the brain: for these may be wounded, or even wholly removed, in a living animal, without any indication of suffering. Both Le Gallois and Dr Philip removed by successive slices the whole of the upper and anterior parts of the brain, without affecting the muscles of voluntary motion, or apparently giving any pain. The knife excited these actions only when it approached the origin of the nerves, and the spinal marrow. The part which performs the office of the sensorium, that is, whose changes are the intermediate links between the perceptive soul and the material body, appears to differ in different animals. In man, and the tribes of mammalia most allied to him in structure, it is chiefly situated in the medulla oblongata and upper portion of the spinal marrow, at the origin of the principal nerves of the organs of sense, and of their muscles. In proportion as we descend to the inferior orders, it seems to be

more diffused over the upper portions of the spinal marrow; but in no case does it belong to any precise point, being always diffused over a certain extent of medullary matter.

It is also generally believed, and Le Gallois professes to be of this opinion, that the power of determining resides exclusively in the brain. If a salamander, says he, be decapitated at the first vertebra, it continues to live several days; but although the muscles of the trunk and limbs be moved with a force sufficient for all the purposes of progressive motion, the animal remains on the same spot, and may be left on a plate, with a little water, without risk of its escaping. On examining its movements we may perceive that they are without order, and without any apparent object. The feet move, each in different ways, without concert; so that, if any advance happen to be made in one direction, it is presently defeated by a movement in the opposite quarter. The same remark applies to decapitated frogs; they are no longer capable of leaping; or if any leaps are made, it is only by a sort of accident, when their hind legs act against any fixed obstacle. When placed on their back, they occasionally agitate their limbs, as if from a desire to change their situation, but they remain as they were placed, from their incapacity to combine the movements necessary for that purpose. But all animals under these circumstances move but little, unless they are touched; and this is readily conceivable, since of all the senses the touch is the only one that remains to transmit impressions. Decapitation is not necessary for the exhibition of these phenomena; division of the spinal marrow will present them, and afford the singular spectacle of the two portions of the same body animated by different principles of action, each having a sensorial existence independent of the other.

Yet many actions of the living trunk of an animal appear to be governed by a sort of instinct, or obscure volition. Guinea-pigs and kittens, after they have recovered from the stupor produced by decapitation, seem strongly to feel pain from the wound in the neck, as appears by the alternate motions of their hind-feet towards that part. Sir Gilbert Blane reports that he divided the spinal marrow in a kitten a few days old, by cutting it across at the neck. The hind-paws being then irritated by pricking them, and by touching them with a hot wire, the muscles belonging to them were thrown into contraction, so as to produce the effect of shrinking from the injury. The same effects were observed in another kitten, after the head was entirely separated from the body. In repeating this experiment he found that when the spinal marrow was cut through between the lumbar vertebrae and the os sacrum, the posterior extremities no longer contracted, but the tail retained its sensibility. It is very certain that birds continue not only to live, but to walk and run, for some time after decapitation. The feats of the Emperor Commodus, who amused himself with striking off the heads of ostriches while they were running across the circus, by shooting at them arrows having a cutting edge, are well known: these animals, though headless, continued to run as before, and reached the end of the area before they dropped. Many physiologists

Physiology. have obtained similar results with turkeys, cocks, ducks, and pigeons. A frog will often be found, some hours after it has been decapitated, sitting in its usual posture, and extremely sensible to any injury inflicted on any part of it. This experiment, however, is somewhat fallacious; for, if care be not taken, great part of the medulla oblongata remains with the trunk, after the operation. It is well known that insects will survive for some time the loss of the head; and that the trunk in such cases shows unequivocally by its actions that it retains the powers of sensation and volition; for the brain in animals of that class is not situated in the head, but near the œsophagus. Yet in many of the vermes these indications are afforded by each portion into which the body is divided. It may be concluded from these and other similar facts, that although in the larger animals the brain appears to be the principal source and seat of the sensorial powers, yet that the exercise of these powers is not absolutely confined to that organ, but extends, in a great number of animals, to the spinal marrow, and that this is more and more the case as we descend in the scale of animals. No evidence as yet exists of the degree in which this extension takes place in man, and we are obviously precluded from ascertaining it by any direct experiment.

effect had been completely produced, the ligature Physiology. be removed, the powers of sensation and motion were gradually recovered, in proportion as the circulation was restored. He found also that the nervous and sensorial powers may be preserved even in a small portion of the trunk isolated from the rest of the system, by keeping up the circulation in that part, for which purpose, the maintenance of artificial respiration, and previous decollation, are indispensable. He seems to think that each portion of the spinal marrow might thus be made a separate centre of sensation and of life. In dogs, Sir Astley Cooper found that a ligature upon the aorta produced only slight weakness in the hinder extremities.

The necessity of a renewal of blood differs considerably in degree in different classes of animals. The amphibia are well known to be remarkably tenacious of life, in all its leading features. Both the nervous and the sensorial powers remain entire, in these animals, for a considerable time after the heart has been taken out, and the vessels drained of their blood. It has been found, indeed, by Dr Philip, that an obscure kind of circulation is kept up in the capillary vessels, after their communication with the heart is intercepted; but it appears probable that even this imperfect circulation, or rather oscillation of fluids in the vessels, must be exhausted in a much shorter period than that during which we see the nervous functions still survive. Greater tenacity of life exists in the nervous systems of animals in proportion as they are young; and it appears to be greater in the smaller than in the larger mammalia. The head of a rabbit, when severed from the trunk, shows signs of sensibility by the motion of the eyes and jaws, which latter are repeatedly opened and closed, as if vainly gasping for breath. Le Gallois observed, in rabbits decapitated on the day of their birth, that these movements continued for about twenty minutes. If the operation be performed at the end of fifteen days, they do not last above three minutes; and in rabbits of a month old, they cease in one minute, or in a minute and a half. The period during which sensibility remains in the trunk is generally less than in the head; but in both it is considerably longer in young than in adult animals.

V.—SECRETION, and other organic changes taking place in the living body, imply a complex series of operations, which the present state of our knowledge affords us no adequate means of analyzing satisfactorily. It is conceivable that a simple mechanical process, analogous to filtration, might effect the separation of some of the simpler fluids, such as serum, from the blood; a purpose which would be answered by a finer set of vessels, admitting only the passage of the thinner portions of the blood. Such a process would, therefore, imply only the mechanical agents and forces concerned in the circulation, and the organization of some of the secreting organs appears well adapted to this simple object. The existence of minute series of capillary vessels in these organs has been established by an experiment of Bleuland's, who injected, through the mesenteric arteries, a mixture of two differently coloured fluids, and found that the thinner fluid had penetrated into a net-work of

Unity of the Sensorium incapable of Proof. Although the unity of the power of perception of which we are conscious, naturally suggests the idea of some central organ in which the corresponding corporal impressions may be united, it is yet obvious that the necessity of such a union of parts does not admit of proof; and that it may be very possible to conceive the different parts of the sensorium disseminated among the organs at considerable distances from each other, and still to be capable of performing their functions, provided they were in sufficient correspondence with each other by nervous connexions.

Requisite Conditions in the Sensorium. The physiological conditions of the sensorium necessary for the exercise of the sensorial powers, besides the proper organization, chemical composition, and temperature, are freedom from compression, and a due supply by circulation of blood having the arterial qualities. Galen had proved by experiment that the ligature of both carotids in animals produced but little inconvenience; the circulation being in that case kept up by the vertebral arteries. Richerand succeeded in placing a ligature around these arteries, after the carotids had been tied, in a dog. Death in a few seconds was the consequence of this total interruption of circulation in the brain. In fainting, the loss of sensibility proceeds, in like manner, from the deficiency of blood in the brain. The insensibility which supervenes on the interruption to respiration is owing to another cause, namely, the presence of venous or carbonized blood in the arteries of the brain, in consequence of the circulating fluid being prevented from undergoing the usual salutary changes in the lungs. A similar loss of power in the spinal marrow is the consequence of depriving it of its circulating blood. This is proved by an experiment of Steno's, in which the tying of the aorta at the first lumbar vertebra was soon followed by paralysis of the posterior part of the body. Le Gallois, on repeating the experiment, found that if, after this

Physiology. vessels of a different order from those admitting the thicker fluid; and their course could be traced as they arose from the minute arteries, and terminated in the veins. But this explanation can apply but to very few of the animal secretions; since by far the greater number exhibit changes of chemical composition quite independent of mechanical separation.

No anatomical examination of the minute structure of secreting organs can be expected to throw much light on the means employed in this process, because those means transcend mere mechanism. The series of vessels, which, ramifying into tubes of smaller and smaller diameter, must have the effect of subdividing the blood, as by a strainer, to a certain degree of tenuity, probably prepare it for the changes it is to undergo in that part of the process in which the real chemical change consists; but farther than this we cannot venture to speculate, since we, in most cases, know so little what are the exact changes produced, and still less what are the particular affinities which must be called into play in effecting these changes. Their operation probably takes place in the parts where the vessels terminate, and beyond the influence of the power which propels the blood; for they occur in insects, in the system of which, it is now well ascertained, no circulation of blood exists, nor can even any vessels be traced, except such as convey air.

as also
Nutrition,
Growth, &c. The appropriation of the materials thus elaborated to the purposes of growth, and the reparation of the solid structures of the body, is another stage of the same mysterious process, to the solution of which no conceivable mechanical or chemical hypothesis is at all adequate. The analogy of crystallization, although referred to in the celebrated definitions of Linnæus, is far too vague and remote to engage our serious attention: the growth of an animal or a plant being a phenomenon of a totally different class from the accretion of a stone or the shooting of a crystal. The phenomena of secretion and nutrition, inexplicable as they are at present, are sufficiently allied to each other to justify our reference of them to the same general head, until the progress of discovery may enable us to establish correct distinctions between them. Bichat, who has classed them together, subdivides them into two orders, as effects of organic sensibility and of insensible organic contractility, implying a distinction for which there appears to be no clear foundation. The terms themselves are inappropriate: we have already objected to the extension of the term sensibility to changes of which sensation forms no part; and contractility is, for the same reason, improperly implied to any organic phenomena in which contractions are neither apparent nor necessarily implied. We have selected the term organic affinities as best adapted to express the powers which produce these organic changes. Whether the coagulability of certain animal fluids, such as the blood, ought to be ranked under this head, or be regarded as a new and specific power, might perhaps admit of discussion; but we have not room to enlarge upon this question.

and Animal
Tempera-
ture. Increase of temperature is a phenomenon commonly attendant on the exercise of the organic affinities; that is, it usually accompanies the chemical

changes that are continually taking place in the living body. It is evidently, however, only a concomitant, not an essential circumstance; for in some cases the contrary takes place, and a reduction of temperature occurs. It has lately been the fashion to speak of caloric as being a secretion, and to regard its evolution as a phenomenon referable to the class of secreting processes. The chemical theories that formerly prevailed with regard to animal temperature have, no doubt, been in some measure shaken by the experiments of Mr Brodie and of Dr Philip; but they are, perhaps, not so completely overthrown as some would endeavour to persuade us.

VI.—Having thus endeavoured to trace the distinctive characters of each of the classes of vital powers, we have further to inquire into their mutual connexions and relative dependencies on each other.

Mutual Connexions of the Vital Powers. Physiologists have endeavoured in vain to discover whether any one of these powers might be considered as the source of the rest. The evolution of the embryo has been anxiously studied with a view to this question. The first powers that appear to be called forth in the original development of parts are the organic affinities; but, long before the period to which any accurate observation can reach, the muscular and the nervous powers have both displayed their energies. Already has the punctum saliens vibrated with the excitements of the fluid which it urges forwards; and the spinal marrow and brain, yet in a semifluid state, have already exerted their influence on the nascent organization. Tiedemann has bestowed great pains in the investigation of the successive stages of evolution of the nervous system in the fetus, and has surmounted many difficulties which had stopped the progress of former inquirers. The late researches of Mr Serres have also brought to light many new facts. An abstract of the labours of these anatomists is given by Béalard. The first part of the nervous system that appears to be formed, or at least that can be distinguished, is the spinal marrow, the upper extremity of which is slightly enlarged. The formation of the brain succeeds, but this organ remains long very little developed in comparison with the spinal marrow; in the more advanced periods of gestation it increases rapidly in size. The sensorial powers are evidently not developed till the others have been matured, and till the frame-work of the body has made considerable advances towards its perfect state.

Phenomena of Evolution; The origin of the vital powers being thus veiled in impenetrable obscurity, the inquiry has been directed to the order of their extinction on the approach of death. But here, also, very little satisfactory information can be gleaned. We learn, however, that the sensorial powers, as they were the last to be developed, are invariably the first to disappear; for their continuance seems to require the most perfect co-operation of the sanguiferous and nervous powers. But the nervous power survives their destruction, and is still capable of performing all its functions, except that it can no longer give evidence of conveying impressions to the sensorium, since the functions of the sensorium are abolished.

Physiology. No impression made on any part of the body is perceived, nor followed by any visible act of volition. The muscular power still remains, for if either the heart, or the muscles of voluntary motion be stimulated, they possess the power of contracting; a power which they lose only by slow degrees, a considerable time after the sensorial power has ceased to exist. It is also manifest that a certain portion of nervous power still remains; for if the nerves themselves, or those parts of the brain and spinal marrow from which they originate, be irritated, the corresponding muscles will be thrown into action—a proof that the nerves retain the power of conveying impressions. Haller made a variety of experiments to ascertain the comparative permanence of the irritability of different muscles; and a still more numerous series with the same view has been lately made by Nysten, in his Recherches de Physiologie, et de Chimie Pathologique. The order which he establishes is somewhat different from that of Haller; but it varies according to the mode of death, and the nature of the stimulus employed as a test. There is, however, no doubt that in the more perfect animals, the vital powers are so connected, that no one can exist long without the others. They are all, more or less, dependent for their continuance on the uniform supply of blood having the arterial qualities: a condition which involves the continuance of the circulation and of respiration; two functions, again, for the exercise of which the muscular power is essential. Thus an interruption to any one function soon reacts in a circle upon all the others, and involves in a common destruction all the vital energies; nothing remaining but those properties which the parts of the body possess in common with inert matter, and which are immediately dependent on their mechanical and chemical constitution.

Although, by this reciprocity of functions, the vital powers are intimately connected with one another, it is still very conceivable that they may all of them be essentially independent of each other: and this is perhaps the simplest hypothesis that the subject admits of. Setting out, then, with this supposition, it remains to be seen what modifications it must undergo. The sensorial power, it is evident, can never be manifested but through the medium of the nervous power; yet it may be conceived as existing separately and independently, although all proof of its separate existence is wanting. Muscular irritability, and the organic affinities are also considerably influenced by the nervous power; but it is not easy to determine the nature and extent of their connection. Prior to the time of Haller, the nervous system was considered as the general source of power in the body; and the contractile power of the muscles was regarded as derived altogether from this system, which was supposed to transmit this power to the muscular fibre in proportion as it was called for, and to regulate the quantity supplied. Haller, as we have seen, contended for the existence of a vis insita, or power essentially residing in the muscles themselves, independently of any condition of the nervous system, and only called into action by stimuli, of which, in the case of the voluntary muscles, the nervous influence is one, contributing, like all

other stimuli, to exhaust it, instead of furnishing any fresh supply. Meckel adopted a sort of intermediate opinion, regarding the nervous influence as one of the conditions necessary for muscular contraction, just as the due circulation of blood is one of those conditions; and, at the same time, admitting the separate existence of a vis insita.

Upon the Hallerian doctrine of the independence of irritability, it is easy to explain the fact that a muscle detached from the body, such as the heart, will still contract when stimuli are applied. If all the nerves supplying the limbs of a frog be divided, and cut out close to the place where they enter the muscles, the latter still retain their contractility in as great a degree as when the nerves are entire. To this it has been replied, that the stimulus may still act through the medium of the portions of nerves that must always remain attached to the muscle, however carefully we may endeavour to dissect them away; and which nervous fibres may perhaps even constitute an essential part of the muscular fibre. This objection, though often urged, was never satisfactorily answered by Haller. Dr W. Philip has endeavoured to remove it by the following experiment, made with a view to ascertain whether a similar exhaustion of irritability would arise if the excitation of a muscle were produced through the medium of the nerves, or by other stimuli. "All the nerves supplying one of the hind legs of a frog were divided, so that it became completely paralytic. The skin was removed from the muscles of the leg, and salt sprinkled upon them, which being renewed from time to time, excited contractions in them for twelve minutes; at the end of this time they were found no longer capable of being excited. The corresponding muscles of the other limb, in which the nerves were entire, and of which consequently the animal had a perfect command, were then laid bare, and the salt applied to them in the same way. In ten minutes they ceased to contract, and the animal had lost the command of them. The nerves of this limb were now divided, as those of the other had been, but the excitability of the muscles, to which the salt had been applied, was gone. Its application excited no contraction in them. After the experiment, the muscles of the thighs in both limbs were found to contract forcibly on the application of salt. It excited equally strong contractions on both sides. In this experiment the excitability of the muscles whose nerves were entire was soonest exhausted. From this experiment it is evident that the nervous influence, far from bestowing excitability on the muscles, exhausts it like other stimuli. The excitability, therefore, is a property of the muscle itself." (Experimental Inquiry, 2d edit. p. 99.)

While the theory of Haller so easily explains the phenomena of the voluntary motions, many difficulties lie in the way of its application to the actions of the involuntary muscles, such as the heart, blood-vessels, stomach, intestines, gall-bladder, &c. These latter organs are usually excited to contract by stimuli of a mechanical or chemical nature applied directly to them, and generally by the mere distension resulting from the accumulation of their contents.

But if these were the only occasions which called them into action, what uses must be assigned to the cardiac nerves, which establish a connection between the heart and the nervous system; and how it is subject to the influence of the passions, except through the medium of the brain or sensorium, acting through the intervention of these nerves? On the other hand, it is asked, if the nervous power, derived from the brain, be essential to the motion of the heart, how is the circulation maintained in acephalous monsters; and how are we to account for the fact that the interruption of all communication between the brain and the heart does not stop the motion of the latter? The theory of Haller explains perfectly these latter circumstances, but does not accord with the former: the opposite view of the subject is consistent with the former, but is opposed by the latter set of phenomena.

Various opinions have prevailed at different times as to the influence of the nervous system on the motions of the involuntary muscles, and especially of the heart; for an account of which we shall refer our readers to the report made to the class of Physical and Mathematical Sciences of the Institute of France, on the work of M. le Gallois, entitled, Expériences sur le Principe de la Vie, notamment sur celui des Mouvements du Cœur, et sur le Siège de ce Principe, and of which a translation is given in Dr Philip's Experimental Inquiry. Le Gallois conceived that he had proved that the power of the heart is derived altogether from the spinal marrow, and not, as formerly supposed, from the brain. The following are the leading facts from which he draws this inference. He found that, by crushing the spinal marrow, the power of the heart is so enfeebled, that it can no longer propel the blood; but that, after the removal of the brain, the power of the heart still continues, and may even be preserved a considerable time by artificial inflation of the lungs, after the whole head has been separated from the body. He was thus at no loss to explain the use of the cardiac nerves; the heart being influenced by the spinal marrow through their intervention, and being also subject to the influence of the passions, because the spinal marrow is itself influenced by the brain. Dr Philip has shown that the above facts by no means warrant these inferences, and has established satisfactorily, by direct experiment, that the brain has just as much influence over the motions of the heart as the spinal marrow has, when the circumstances of the experiment are precisely the same. The removal of the spinal marrow, like that of the brain, if the experiment be performed with caution, and by slow degrees, does not sensibly affect the motion of the heart, the animal being previously deprived of its sensibility. In these experiments the circulation ceases quite as soon without as with the destruction of the spinal marrow. Loss of blood seems to be the chief cause of its cessation; and pain would also contribute to the same effect, if the animal were operated on without being rendered insensible. The results are the same in frogs, only they are more distinct, because less immediately affected by the loss of blood. In these animals, if the head and spinal marrow be removed, the heart con-

tinues to perform its functions perfectly for many hours, and seems not to be immediately affected by their removal. Mr Clift made a series of experiments to ascertain the influence of the spinal marrow on the action of the heart in fishes, and found that, whether the heart be exposed or not, its action continues long after the spinal marrow and brain are destroyed, and still longer when the brain is removed without injury to its substance.

On the other hand, when the brain is suddenly crushed, as by a blow, which at once destroys its texture, the power of the heart is instantly so enfeebled, that it can no longer propel the blood. The same effect takes place if a similar injury be inflicted on the spinal marrow, as happened in Le Gallois' experiments. Dr Philip reports, that when the brain of a frog was crushed by the blow of a hammer, "the heart immediately performed a few quick and weak contractions. It then lay quite still for about half a minute. After this, its beating returned, but it supported the circulation very imperfectly. In ten minutes its vigour was so far restored that it again performed the circulation with freedom, but with less force than before the destruction of the brain. The spinal marrow was then crushed by one blow, as the brain had been. The heart again beat quickly and feebly for a few seconds, and then seemed wholly to have lost its power. In about a minute and a half it again began to beat, and in a few minutes acquired considerable power, and again supported the circulation. It beat more feebly, however, than before the spinal marrow was destroyed. It ceased to beat in about an hour and a half after the brain had been destroyed." In common cases of hemiplegia, the muscles carrying on the vital functions are seldom impaired: and Dr Cheyne of Dublin relates a case in which, while one half of the body was paralyzed, the uterus performed its function perfectly, by acting so as to expel a living fetus.

It has generally been imagined that the action of the heart cannot be influenced by stimuli applied to the brain or spinal marrow; for it would seem an inconsistency to hold this opinion, and at the same time to admit that it is not influenced by the total removal of those organs. But Dr Philip found that the application of alcohol to the brain or to the spinal marrow of a rabbit, produced immediately a great increase in the action of the heart. An effect of the same kind, but in a much smaller degree, took place from the application of a watery solution of opium, and a still smaller effect from an infusion of tobacco. The increased action which had been excited was soon succeeded by a more languid action of the heart, than that which had existed before the application was made. Little or none of the debilitating effect was observed when alcohol was used, and the action of the heart, in this case, returned to its usual state. Effects in every respect similar were observed to take place, when the same experiments were repeated on frogs.

It appears, then, that there is no essential difference between the irritability of the muscles of voluntary and of involuntary motion, in as far as their independence on the nervous system is concerned;

Physiology. but yet they are influenced by different stimuli, or at least by stimuli applied in different ways. Hence the laws which regulate the effects of stimuli applied to the brain and spinal marrow on the heart and muscles of voluntary motion are different. Mechanical stimuli, such as cutting instruments, applied to the brain or spinal marrow, produce no effect on the muscles of voluntary motion, unless they are applied to those parts where the nerves originate; they then excite the most violent spasmodic actions. The heart, on the other hand, is but slightly accelerated in its motion by mechanical injury done to the brain or spinal marrow, in what part soever that injury be inflicted; but the heart is affected in proportion to the extent of the parts that are injured. It is most excited when the brain is wounded rapidly in many directions. Chemical stimuli, on the contrary, such as alcohol, applied to any part of the brain or spinal marrow, produce considerable and immediate increase of the motion of the heart; while the voluntary muscles are, at the same time, not at all affected, and the animal betrays no sense of pain. The general conclusion to be deduced from these facts is, that the heart is excited by all agents applied to any considerable part of the brain or spinal marrow, while the muscles of voluntary motion are excited only by more powerful agents applied to certain definite parts of these organs. Hence we may easily derive the explanation of the apparent anomalies formerly mentioned. The heart may continue its action when removed from the body, and when no brain or spinal marrow exists, because it has no direct dependence on any part of the nervous system. It is supplied with nerves, and subject to the influence of the passions; because, although independent of this system, it is capable of being influenced through it, especially by such causes as, like the passions, affect a considerable portion of the nervous system.

The same conclusions, derived from a series of analogous experiments, are found applicable to the powers of the vascular system employed in carrying on the circulation, and even to those of the minutest vessels which can be seen by a powerful microscope. The doctrine, indeed, may be extended generally to all the muscles of involuntary motion, or those parts possessing what Bichat would call contractilité organique sensible. The irritability of the stomach and intestines, from whence arises their peristaltic motion, is, like that of the heart and blood-vessels, independent of the nervous system, though capable of being influenced through it. It survives the destruction of the spinal marrow, and of the brain, or of both these organs. In rabbits the peristaltic motion continues till the parts become cold; so that, when the intestines exposed to the air have lost their power, that of those beneath still remains. The effects of the passions on the alimentary canal leave no room to doubt that its muscular fibres are capable of being stimulated by the direct influence of the nerves. But, from the extreme irregularity of their movements, we cannot so well ascertain whether they are subject to the influence of the different parts of the brain and spinal marrow, as in the case of the heart and blood-vessels.

Vital Powers con- Respiration is a function which requires for its

perfect performance the combined agency of three Physiology. great classes of vital powers, the sensorial, the nervous, and the muscular. It ceases immediately on the destruction of the sensorial power; a fact which is unexplained on the hypothesis of Le Gallois, that the nervous power alone is concerned. How it happens, that, after decapitation, the movements of inspiration are the only set of actions relating to this function that are annihilated, while the rest remain, is, according to that physiologist, "one of the greatest mysteries of the nervous power, the revealing of which would throw the strongest light upon the mechanism of the functions of this wonderful agency." The difficulty vanishes, according to Dr Philip, if we admit the sensorial power as acting a part in this process, and as being necessary to call into action the nervous and muscular powers. He considers the muscles of respiration as, in the strictest sense, muscles of voluntary motion. By a certain sensation originating in the lungs, a wish is excited to expand the chest. This is an act of the sensorium, and until it takes place, the nervous as well as the muscular power, by which its expansion is affected, remain inert. It is in vain that these powers survive, if the power which calls them into action be lost. Thus we can understand why the removal of the brain, or the injury to that part of the brain where the par vagum originates, or the division of the spinal marrow above the origin of the phrenic nerves, immediately puts a stop to respiration, and the animal perishes unless the lungs be artificially inflated.

The number of muscles which have a relation to System of this function, and which may occasionally be called Respiratory into play as auxiliaries, when, in consequence of Nerves. some impediment to the natural motions, the muscles usually employed are inadequate to produce the full expansion of the chest, is extremely great. "When a post horse," says Mr Bell, "has run its stage, and the circulation is hurried, and the respiration excited, what is his condition? Does he breathe with his ribs only; with the muscles which raise and depress the chest? No. The flanks are in violent action; the neck as well as the chest is in powerful excitement; the nostrils as well as the throat keep time with the motion of the chest. It is quite obvious that some hundred muscles thus employed in the act of breathing, or in the common actions of coughing, sneezing, speaking, and singing, cannot be associated without cords of connection or affinity, which combine them in the performance of those actions: the nerves which serve this purpose I call the respiratory nerves." Some of the peculiarities in the structure of these nerves have already been noticed. The following are enumerated by Mr Bell as composing this system: the par vagum; the portio dura of the seventh pair, or respiratory nerve of the face; the spinal accessory nerve, or superior respiratory nerve of the trunk; the phrenic, or great internal respiratory nerve; the external respiratory nerve; and also, the glossopharyngeal nerve, or ninth of Willis, and the branches of the par vagum to the superior and inferior larynx. Mr Bell has deduced his theory from a great number of experiments made on animals. Thus, on dividing the portio dura on one side of the head, in an ass,

Physiology. the motion of the nostril accompanying respiration immediately ceased on that side, while the other nostril continued to expand and contract in unison with the motions of the chest; and sneezing took place only on that side. But the voluntary motions of the lips, and other parts of the face, remained on both sides, and the animal could eat without impediment. The reverse took place on the division of the superior maxillary branch of the fifth pair; no change occurred in the motion of the nostril; the cartilages continued to expand regularly in time with the other parts which combine in the act of respiration, but the side of the lip was observed to hang low, and it was dragged to the other side. When the nerves on both sides were divided, the power of elevating and projecting the lips, necessary for gathering food, was lost. The effects of partial paralysis, which affects sometimes one set of nerves, and sometimes another, strikingly illustrate the truth of Mr Bell's theory, and have been accurately traced by Mr Shaw, in his papers in the Journal of Science, and more recently in a Memoir published in the twelfth volume of the Medico-Chirurgical Transactions.

Influence of Nervous Agency on the Organic Affinities. It remains to be inquired, whether the organic affinities which give rise to the phenomena of secretion, nutrition, and animal temperature, are in immediate subordination, or are only occasionally under the influence of the nervous power. In as far as the materials on which the observed chemical changes are produced by the operation of these powers, are furnished to the secreting organs by the blood-vessels, secretion must evidently be influenced by all those causes which affect the circulation; many of which causes, we have seen, act through the medium of the nervous system. But with regard to that part of the process, which is conducted after the fluids have passed out of the capillary vessels, it is conceivable that they may still be carried on after the circulation has ceased, and may be influenced by totally different causes. The evidence of such processes being continued after the circulation is at an end, is obscure and defective. Stories have been told of certain secretions occurring after death; and of the hair and nails continuing to grow; but they have seldom been stated on authority to which any credit can be attached. Majendie, however, reports the fact of these parts growing for several days after death; and states, that he has seen a similar phenomenon with respect to the secretion of mucus. If the power which effects secretion, be independent of the other vital powers, these facts would admit of explanation; for it might survive the destruction of the vital powers for a certain period, in the same way that muscular contractility survives for a short time, the destruction of the sensorial and nervous powers. Hunter suspected that a vital action, referable to this class, continued in the stomach for some time after death, occasioning "an action and probably a secretion in the stomach." This he inferred from the well known fact that the coats of the stomach are often found digested, when examined after death. But Dr Philip remarks that this phenomenon is evidently the effect of a chemical, and not a vital process.

Dr Philip, who has so ably supported the doc-

trine of the independence of the muscular and nervous powers, has advanced an opinion directly the reverse of this with regard to the power of secretion, which he maintains is completely dependent on the nervous power. Such is the inference he deduces from a series of numerous experiments on the effects of dividing the par vagum on both sides of the neck in warm-blooded animals, and particularly in rabbits, in performing which, he was assisted by Dr Hastings. If this be done with proper precautions, the secretion of the gastric juice, and, consequently, the process of digestion is entirely suspended: great difficulty of breathing succeeds, and the air-cells of the lungs, with the bronchiae, become clogged with a frothy mucus. Mr Brodie had already found that arsenic introduced into the system, after the division of these nerves, does not produce the copious secretion from the stomach and intestines, which it is found to do under ordinary circumstances; and he met with a similar result when he divided the stomachic nerves immediately above the cardiac orifice of the stomach. The destruction of any considerable portion of the spinal marrow also deranges the secreting power of the stomach. When the nervous influence is withdrawn, the capillaries continue to convey fluids to the secreting organs, because their action is independent of that influence: but the changes constituting secretion no longer take place; it is thence inferred, that the power of secretion is immediately dependent on the nervous power. Dr Alison, on the other hand, has contended, in some very ingenious essays published in the Journal of Science, that this is not a legitimate inference because, in the experiment of the division of the par vagum, other causes than the interruption of the influence of the brain, such as the immediate injury done to the nerves by the act of dividing them, might be assigned for the observed effects of this operation on secreting surfaces. Mr Shaw thinks, that the only inference that can be drawn from the phenomena of this experiment is, that, in consequence of the bond of connection between the stomach and the organs of respiration and circulation, being destroyed, the functions of the stomach will necessarily be more or less disturbed.

Dr Philip has been further led to conclude, that galvanism is identical with the nervous agency. This opinion is founded on the result of the experiments we have already alluded to, in which the galvanic influence, transmitted through the lower portions of the divided par vagum, restored the secretion of the gastric juice, and the digestion of the contents of the stomach, which would otherwise have ceased. The accuracy of the experiments was for a long time disputed, but has lately been satisfactorily established by their careful repetition at the Royal Institution, by Dr Philip, in conjunction with Mr Brodie. But though the fact be admitted, there appears reason to doubt whether it warrants the conclusion, which Dr Philip has boldly drawn from it, relative to the identity of the nervous and galvanic agencies. Dr Alison, in the papers above alluded to, has discussed this question with great acuteness. Our limits preclude us from entering into the arguments employed in the controversy. We shall only, therefore, add, that Dr Philip ascribes the

Physiology. evolution of animal heat to the operation of the nervous power, and endeavours to show, by experiment, that the galvanic influence occasions an evolution of caloric from arterial blood, if subjected to it as soon as it leaves the vessels, but that it produces no such effect on venous blood; and also that the destruction of any considerable portion of the spinal marrow is followed by a reduction of the temperature of the animal.

Nervous Communications by Ganglions, &c. In order that any one set of organs may be subjected to the influence of every part of another set, it is necessary that some very extensive mode of nervous communication should exist between them. Such appears to be the object of that complex system of ganglionic nerves, of which the branches of the great sympathetic compose so large a portion. All the organs which have muscles of involuntary motion, and which, as we have seen, are influenced by every part of the brain and spinal marrow, receive nerves from a chain of ganglions, to which filaments of nerves from all the parts of the brain and spinal marrow are sent. Thus the nerves issuing from those ganglions are made up of filaments from an infinite number of sources, and transmit to the organs in which they terminate the united influence of all the nerves which the ganglions have received from the brain and spinal marrow. Each ganglion may accordingly be regarded as a secondary centre of nervous influence, receiving supplies from all the parts of the brain and spinal marrow, and conveying to certain parts the united influence of these organs. On the other hand, as the muscles of voluntary motion are to be subjected to the influence of only small portions of these central parts of the nervous system, they receive their nerves directly from those parts; generally without the intervention of ganglia, and with comparatively few intermixtures of nervous filaments. The system of ganglionic nerves appears to be quite as extensive as that of nerves proceeding directly from the brain and spinal marrow. These views, developed by Dr Philip, are directly opposed to those of Bichat, who maintains that the ganglions are centres of nervous influence, independent of the brain and spinal marrow, and incapable of transmitting their influence; but they coincide with Bichat's opinion, that the great sympathetic derives its origin from the spine and not from the brain, with which it has only very slender communications.

General Conclusion. We have thus attempted to delineate the characters of those powers by which the living animal body is distinguished from those of the inert materials that enter into its composition, and of which the combined effects constitute what is called life; and we have further endeavoured to investigate the laws of their mutual connections and dependencies. If the views we have presented are correct, they will enable us to detect the fallacy of those definitions of the vital principle, which have been from time to time advanced, in violation of the just rules of philosophical induction. The real state of the science does not yet authorize that degree of generalization which these definitions would imply. We are not warranted by the phenomena already known, in regarding life as the effect of any single power.

The attempts of Brown, of Hunter, and of Bichat, Physiology. to reduce the science to this state of simplification are premature, and have been the means of retarding, instead of promoting, the progress of real knowledge. The error has sometimes been that of generalizing partial views; so that one class of facts has been assumed as comprehending all the rest; but most frequently it has consisted in including a variety of dissimilar phenomena under the same common principle. We may take as an instance of the former, the definition of Le Gallois, which takes cognizance only of the nervous power, as if this were the sole characteristic of life. "Life," says he, "is owing to an impression made by arterial blood on the brain and spinal marrow, or to the principle which results from this impression;"—a definition which would totally exclude the life of animals that have neither brain, nor spinal marrow, nor arterial blood, nor circulation, which is the case with so large a proportion of beings in the inferior ranks of the animal creation. As an example of the latter error, we may adduce the language of Hunter, who talks of life as the effect of a peculiar and subtle matter, the materia viva diffusa; a hypothesis analogous to those in natural philosophy, by which the phenomena of matter are attempted to be explained by the intervention of an ethereal fluid, diffused throughout all space. To pursue these speculations, would be to wander in the regions of visionary hypothesis, where fancy assumes the garb of science, and where truth is obscured by the clouds of mysticism. The attempt to reduce the vital phenomena to a single law, is as vain as would be a similar endeavour with regard to the phenomena of the inorganic world. The effects of gravitation, electricity, magnetism, cohesion, elasticity, and chemical affinity, have been sometimes regarded as modifications of a single principle of attraction. But this is a simplification not warranted by the facts; although future researches may possibly enable us to recognize the identity of some of these principles with others. The recent discovery of Professor Grsted, indeed, entitles us to hope that such will ere long be the result with regard to electricity and magnetism. In like manner, the muscular, the nervous, the sensorial, and the organic powers, may be established as separate agencies in the living body; and no speculative ingenuity is able to reduce them to a single power; unless, indeed, we call to our aid another and a totally different class of relations, namely, those which they bear to the general object they concur in producing. But against such a substitution of final for physical causes, we have already entered our serious protest. The latter are the only legitimate objects of philosophical analysis, and no other can furnish a solid basis for the physical sciences. This foundation being once firmly established in physiology, subsequent inquiries will consist in the analysis of complex phenomena into the several principles from which they result; and we may then pursue with safety the more fascinating study of final causes, and trace the admirable subordination of purposes, and the skilful combination of means for attaining distant ends, which marks the whole series of phenomena presented to us by living beings. (w.)