The Encyclopædia contains such an account of the discovery and construction of this most valuable instrument, as could be drawn from the popular treatises of natural philosophy in the English language. But, unfortunately, our compilers of elementary works have seldom taken the trouble to remount to the original sources of information, and have frequently, by substituting their own fancies, or servilely copying the mistakes of others, contrived to disfigure egregiously the relation of facts, and the history of the progress of invention. We now purpose, therefore, as far as our limits will admit, to remodel the article; and, passing rather slightly over the description of the different kinds of barometers, and other practical details already given, to dwell more especially on the successive steps which led to the fine discovery of atmospheric pressure, and its application to physical science.

The opinions entertained by the ancients concerning physical subjects, appear at best only splendid visions. They speculated boldly in cosmological theories, but were easily satisfied with those conclusions which merely soothe the fancy. Many of the philosophical notions, however, adopted in remote ages, have left a durable impression in the structure of language, and still continue to exert a visible influence in moulding the current sentiments of mankind. The early sages of Greece distinguished matter into the four primary elements, of earth, water, air, and fire, which, by their various combinations, were supposed to produce the animated spectacle of the universe. With these elements were associated corresponding qualities, in a binary conjunction: Hot and cold; dry and moist. Earth and water were considered as ponderous and inert; but air and fire, endowed with elastic virtue, were imagined to possess lightness and activity. Fire, though extracted from all bodies by the operations of nature or of art, was yet conceived to be derived, by invisible emanation, from that diffuse lambent fluid, which, under the name of Æther, occupied the highest heavens, and constituted the substance and nutriment of the celestial bodies. While the earthy matter would, therefore, naturally settle towards the centre, and the aqueous fluids roll along the surface, of the solid globe; the air and fire soared aloft, the former occupying the whole of the region below the moon, and the latter streaming through the boundless extent of space. This sublunar scene is exposed to incessant change, calamity, and decay; but above it was supposed to reign a perpetual calm, the seat of bliss, and of divine and imperishable essence.

Aristotle, and some other philosophers, viewing Æther as altogether distinct from culinary fire, were disposed, however, to consider it as a fifth element, of a pure, divine, and incorruptible nature; an opinion of which afterwards gave occasion to the famous Quinta Essentia, or Quintessence of the schoolmen. The alchemists, who sprung up nearly about the same benighted period, in adopting those notions, modified them to suit their own peculiar views. To the elements commonly received, they joined the active auxiliaries of mercury and sulphur. For quintessence they substituted spirit and elixir; the former, drawn off by the application of fire, being conceived to represent the animating principle of each body; while the latter, extracted by the combined action of heat and moisture, was supposed to exhibit its concentrated and most select qualities.

Some of the ancient cosmologists supposed a vacuum beyond the shining expanse of æther, destined to receive the exhalations from this nether world. Others denied the existence of a separate void, but admitted small vacuities interspersed through bodies. Aristotle, however, maintained the necessity of a plenum, asserting that our idea of space or extension is inseparable from that of body. To this principle he ascribed the suspension of water in a tubc, when the finger is applied to shut the upper end. Yet the very contempt in which that philosopher, from a consciousness of his own superiority, was accustomed to hold the received opinions, might have led him to take juster views. He rejected the notion, that air has levity inherent in its nature; nor would he admit the more plausible idea, that a fluid so easily moved must possess the quality of perfect indifference, and be neither light nor heavy. Aristotle not only maintained that air is ponderous, but did not scruple to appeal to direct experiment in support of his assertion. A bladder, he says, will be found to gain some weight, on being blown or filled with air. But this was evidently a mere random assertion, betraying his ignorance of the constitution of fluids. A bag filled with air, and suspended in a like medium, it is obvious, from the laws of hydrostatics, must weigh exactly the same as before. If it be alleged that, in blowing up the bladder, a portion of air would be introduced immediately from the lungs, and containing, therefore, a small admixture of carbonic acid gas, which is specifically lighter than the common atmospheric fluid; the additional weight, amounting scarcely perhaps to a grain, would be too minute to be detected by any of the jeweller's balances constructed in ancient Greece.

The mutual opposition of the leading philosophical sects of antiquity had, in general, most fatally discouraged the application of mathematical reasoning to the system of the material world. The Academi- Barometer. cians, or the disciples of Plato, who cultivated geometry with ardour and brilliant success, were disposed to regard that science as a pure intellectual contemplation; and resigning themselves to the illusion of their lofty dreams, they turned with disdain from the investigation of individual facts and all the vulgar realities of life. The mind of Aristotle was of a more sober and practical cast; acute, profound, and discriminating, it ranged, with incredible industry, over an immense field of inquiry. That judicious philosopher recommended a careful and constant appeal to external observation, as the only sure ground on which to erect the structure of physics; but unfortunately his scholars neglected too much the study of mathematics, the most powerful instrument for conducting physical research. The precepts of Aristotle, though excellent in some respects, were hence in the sequel unproductive of any genuine fruit. On the contrary, the weight of his opinions, during a long course of ages, confined and repressed the efforts of human genius.

It must be gratefully acknowledged, that the alchemists, styled also philosophers by fire, were the first among the moderns who dared to explore new paths of science. Their projects were, indeed, highly chimerical, but they had the merit at least of setting the example of investigating the properties of matter by actual experiment. They likewise formed associations among individuals, for the more effectual prosecution of such researches. Hence the origin of that obscure sect, known by the fanciful title of Rosicrucians, who sprung up in Germany, and insensibly spread their influence over the Continent. Those principles were afterwards transplanted into the matured soil of Italy, where philosophy, succeeding to the cultivation of letters, wore a more attractive garb. Baptista Porta, a Neapolitan nobleman, who flourished about the latter part of the sixteenth century, was especially distinguished by his zeal in promoting such pursuits. Having spent many years in travelling over Europe to gain information respecting natural objects, he invited a few individuals of a similar taste to assemble, at stated times, in his house, and assist him in making new experiments. These meetings, however, gave umbrage to the watchful jealousy of the clergy, and they were soon suppressed by a mandate from the Court of Rome. But the example was imitated in other parts of Italy, where the papal authority enjoyed less respect; and academies, for the promotion of natural science, were successfully instituted under the patronage of different princes, particularly those of the illustrious house of Medici.

In this ferment of inquiry, Galileo arose, a man fitted alike by the gifts of nature, and the lights of education, to be the founder of experimental science. His elegant genius was invigorated by the study of the Greek geometry; and he conceived the happy and prolific idea of employing that refined instrument to explore facts and combine the results. Archimedes, indeed, among the ancients, had anticipated this road of discovery, having most successfully applied the powers of geometrical analysis to the investigation of some parts of mechanics and hydrostatics. But his was a solitary instance, unheeded by succeeding ages. The ingenuity of Galileo prepared a complete revolution in science. By means of a few simple but striking experiments performed on the lagoons of Venice, he established the laws of motion, which he now transferred from the surface of our globe, to direct the revolutions of the heavenly bodies. The publication of his Dialogues, which unfold the right process of induction, and are not less distinguished by fineness of conception than beauty of diction, form a new era in the annals of philosophy. He was the first that attempted to ascertain the weight of air by actual experiment; and considering the nicety of the operation, and the rudeness of the instruments constructed at that period, he made a very tolerable approach to the truth. It had been known for many ages, that air is capable of being highly condensed; and Ctesebius of Alexandria had invented an engine, which, by the force of the sudden expansion of this compressed fluid, hurled missile weapons. This was afterwards improved into the wind or air-gun, which seems to have been not uncommon in Europe as early as the fifteenth century, though soon afterwards generally superseded in practice by the introduction of fire-arms. Galileo, being led by a different path from that pursued at present, set himself to examine the weight which air acquires by condensation. Having fitted a large copper vessel with a valve, he injected air into its cavity by means of a syringe, and then suspended it to a balance. The additional increase of weight being thus found, he opened the valve under an inverted glass receiver full of water, and measured, by the displacement of this liquid, the surplus quantity of air which had been injected into the copper vessel. He thence concluded that air is 400 times lighter than water, being about the double of the true estimate.

After he had, by such researches, acquired celebrity in the scientific world, Galileo accepted an invitation, with a very handsome appointment, from Cosmo de Medici; and devoting himself intensely to astronomical observations, aided by the telescope, which, from an obscure hint, he had recently constructed, yet occasionally unbending his mind with elegant recreation, he spent almost the whole of the evening of his life, at the villa of Arcetri, near Florence, in a style of comfort and even splendour. But, while occupied with those delightful pursuits—exploring the planetary phases—and discovering new worlds—he was for a moment recalled to his early studies, by an incident destined to form an epoch in the history of physical science. Some artisans, in the service of the Grand Duke, having been employed to construct a lifting or sucking pump for a very deep well, found, with equal surprise and vexation, that, in spite of all the pains they had taken in fitting the piston and valves, the water could by no effort be made to rise higher in the barrel than eighteen palms, or thirty-two feet. In this dilemma, they applied to Galileo for an explication of the cause of a failure so unexpected and perplexing. But the philosopher was not yet prepared to encounter such a discordant fact. The Aristotelian tenet of the impossibility of the existence of a void, was, at this period, universally received as an unquestionable truth. It had become a favourite axiom of the schoolmen, deceiving Barometer. themselves—as Leibnitz did afterwards, in proposing his principle of sufficient reason—by the glimmer of a metaphorical expression, the fugae vacui, or nature's horror of a void. To create a vacuum, they gravely maintained, would require the hand of Omnipotence, transcending the utmost power of men or even devils. But Galileo, though borne along by the current of opinion, saw the necessity of at least modifying the general principle. Without questioning nature's abhorrence of a vacuum, he supposed the influence of this horror to be confined within certain limits, not exceeding the pressure of a column of water eighteen palms in height. This was evidently evading, rather than meeting, the difficulty proposed for his solution. Yet, in the last of his Dialogues, he actually mentions an experiment to ascertain this power or virtu, as he calls it, of a vacuum. A piston, exactly fitted into a smooth hollow cylinder, was rammed quite to the end, and this carefully shut up; then placing the cylinder in an upright but inverted position, successive weights were appended to the rod, till it was drawn from the close end, and pulled down. It may seem strange, that the Tuscan philosopher, after advancing so far, should have stopped on the verge of a great discovery. He had already weighed the air, and it was only another small step thence to infer the effect of its incumbent mass. But the atmosphere was still supposed to reach to the moon, and the pressure of columns of such enormous height seemed to mock all calculation, and overwhelm the imagination.* Yet, on reconsidering the subject, Galileo began to suspect the solidity of the explication which he had given; but it was now too late for him, in his advanced age, loaded with bodily infirmities, and dispirited by clerical persecution, to attempt any farther innovation in science. Recommending it earnestly to his friend and pupil Torricelli to resume the investigation, this illustrious precursor of Newton expired in 1642, the very year in which the English philosopher was born. His uniform kindness and urbanity rendered him extremely beloved; and his disciples, particularly Torricelli, Viviani, and Ricci, venerating his memory, caught the same taste, and followed similar pursuits.

Torricelli's famous Experiment. Torricelli now conceived the happy idea of exhibiting the action of a pump on a contracted scale, by means of a column of mercury, which is nearly fourteen times heavier than water. This experiment he first communicated to his friend Viviani, who performed it with success in 1643; and he afterwards repeated and varied it himself. The method which he took brought very neatly under one view all the circumstances affecting the question. Having selected a tube about a quarter of an inch wide, and four feet long, he sealed one of the ends hermetically, or closed it under the Barometer flame of a lamp; he then filled the cavity of the tube with mercury, and applying his finger to the open end, he inverted it in a bason likewise containing mercury, though covered with a portion of water. The mercury instantly sunk to nearly thirty inches above the lower surface; but on raising the tube, till its orifice communicated with the layer of water, the mercury run all out, and the water now sprung up to the top, and occupied the whole of the cavity. It was thus proved, that the water and mercury are each supported by the same equipoise, which Torricelli, after some hesitation, at last concluded to be the pressure of the external atmosphere. He next converted the mercurial column into a form adapted for observation, by bending the lower end of the tube, and constructing what has since received the name of the syphon barometer. (See fig. 1. Plate XXXII.) Thus armed with a commodious instrument, he soon detected the variation of atmospheric pressure, which depends on the change of weather. These important results were published in the year 1645; but Torricelli did not live to enjoy the fame of his great discovery, for this most promising genius was snatched away by a putrid fever in the flower of his age.

The report of Torricelli's first experiments having been carried to France before he had ventured to draw his capital conclusion, set philosophers to speculate on the cause of such an unexpected fact. Descartes, with his usual rapidity and boldness of conception, did not hesitate, in his correspondence with Mersenne, to refer the suspension of the mercury in the tube at once to the pressure of the external atmosphere. But this influence appears not very consistent with his system, which assumed the existence of an absolute plenum, and only supplied the place of a void by the diffusion of subtle abraded particles of matter. He suspected also the accuracy of Galileo's estimate of the weight of the air, which he thought was scarcely appreciable by experiment.

But, in the same country, the subject was now Pascal's Exp. pursued with deliberate caution, and through all its periments details, by another genius of the highest order; one of the finest and most original that France has ever produced. Pascal had shown premature and extraordinary talents, which were encouraged by his father, a man of learning, who lived in habits of intimacy with the literati of Paris. The young philosopher happened to be residing at Rouen, in 1646, when he was informed of the famous Italian experiment. Having access, fortunately, to a glass-house, he resolved immediately to repeat the observations on a large scale. He had already suspected the justness of the principle, that "nature abhors a vacuum," and thought that the condensation and rare-

* This narrative, which marks so well the slow and timid steps whereby men, even of the highest intellectual endowments, usually advance in the search after truth, is drawn from the writings of Galileo himself. The carelessness of some authors in mis-stating facts, and imputing unworthy motives to those patriarchs of science who could not open their eyes all at once to the bright effulgence of day, deserves severe reprehension. We may remark, in passing, that M. Biot, who ranks now among the first mathematicians and philosophers in France, has not scrupled, in a recent bulky compilation on physics, to allege that Galileo only joked with the artisans who asked him the reason of the failure of their pump; that he had an idea of the true explication, but chose to keep his secret, and suffered it to die with him. Such a contemptible conduct would certainly have been a reproach to Galileo's acknowledged candour. Barometer. faction of the air point to a different, or at least a modified conclusion. With a view to clear up this subject, Pascal performed a number of satisfactory experiments, of which we shall cite a few of the more striking, nearly in his own language, tintured evidently with the prevailing opinions of the age: 1. Having fitted a piston to an open glass tube, and rammed it quite down, he applied his finger close to the lower end, and plunged the whole under water; then drawing back the piston, which was done with ease, the finger felt strongly and rather painfully attracted, while an apparent vacuity was formed above it, and continued to enlarge: but instantly on removing the finger, the water, contrary to its nature, darted up and filled the whole of the cavity. 2. A glass tube, about fifty feet long, sealed hermetically at one end, and filled with water, or rather red wine, as a more visible fluid, was inverted perpendicularly in a bason of the same. The liquid immediately subsided, leaving a vacant space of thirty-five feet; but, on gradually reclining the tube, the liquid rose again, and continued to mount, till it struck a sharp blow against the top of the glass. 3. A syphon, having one leg fifty-five feet high, and the other only fifty, being filled with water, and planted in two basons containing the same, such that the shorter branch had a perpendicular position, the water sunk in both to the same level, without being attracted, as usual in syphons, to the longer branch; but, on leaning the syphon back, the columns rose till they united at the top, and then the water began to flow towards the lower bason. The same experiment was also performed with mercury, the syphon having one leg ten feet, and the other only nine feet and a half in length, the mercury being found to divide itself into two columns, which continued suspended at an altitude of about thirty inches. 4. Having nicely fitted a piston to a long glass syringe, and pushed it down to the end, he immersed this in a bason of mercury, and held the tube in a vertical position; on gently drawing up the piston, the mercury closely followed it to the height of twenty-nine inches, but then stopped, leaving the piston to form above it an apparent vacuity. In this state, also, the syringe weighed exactly the same, whatever was the magnitude of the vacant space.

From these and other similar experiments, Pascal led his inductive process, with a degree of caution that might seem to border on timidity. He inferred that all bodies have a reluctance to a visible separation, or that nature abhors an apparent void; that this reluctance is exactly the same for a small as for a great vacuity; and that the force is limited, and exceeds not the pressure of a column of water thirty-three feet in height. He next ventured one step farther, and concluded, that this apparent vacuity was not filled by air lodged in the pores of the glass, or derived from external filtration; that it contained no subtle matter secreted from the atmosphere, and was not occupied by mercurial vapours or spiritous exhalations; in short, that a real and absolute vacuum had been formed.

Pascal, then only twenty-four years of age, proposed to write a treatise on the subject of those inquiries; but thought proper, in the meantime, to publish a short abstract of it, which appeared in 1647, and involved him in a wretched controversy. Father Barometer. Noël, rector of the Jesuits' College at Paris, keenly attacked it, armed with all the miserable sophisms of the schools, and the absurd dogmas of the Romish church. He contended, that the space above the mercurial column was corporeal, because it was visible and admitted light; that a void being a mere non-entity, cannot have different degrees of magnitude; that the separation produced in the experiments was violent and unnatural; and he presupposed that the atmosphere, like blood, containing a mixture of the several elements, the fire and the finer part of the air were detached from it, and violently forced through the pores of the glass, to occupy the deserted space. To enforce these puerile arguments, the reverend Jesuit did not scruple to employ the poisoned weapon which his order has often wielded with deadly effect,—the hinting an oblique charge of heresy. This rude attack only roused Pascal, and disposed him boldly to throw off the fetters of invertebrate opinion. He began to perceive that "abhorrence" cannot, in strict logic, be applied to nature, which is a mere personification, and incapable of passion; and was inclined, by degrees, to adopt the clear disembarrassed explication of Torricelli, referring the suspension of the mercurial column to the pressure of the external atmosphere. In stating this conclusion, he makes some remarks which would deserve the serious attention of philosophers in the present age. "When the weakness of men is unable to find out the true causes of phenomena, they are apt to employ their subtlety in substituting imaginary ones, which they express by specious names that fill the ear, without satisfying the judgment. It is thus that the sympathy and antipathy of natural bodies are asserted to be the efficient and unequivocal causes of several effects, as if inanimate substances were really capable of sympathy and antipathy. The same thing may be said of the antiperistasis, and various other chimerical causes, which afford only a vain relief to the avidity of men to know hidden truths, and which, far from discovering them, only serve to conceal the ignorance of those who invent such explications, and nourish it in their followers." These remarks, equally judicious and profound, are the more striking, since Lord Bacon, while he proposed to reform and new-model the whole structure of human learning, yet complied with the taste of his age in retaining much of the jargon and barbarous distinctions of the schools.

But Pascal did not rest satisfied with mere reasoning, however strictly conducted; and he soon devised an experiment which should palpably mark, under different circumstances, the varying effects of atmospheric pressure. It occurred to him, that, if the mercury in the Torricellian tube were really supported by the counterpoising weight of the atmosphere, it would be affected by the mass of superincumbent fluid, and must therefore partially subside in the higher elevations. He was impatient to have his conjecture tried in a favourable situation, and, in November 1647, he wrote a letter communicating those views to his brother-in-law, Perier, who filled an office of considerable trust in the province, and commonly resided at Clermont in Auvergne, in the immediate vicinity of the Puy de Dôme, a lofty coni- Barometer.

cal mountain, which rose, according to estimation, above the altitude of 500 toises. Various avocations, however, prevented that intelligent person from complying with his instructions, till the following year. Early in the morning of the 19th of September 1648, a few curious friends joined him in the garden of a monastery, situate near the lowest part of the city of Clermont, where he had brought a quantity of mercury, and two glass tubes hermetically sealed at the top. These he filled and inverted, as usual, and found the mercury to stand in both at the same height, namely, 26 inches and 3 3/4 lines, or 28 English inches. Leaving one of the tubes behind, in the custody of the subprior, he proceeded with the other to the summit of the mountain, and repeated the experiment, when his party were surprised and delighted to see the mercury sink more than three inches under the former mark, and remain suspended at the height of 23 inches and 2 lines, or 24.7 English inches. In his descent from the mountain, he observed, at two several stations, that the mercury successively rose; and, on his return to the monastery, he found it stood exactly at the same point as at first. Encouraged by the success of this memorable experiment, Perier repeated it on the highest tower of Clermont, and noted a difference of two lines at an elevation of 20 toises. Pascal, on his part, as soon as the intelligence reached him at Paris, where he then chanced to be, made similar observations on the top of a high house, and in the belfry of the church of St Jacques des Boucheries, near the border of the Seine; and so much was he satisfied with the results, that he proposed already the application of the barometer for measuring the relative height of distant places on the surface of the globe.

The investigation of the existence and effects of atmospheric pressure was now completed, and it threw a sudden blaze over the whole contexture of physical science. The fame of the experiments performed in Italy and in France, quickly spread over Europe. Yet such is the force of habit and early prejudice, that, after the first moments of surprise and confusion, few of the learned at this period had the courage to open their eyes to the light which had so unexpectedly burst upon them; but, secretly cherishing their inveterate notions, they sought to comfort themselves, by starting a variety of captious objections. Father Mersenne, though a man of some abilities, conceived that suction was occasioned by certain hooked particles dispersed through the atmosphere, which laid hold of any fluid in contact with them, and drew it towards the general mass. Father Linus, plunging still deeper in mysticism and absurdity, gravely proposed the funicular hypothesis, which attributes the suspension of the mercerual column to the agency of certain small invisible threads. But others of the clergy attacked Pascal with envenomed bitterness. The Jesuits of the college of Montserrat scrupled not, in their public theses, to pervert his expressions, and even contest the originality of his experiments. The philosopher was justly incensed at their base conduct; and those repeated provocations served, no doubt, to give a keener edge to his wit, when he afterwards directed it with such overwhelming energy against that insidious and formidable order of priesthood. He composed in 1653, though they were not published till after his death, Barometer's two short treatises, On the Equilibrium of Liquors, and On the Weight of the Mass of Air, remarkable for their neatness, perspicuity, and lucid order. The laws of the equilibrium of fluids are there beautifully deduced from a single principle, which suggests a variety of original views and admirable remarks. In those tracts, he likewise gives a description of the Hydraulic Press, a most useful and powerful machine, which has lately been revived in this country, and considered as a new invention.

A similar discovery, which was made about the Discovery same time in Germany, came seasonably to support of the Air, the triumph of innovation. Otto Gürické, a wealthy pump by magistrate of Magdeburg, who amused his leisure by constructing pieces of mechanism, and instituting curious physical inquiries, finding that the belief in the impossibility of a vacuum, with other scholastic tenets, was on the gradual decline, had the boldness to conceive that the forming of a void was a task perhaps within the reach of human ingenuity. Fired with the idea of accomplishing what for ages had been deemed unattainable, he directed all his efforts to compass that end. In his first trials he failed, as might be expected; but, by perseverance, he was enabled to surmount every obstacle. Having filled a wooden cask with water, he attempted to extract this again, by means of a small sucking pump, introduced at the bottom of the cask, and worked vigorously by three stout men; a hissing noise was heard like that of boiling water, the air entered from above through the interstices of the wood, and the water flowed out. The more effectually to exclude the air, he next took a smaller cask, with a sucker attached to it, and placed it within a larger one, having filled up the space between them with water. On working the pump as before, the water was forced through the pores of the wood into the inner cask, but none was extracted by the action of the piston. Foiled in these attempts with wooden casks, he had recourse to a copper ball, to the under part of which he screwed an inclining sucker; and, with this apparatus, he at last succeeded in extracting the air. He continued the operation, till no farther portion of air was perceived to issue from the vent. On opening the cock again, the air rushed into the cavity of the ball with violence; and the same effect took place, with scarcely any diminution of power, after an interval of a day or two. The construction of the machine was afterwards rendered more perfect, by substituting a large inclined metal sucker, with its joints secured by immersion in water.

Such was the origin of that most valuable addition to philosophical apparatus—the air-pump, which long retained its earliest rude and simple form on the Continent. By help of this new and powerful instrument, Gürické was enabled to perform some interesting and very important experiments. One of these, which demonstrates in a very striking way the pressure of the atmosphere, has been since deservedly styled the Magdeburg Experiment. It was performed with two hollow copper hemispheres, closely fitted together, and the air exhausted from their cavity. This singular experiment Gürické had the honour of exhibiting, in the year 1654, before the princes of the empire and the foreign ministers, assembled at the diet of Ratisbon. The force of two teams, each consisting of a dozen of horses, made to pull in opposite directions, was found insufficient to separate the hemispheres. It was now that the Burgomaster of Magdeburg heard, for the first time, of Torricelli's great discovery, and the intelligence must have appeared quite delightful to him, who, by a path so different, had yet arrived at a similar conclusion.

After his return from this splendid assembly, Gürické pursued at home various pneumatical researches. He showed the diminished pressure of the atmosphere at an elevation above the surface, by means of a hollow ball fitted with a stop-cock; having carried this to a height, a portion of the contained air rushed out on turning the cock; but when it was brought down again and opened, the same measure of air apparently flowed into its cavity. He actually weighed the air, by ascertaining, by a nice balance, the loss which a large bottle sustained on being exhausted, and found that air is 970 times lighter than water, a very near approximation, if allowance were made for the residuum of air still left in the bottle. He was the first who proposed the Statical Balance for measuring the variations of atmospheric density, consisting of a hollow glass ball about a foot in diameter, hermetically sealed, and freely suspended in the air, to indicate by its different buoyancy the changes which take place in the gravity of the external fluid.

But Gürické took great pleasure in a huge water barometer erected in his house. It consisted of a tube above thirty feet high, rising along the wall, and terminated by a tall and rather wide tube hermetically sealed, containing a toy, of the shape of a man. The whole being filled with water, and set in a basson on the ground, the column of liquid settled to the proper altitude, and left the toy floating on its surface; but all the lower part of the tube being concealed under the wainscoting, the little image, or weather-mannikin, as he was called, made its appearance only when raised up into view in fine weather. This whimsical contrivance, which received the name of anemoscope, or semper vivum, excited among the populace vast admiration; and the worthy magistrate was in consequence shrewdly suspected by his townsmen of being too familiar with the powers of darkness.

The taste for experimental science was about this time introduced from the Continent into England. The great struggle for the security of private rights had called forth the national energy, and its triumphant success had infused among all classes of men a spirit of boldness and enterprise most favourable to the reception of the new philosophy. The parliamentary commissioners, by removing the more violent and bigoted members of the universities, contributed, on the whole, to encourage a more liberal tone of thinking in those opulent seminaries. Near the close of the civil war and during the vigorous administration of Cromwell, the philosophy by experiment found some proselytes at last among the cloisters of Oxford, where the mass of antiquated opinions had lain so long embalmed and protected by religious awe. A small association was there formed, for combining together the efforts of individuals in the prosecution of such inquiries; and the fruits of this mutual compact were afterwards visible in the composition of various philosophical works. But the Restoration, by which the nation, in a burst of inconsiderate loyalty, surrendered the privileges which it had purchased with torrents of blood, threw the government of the universities again into the hands of men decidedly hostile to the very shadow of improvement. Experimental science withdrew to a more congenial soil, and sought shelter and support in the wider scope of the capital. The college founded by the munificence of Sir Thomas Gresham, for the benefit of the citizens of London, though now unfortunately sunk in absolute neglect, had the merit of first extending its protection to the pursuits of inductive philosophy. It produced a succession of professors, eminent in mathematical learning, which is so closely allied with experimental research. A more extensive association was accordingly formed in London, which regularly met at the apartments within the Exchange, and was afterwards, at the suggestion of Oldenburg, the resident from the city of Hamburg, and in imitation of the foreign academies, constituted by charter into the Royal Society. Such Royal So- was the humble beginning of that illustrious body, ciety, and such was all the countenance it received from a needy and profligate government. The institution, however, proved at first eminently useful, by the influence it had in directing the public opinion, and the shelter it afforded to experimental philosophy against the jealousy and declared hostility of the clerical and scholastic seminaries. The union of rank, or wealth, or talent, though still very limited in its range, bestowed a degree of lustre on the infant society, that was quite necessary for its defence against the attacks of ignorance, and the mining of bigotry.

One of the most active members of the Royal So- Boyle and ciety was Mr Boyle, who, having become acquaint- Hook. ed with experimental researches in the course of his travels, devoted, after his return home, his time and his fortune to such calm but engaging pursuits. In this occupation, he derived the most essential aid from Dr Hook, whom he had the discernment to engage as his assistant,—the most skillful mechanician, and the best practical philosopher, of the age. The same ingenious person was likewise employed as operator to the society, and undertook to produce at each meeting some new experiments for the instruction and entertainment of the members. One of the favourite subjects was to exhibit the properties of the atmosphere. Dr Hook, at the instance of Mr Boyle, had given a more convenient form to the air-pump, and had materially improved its construction, especially by the application of oil to the joints and valves. With this improved machine, a more perfect vacuum was procured than Gürické had obtained; and the English philosophers were thus enabled to perform a variety of delicate and interesting experiments, which extended the influence of the original discovery.

In those early meetings, too, of the Royal Society, the suspension of the mercury in the Torricellian tube had still the attraction of novelty. The famous Barometer. Italian experiment, as it was called, was frequently repeated and varied in the presence of a few of the more assiduous members, who, though delighted with the exhibition, continued to reason and to doubt concerning the cause of the phenomenon. These doubts acquired new force from a singular experiment which the celebrated Huygens some years afterwards communicated, during a visit he made to London. Having filled a glass tube eighty inches long with mercury, and carefully expelled whatever air was lurking about the sides, he gently inverted it, as usual, in a bason; when the mercury notwithstanding remained still hanging from the top of the tube, and did not subside to the proper height, till it was struck with a slight blow. This anomalous fact appeared then extremely puzzling. The experiment, indeed, requires great nicety and address on the part of the operator, and evidently depends on a concurrence of circumstances which have not yet been sufficiently explained. There can, at present, exist no doubt that this extraordinary suspension of the mercury is occasioned by its obstinate adhesion to the inside of the tube, which, in the process of purging the air, becomes probably lined with a very thin film of mercurial oxyd. But Huygens, who had embraced the leading principles of the Cartesian philosophy, was inclined to draw a very different conclusion. He thought that the fact proved the existence of another fluid, besides the atmosphere, and one possessed of such extreme subtlety and power, as to be capable of permeating the grosser bodies. In ordinary cases, this fine ethereal substance might be supposed to escape through the pores of the glass, and leave the mercurial column to the mere pressure of the atmosphere. Such was the unfortunate introduction of that ideal being—an ether—into experimental science, which it has continued to infest with mysticism, and to dazzle with a false glare. Similar notions are perpetually renewed by a certain class of superficial inquirers, and have exercised a visible and most pernicious influence in retarding the progress of sound philosophy.

Cistern Barometer. It was soon perceived, that the syphon barometer of Torricelli has a disadvantageous form. Both branches of the tube being supposed of the same width, the mercury must evidently sink as much in the one as it will rise in the other; so that the variations in the height of the column are thence reduced to half the true quantity. A small bason, or semi-circular wooden box, to hold the surplus mercury, was therefore attached to the frame of the instrument; and this construction, with very little change, was adopted, during the course of a century, by the makers of the ordinary barometer. But the syphon barometer itself was afterwards materially improved by having its lower branch blown into a wide bulb for holding the charge of mercury. (See fig. 2. Plate XXXII.) This form of the barometer is not quite accurate, owing to the smallness and unequal shape of the round bulb; but being very convenient for carriage, it has grown into general use, at least for the cheaper and more common sort of instruments.

Barometer of Descartes. As soon as the barometer came to be regarded as a weather-glass, ingenuity was set at work to devise the means of enlarging its scale of variations. Descartes first proposed a simple method for effecting that object, by combining a mercurial with a water barometer; which arrangement, though subject to imperfection, has led to many of the subsequent improvements. (See fig. 4.) He directed two short barometric tubes to be cemented, the one into the bottom, and the other to the neck of a phial; or, still better, that the tubes should be joined, by the flame of a lamp, to the opposite ends of a wide and regular cylinder. The lower tube, and a portion of the cylinder, were then to be filled with mercury, and above it was to be introduced pure water, reaching to the top of the upper tube, and there sealed close. When this compound tube was inverted in a bason of mercury, it is evident that the columns both of mercury and of water would sink, till their joint pressure became just equal to the superincumbent weight of the external atmosphere. But the variation of this weight would afterwards be indicated chiefly by the large motion of the water; since the mercurial column, spreading out above into a broad surface, must, in any case, experience a very slight difference of altitude. Thus, suppose the cylinder to have eight times the diameter of the upper tube, or a section sixty-four times greater, mercury being 13.6 times denser than water: For each inch of increase of altitude which the ordinary mercurial column gains, the top of the water would be raised in the tube 11.4 inches, its own rise being 11.18 inches, and that of the wide mercurial cylinder only .18 of an inch, yet equal in pressure to 2.4 inches of water. But Descartes, generally satisfied with mere theory and speculation, did not live to see his construction of the barometer carried into effect; and Chanut, the French resident at Stockholm, to whom he had imparted his views, met with such difficulty in the execution of the project, that, after some fruitless attempts, he abandoned it altogether.

Huygens was more fortunate; and succeeded, by dint of perseverance and skill, in constructing the Double Cartesian barometer. But he had the mortification to find that, in spite of all the pains he could take, the water, after it was relieved from the pressure of the atmosphere by the sealing up of the tube, constantly discharged a portion of air, which collected at the top, and by its elasticity depressed the compound column below its due altitude. Convinced that this source of imperfection is irremediable, he sought to rectify the construction of the instrument, and produced his Double Barometer; a form of combination frequently used, especially when the object is rather to make the variations very sensible than to obtain delicate results. (See fig. 5.) He joined a barometric tube of the usual length by the flame of a blow-pipe, to two wide cylinders, the one sealed at the top, and the other annexed likewise hermetically to a tall and narrow tube, open at its extremity; he then bent the thicker tube a little above the lower cylinder, and brought the two branches to be parallel. The instrument being thus formed, he filled the first branch with mercury, and introduced above, in the second branch, some liquid of comparative lightness. Alcohol would, in this respect, answer extremely well, if it were not so liable to waste by evaporation. Barometer. An alkaline lye, or the deliquiate salt of tartar, which also readily admits of being coloured, was, therefore, on the whole, preferred.

The principle of this construction is evidently the same as in that of Descartes; but the vacuum lying contiguous to the mercury itself, can have no admixture of disengaged air or of aqueous vapour. Since the cylinders are made very much wider than the bore of the annexed tube, the variation of pressure will be produced almost entirely by the change of altitude which the alkaline liquor undergoes, the mercury suffering only a very minute alteration of ascent or descent. The divisions of the ordinary scale will be about tenfold enlarged, if a section of each cylinder should exceed twenty times that of the tube in which the liquor plays.

A barometer of this construction has decided advantages with respect to the extent of its changes, but still it is not exempt from considerable defects. The moisture on the inner surface of the cylindrical reservoir increases the adhesion of the mercury, and retards its movements. But a much greater source of error proceeds from the influence of heat in extending the volume of liquor contained in that reservoir, and rising into the narrow stem. This instrument, therefore, to a certain extent, blends the indications of the barometer with those of the thermometer, which are essentially different, and can seldom accord.

About the same period, Dr Hook likewise proposed a double barometer, of a similar construction. He afterwards resumed the subject, and with a view to correct the defect of the former arrangement he produced, in 1685, an instrument of a more complex form, but very ingeniously conceived. (See fig. 6.) To the upper end of the open stem, he joined a third cylinder of the same dimensions as the two former, but tapering away to a fine orifice at the top. The principal tube being filled as usual with mercury, extending to occupy the bottoms of both the connected cylinders, he introduced a liquor immediately over the mercury in the second cylinder, rising partly into the stem; above this, again, he poured another liquor specifically lighter and differently coloured, filling up the rest of the stem, and mounting into the third cylinder. By this artificial and delicate combination, the mercury is left perfectly stationary, and all the movements corresponding to the changes of atmospheric pressure, are performed by the counterpoising liquors, and marked by their line of mutual separation. Since the stem or narrow tube remains constantly full, the variation of its pressure must depend on the different proportions of its length occupied by the two fluids. If the weight of external atmosphere should, for instance, increase, the denser liquor will rise, and consequently cause the lighter liquor to contract its column. The action of this compound barometer, being thus produced merely by the difference of the gravity of the two fluids, might, therefore, be augmented indefinitely. Suppose the liquid resting on the mercury to be pure water, and the superincumbent liquid to be olive oil, which is about one-twelfth part lighter; the scale would be enlarged no less than one hundred and sixty-three times, or an alteration of one-tenth in the altitude of the common mercurial column, would be marked by a motion through \( 12 \times 1.36 \) inches, or 16.3 inches. But such a vast enlargement of the scale is far greater than would ever be desirable in practice. It were better, therefore, to introduce next the mercury some fluid which is denser than water. If oil of sassafras were combined with oil of oranges, the divisions of the scale would be augmented only sixty-eight times, and consequently the whole range might not exceed ten or twelve feet. Those oils, however, would move rather sluggishly, especially in cold weather, and might, from their incessant shifting, during a lengthened period, become insensibly mixed. On the other hand, fluids of distinct characters are seldom free from chemical action; they expand differently with heat, and by coating with other traces the inside of the tube, they are the more apt to retard the motion of the column. In general, the advantage of any very great augmentation of the scale is counterbalanced, as the fluids then work by irregular starts; and the instrument loses in delicacy whatever it has gained in extent of action.

Another method of augmenting the variations of the barometer was invented by the same fertile genius, which has the advantage of uniting great simplicity with tolerable accuracy. (See fig. 7.) Resuming the syphon barometer, he made a small float of iron or glass to rest on the exterior surface of the mercury, and suspended by a slender thread passed round a small wheel or cylindrical axis that carried an index. Though the varieties of the height of the mercurial Wheel Barometer are in a tube of this form, reduced to half the ordinary measure; yet, from the great length of the index compared with the diameter of its axis, the divisions on the circumference of the circle in which it travels are much amplified. The little machinery being concealed within the frame of the instrument, the index only is brought into view, protected by a circular plate of glass. Thus fitted up, the whole forms rather a handsome piece of furniture. The Wheel Barometer, as it is called, has long maintained its reputation among ordinary observers.

A very simple mode of enlarging the divisions of the Inclined Barometer is commonly ascribed to Sir Samuel Moreland, the same person who had invented, or perhaps only revived, the Speaking Trumpet. (See fig. 8.) It consisted in merely bending the upper part of the tube into a very oblique position. By this plan, however, the scale, which depends on the perpendicular altitude, cannot be augmented beyond three or four times, without incurring evident risk of inaccuracy. This instrument is called the Inclined or Diagonal Barometer. The form has been sometimes varied by the fancy of artists, who, repeating the inclination of the tube, have occasionally given the upper part a zig-zag appearance.

The most ingenious barometer, filled with mercury Square Barometer, only, and yet admitting a scale of any extent, was invented by Cassini and by John Bernoulli, who first gave the description of it in 1710. (See fig. 9.) A wide cylinder is annexed to the top of the main tube, at the bottom of which there is joined at the right angles another long and narrow tube. The mercury, in ascending or descending within the wide cylinder must, therefore, run along this horizontal tube. If that cylinder have a diameter only four times greater than the bore of the tube, the scale of variation will be augmented sixteen times. This instrument is, from its shape, called the Square Barometer. It is not found in practice to answer so well as the theory might lead us to suppose. The mercury creeps along the horizontal tube with difficulty, and by desultory advances; and these irregularities increase, as it becomes, from its motion and exposure, covered with dust and partial oxidation.

The simplest of all the barometers, with an enlarged scale, and, at the same time, one of the most ingenious, is the Conical or Pendant Barometer, invented and described in 1695, by Amontons, a French philosopher, who being afflicted with total deafness, in consequence of a fever in his infancy, had devoted himself to mechanical contrivances. (See fig. 3, Plate XXXII.) This instrument consists merely of a tube, four feet or more in length, with a bore narrower than ordinary, and tapering regularly to the top. The width at the bottom must hardly exceed three-twentieth parts of an inch, while near the top it may be contracted to about one-tenth. A column of thirty-one inches of mercury being introduced, the tube is gently inverted and held perpendicular; the cohesion of such a narrow column is sufficient to prevent it from dividing and admitting the air, unless it be shaken; but overpowering the atmospheric pressure, it descends till it has contracted into the equiponderant altitude, by passing into a wide part of the tube. To obtain equal divisions on the scale, it is necessary that the tube should have an uniform taper. The most accurate construction of a barometer of this kind is, therefore, attained by forming together two tubes that have even but unequal bores, the longer and narrower one being uppermost. If the width of the upper tube were supposed to be to that of the under one as two to three, the scale would be enlarged three times, since, by descending three inches from the top, and consequently two at the bottom, the column would suffer a contraction of one inch in height.

This species of barometer is thus recommended by its simplicity and its ample range. But the bore of the tube being indispensably narrow, the mercury moves with difficulty, and resists the impression of minute changes of external action. When the conical shaped tube is retained, the instrument is liable to some inaccuracy from the influence of the cohesion of the mercury, which varies with the diameter of the column in different parts of the tube.

Amontons likewise proposed another form of barometer, in which the mercurial column is subdivided among several short connected branches. (See fig. 10.) Suppose the instrument were to have only the third part of the usual height; the first, third, and fifth branches enlarged above and below into very short cylinders, are filled with mercury; and the second, fourth, and sixth branches, which may have their bores narrower, are occupied with some light fluid, or simply with air. If the external pressure should suffer any diminution, the three mercurial columns which produce the counterpoise, will each descend and push up the last fluid of the series by their combined effects. It is evident, that, by multiplying those branches, the barometer will have its altitude proportionally reduced. But this construction, though specious in theory, is found to have no practical advantages. The instrument is, from its complication, very difficult to construct; its motions are sluggish, owing to the multiplicity of tubes, and the conjunction of fluids, and they are subject to derangements from the variable influence of temperature. It has, therefore, been generally abandoned.

These different forms of the instrument have been variously modified, and often brought forward with claims of novelty. We may notice, however, the Sectoral Barometer proposed by Magellan, in which the mercury is always made to rise to the same high point of the tube, by drawing this less or more aside from the vertical position. The arc they described will indicate the deviation from the perpendicular, and consequently the actual descent of the mercury. But the difference between the vertical and the oblique line is not measured by the inclination merely; it is proportioned to the versed sine of this angle, or nearly to the square of the arc. The advantage of this mode of observing is, therefore, best perceived in small variations of the mercurial column. In the hands of a skillful observer, the best and most accurate barometer, after all, is that of the original construction, with a tube rather wide, and a broad cistern. To apply minute divisions, is decidedly preferable to any enlargement of the scale. The measuring of such divisions has been since rendered extremely easy, by the adaptation of the differential scale—a most valuable contrivance proposed by Vernier, early in the seventeenth century, but strangely neglected long afterwards. This delicate appendage being once adopted, it became the more desirable to improve the sensibility, and regulate the correctness of the indications of the barometer.

The first object was carefully to cleanse the mercury, and to expel any portions of air or moisture adhering to the inside of the tube. The influence of aqueous vapour in depressing the mercurial column had been observed by Huygens; but other more evaporable fluids were afterwards found to occasion, by their presence, a still greater derangement. Homberg having, about the year 1705, washed a tube with alcohol, to remove the impurities from its internal surface, remarked that the mercury introduced into it stood an inch and half lower than usual; a depression which this ingenious chemist was disposed to attribute to the elasticity of the spiritous exhalations collected above the mercurial column; though other academicians, and Amontons among the rest, misled by their Cartesian prejudices, sought to ascribe the effect to the different sized pores of the glass. These anomalies were removed, by heating or rather boiling the mercury in the tube, till it was completely purged of air and moisture, and brought into close contact with the inside of the tube. But a new fact occurred which long puzzled the mechanical philosophers. The tube Barometer of a barometer, which had been filled with more than usual care, was observed to exhibit a luminous appearance, when moved or slightly agitated in the dark. This curious phenomenon gave occasion to multiplied and prolonged controversies; it was attributed to the subtle matter of Descartes, or ascribed to a native phosphorescence, or a latent fire inherent in the mercury. Our countryman, Hauksbee, in the year 1708, gave the first rational explanation of the fact, by referring it to electricity, which he had just begun to cultivate as a distinct science. It resembles exactly, indeed, the experiment of the exhausted flask, in which an electrical current flashes with a diffuse lambent flame, like the aurora borealis, or the northern streamers. The friction of the mercury against the inside of the tube excites electricity, while the vacuum, or rather the very attenuated vapour, in which the supposed fluid plays, facilitates its expansion. When the vacuum is rendered very perfect, by the careful and accurate boiling of the mercury, the lambent flashing ceases, for want of a fine medium to conduct and disperse the electrical influence.

The next point to which experimenters were led to direct their attention, was the effect of the width of the tube on the altitude of the mercurial column. Plantade, a lawyer at Montpellier, appears to have been one of the first who remarked that the mercury stands always lower in narrow tubes. This fact he communicated about the year 1730 to Cassini, who was then occupied in the south of France, with carrying on the great trigonometrical survey. But the discrepancies observed by Plantade being unfortunately confounded with other collateral circumstances, were for a time overlooked. In tubes having a narrow bore, the depression of the mercury, however, is very considerable, as may be readily perceived in a small glass syphon, of which the one branch is about half an inch in diameter, and that of the other branch less than the tenth of an inch. Thus, if the narrow tube had a width of only the thirteenth part of an inch, the depression of the mercury would amount to half an inch, which is about the third part of the elevation to which water in similar circumstances would be raised by capillary action. This effect has not been sufficiently examined, but it appears to result from the attraction of the particles of the mercury to each other exceeding their attraction to the surface of the glass. Mercury, in contact with glass, therefore, tends to a spherical form, and always assumes a convex surface within a clean tube. Water and other liquids again manifest an opposite character, the mutual attraction of their particles being less than their adhesion to glass. Accordingly, they spread along a vitreous surface, instead of collecting into drops; and in narrow tubes they mount above the level, and invariably have a concave termination. If the bore be so small as to be reckoned capillary, the depression of mercury is, like the elevation of water, inversely as the diameter; but when the bore has a considerable width, the quantity of depression, depending on the curvature of the surface of the mercury, diminishes proportionally faster, and follows nearly the inverse duplicate ratio of the diameter. But on the subject of capillary action, we expect, with no small degree of impatience, to see a paper which was very lately communicated to the Royal Society of London, by Mr Ivory, of the Military College at Sandhurst, one of the most original and profound mathematicians that our island has had the honour to produce.

The influence of the predominating attraction of the particles of mercury to themselves, above their adhesion to the sides of a glass tube, has not been considered with so much attention as it demands. Nothing is more common than to remark that the mercury in the barometer is in the act of rising, if it show a convex surface, but about to fall, if it should appear concave. Now, the top of the mercurial column must always remain convex, if the barometer be properly constructed, the tube perfectly clean, and the mercury purged of all impurities. But if the inside of the tube be any how soiled, whether covered with humidity or stained with mercurial oxyd, the metallic fluid will adhere so obstinately to the glass, as to lose its convexity, and to subside into a flat surface, or even sink into a concavity, like water and other liquids. Hence the danger of boiling the mercury too long in the tube, as it becomes partially oxydated, and the thin crust so formed not only suspends the column higher, but obstructs the freedom of its motion. The same effect is produced by greasing the inside of the tube. Some respectable authors, from not attending to these facts, have hastily inferred that the convex appearance which mercury assumes in the barometer was merely accidental, and consequently removed by a more complete boiling and purification.

In the case of tubes having wide bores, the depression of the mercurial column may, without any sensible error, be disregarded. According to the accurate experiments made by Lord Charles Caven-dish, and published by his son, the celebrated Mr Cavendish, the quantity of depression is only the 200th part of an inch in a tube of 6-10ths of an inch in diameter, the 28th part of an inch in a tube of 3-10ths diameter, and the 15th part of an inch in a tube of 2-10ths diameter. Wide tubes ought, therefore, to be preferred in the construction of barometers, both on account of the facility with which the mercury moves in them, and the smallness of its depression. The only circumstance to overbalance these advantages, would be the necessity and inconvenience of having a very large cistern. A quarter of an inch may be reckoned a good width of tube, and the corresponding depression is only the twentieth part of an inch.

In the syphon barometer, if both branches have the same diameter, the action is exerted on opposite sides, and, therefore, the effect of depression becomes entirely lost. For accurate purposes, this original form of the instrument has been again resumed, and the inconvenience arising from the large variation of the lower level entirely obviated by an ingenious contrivance introduced about forty years since. This consists in the application of a leathern bag, instead of a wooden or ivory cistern, to hold the surplus mer- Besides the barometric tube, there is placed adjacent to it another short one of the same width, communicating with the mercury contained in the bag, which being pressed by turning a screw below, is, at each observation, brought exactly to the same mark. The external atmosphere readily acts through the substance of the leather, but the mercury, from the powerful cohesion of its own particles, cannot be squeezed through the pores of that casing without violent compression. The addition of a bag within a cylindrical box, omitting the lower tube, likewise renders the barometer easily portable; since, for safe carriage, the mercury can be screwed up tight, to fill the whole cavity of its tube, but, on turning the screw again, the column will subside and rest on a broad base.

The last object which required nice observation, was to estimate the effect of heat in dilating the mercury, and consequently increasing the altitude of the equiponderant column. This correction could not be made with any sort of accuracy previous to the application of the thermometer, which, though invented half a century earlier than the barometer, was yet more than another half century in arriving at perfection. Hero, a mechanical philosopher, who flourished at Alexandria about 130 years before Christ, has described in his Spiritalia a sort of huge weather-glass, in which water was made to rise and fall by the vicissitudes of day and night, or rather the changes of heat and cold. This machine had for ages been overlooked, or merely considered in the light of a curious contrivance. But Sanctorio, the inventor of the famous statical balance, a very learned and ingenious Italian physician, who was long professor of medicine in the university of Padua, and had laboured to improve his art by the application of experimental science, reduced the hydraulic machine of Hero into a more compendious form, and thus constructed, about the close of the sixteenth century, the instrument since known by the name of the air-thermometer, which he employed with obvious advantage to examine the heat of the human body in fevers. Some years afterwards, a similar instrument was contrived, perhaps without any communication, by Drebblel, a very clever and scheming Dutch artist, who visited London in the reign of James I., and introduced the knowledge of that instrument into England.

But this air-thermometer was evidently of the same nature with what has been since called the manometer; it could measure only the dilatation or augmented elasticity of the air confined within its bulb, whether occasioned by heat or the diminution of external pressure. It was, therefore, considered merely as a weather-glass, indicating the state of the atmosphere; nor could its blended impressions, which might separately affect both the thermometer and barometer, be then distinguished. Had it been more closely studied, it must have led, by another path, to the discovery of the latter. But those irregularities to which the air-thermometer was hence subject appear to have created such doubts respecting the accuracy of the instrument, as occasioned its being neglected long afterwards.

The same country, however, which had given birth to the thermometer, began its improvement. After the principle of the barometer was established, the members of the Academy del Cimento, founded at Florence in 1657, and supplied with liberal funds by the Grand Duke of Tuscany, had, among other interesting physical researches, resumed the application of the thermometer; and instead of air, they substituted alcohol or spirit of wine, another very expansible fluid not affected by pressure, while they attached to the tube a scale graduated on a regular plan, though directed by no very precise measures. The instrument so constructed, but somewhat varied in its form, being copied by Italian artists, was dispersed over Europe under the name of the Florence Glass. From its careless execution, however, in the hands of itinerant vendors, this thermometer, or rather thermomroscope, appears never to have obtained an established reputation.

The great object was to bring thermometers to an exact correspondence. It was expedient, therefore, obtain a not only to select a proper fluid, but to adopt an uniform and consistent scale. Alcohol, linseed oil, and mercury, had been successively tried. The graduation was at first drawn from the temperature of cellars and deep caves, which, indicating the natural heat of the globe, had long been considered invariable; but more enlarged experience discovered the inaccuracy of that supposition, and showed the mean temperature to be materially modified by the latitude of the place, and its elevation above the level of the sea. Congelation, or rather the inverted process, the thawing of ice, or the melting of snow, was then found to remain fixed; a most important fact, which had been first noticed by Gürické, but overlooked till a considerable time afterwards. A stationary point was hence obtained, from which to commence the thermometer scale. But different modes were pursued for determining the divisions. Amontons, reverting to the air-thermometer in spite of its acknowledged defects, found that the elasticity of air compressed in the bulb, and able at the temperature of melting snow to support a column of mercury fifty-four inches high, was capable of raising this to seventy-eight inches, at the heat of boiling water; and he seemed contented in framing a rude standard, with merely dividing the intermediate space into inches and half-inches.

But about the same, or nearly at the beginning of the eighteenth century, Newton himself cast a keen though rapid glance on the subject of heat, and proposed a thermometer of a much simpler and more elegant construction. Having adopted linseed oil as a fixed and uniform substance, capable of great dilatation, he discovered by experiment, that distinguishing the capacity of the bulb into ten thousand equal parts, the liquid expanded 256 parts, from melting snow to blood heat, and 725 parts to that of boiling water. These numbers, however, being inconveniently large, he reduced them somewhat more than twenty times, adopting 12 and 34 as the proportional divisions on his scale. But oil, being so viscid a substance, was found to trail and collect on the inside of the tube; and this thermometer, though constructed on a right principle, never came into general use.

Röemer. Röemer, the Danish astronomer who made the fine discovery of the progressive motion of light, was the first who proposed mercury as the fittest fluid for thermometers; and Halley and Amontons remarked about the same time, that it expands uniformly with heat, and remains nearly stationary at the point of boiling water. On this principle, Delisle, of St Petersburgh, constructed, in 1733, a mercurial thermometer, with a descending scale, the distance from freezing to boiling water occupying 153, or, in round numbers, 150 divisions, of which the bulb itself contains 10,000. A more ingenious method, but perhaps too refined, for graduating thermometers, was proposed by Renaldini in 1694. It consisted in adopting the scale in the successive temperatures produced by mixtures in the different proportions of twelve parts of water at the moment of thawing and of ebullition. This suggestion led to a very important inference, since it proved that mercury expands uniformly with equal additions of heat, while alcohol swells constantly in a rising progression. But the capital improvement of the thermometer was effected by the skill and perseverance of Fahrenheit, whose name has remained justly attached to the instrument. This ingenious person, originally a merchant at Dantzic, who had the misfortune to fail in business, was induced, by his taste for mechanics and chemistry, to have recourse to the manufacture of thermometers, as the means of gaining a slender livelihood. But not meeting with sufficient encouragement at home, he removed, about the year 1720, to Holland, the great emporium of the arts, and fixed his future residence at Amsterdam. He now preferred mercury to alcohol for filling his thermometers; and, adopting the division of the bulb into 10,000 parts, he reckoned 64 of them as the expansion between freezing to blood-heat, and 32 as the contraction from the same point to what he considered as extreme cold, or that produced by the mixture of salt with snow. These numbers were extremely convenient, being found by a repeated bisection. With respect to the heat of boiling water, Fahrenheit discovered the important fact, that it varies with the state of atmospheric pressure. Taking the mean, however, he reckoned 180 degrees from freezing to ebullition, and, therefore, marked this point at 212 on his scale. The thermometer owes its improvement to Celsius, professor at Upsal, who in 1742 placed the commencement of the scale at congelation, and divided the interval thence to boiling water into an hundred degrees, extending such a portion downwards as might be wanted. This centesimal thermometer is exactly the same as what the French have since called the centigrade, which, from its fitness and simplicity, deserves to be universally adopted.

The thermometer having been thus carried by successive steps to perfection, it was found by delicate experiments, that, between the points of boiling and freezing, the expansion of mercury amounts to the fifty-fourth part of its bulk, or that it acquires, for each degree of heat, an increase of volume amounting to the 5412th part on the centesimal scale, or the 9742d part on the scale of Fahrenheit. A correction, therefore, on the height of the mercurial column in the barometer, becomes necessary according to the changes of temperature which it undergoes. In this climate, the extreme variation arising from that cause will seldom exceed two-tenths of an inch. But if the barometer be suspended in a room, kept at an agreeable temperature, the error occasioned by the expansion of the mercury may, in ordinary cases, be disregarded, since it will scarcely amount to the twentieth part of an inch.

Since the barometer marks the condition of the distant atmosphere, and intimates those internal alterations which are generally connected with the change of the weather, it is particularly valuable at sea, by forewarning the mariner of the approach of a storm. But an instrument of the ordinary construction would not answer this purpose, the agitation of a vessel on a tempestuous ocean being such as will not only throw the ponderous mercurial column into violent oscillation, but communicate those sudden shocks which must infallibly break the tube. Various attempts have accordingly been made to obtain a Marine Barometer, exempt from risk, and yet sufficiently sensible to the variations of the atmosphere. The conical or pendant barometer-being, from the narrowness of its bore, rather sluggish, was first recommended for that purpose, though never adopted into practice. About the beginning of the eighteenth century, Dr Hook and Amontons severally proposed to employ for a barometer on board ship, the manometer or air-thermometer. To obviate the derangement arising from the influence of heat, there was to be placed beside it a spirit-of-wine-thermometer, with a ball so large as to give expansions equal to those of the portion of air confined within the bulb. The difference between the two adjacent columns of liquid would therefore measure the variation of external pressure. But to procure such a nice adaptation would prove so extremely difficult in practice, that most probably this instrument was seldom or ever actually constructed. Besides, the liquid column of the manometer, though light and narrow, would yet be much shaken by the rolling and pitching at sea. Notwithstanding these weighty objections, however, this compound manometer was tried in England, mercury being employed as the fluid both of expansion and pressure, and various adjustments applied by means of a complex machinery.

An ingenious and very substantial kind of marine barometer was above twenty years since recommended by Blondeau, one of the professors of the naval academy at Brest. (See fig. 11. Plate XXXII.) It consisted of an iron tube, bent below into a syphon, and filled carefully with mercury, which carried a float. For this purpose, a musket-barrel, about three feet long, was chosen, having a very smooth and even bore, and an iron breech closely welded to it, instead of being soldered with brass, which might become corroded by the action of the mercury. The lower end of the tube had a collar of leather, to which was screwed a piece Barometer. of iron, perforated through its whole length, and bent into an arch, having screwed likewise, with a collar of leather at its other extremity, a vertical cylinder of iron, four inches high, and of the same bore exactly as the tube. The contracted aperture at the end of the tube, not being exactly in the middle, was not always opposite that of the arch; and, therefore, by turning it occasionally aside, the communication could be contracted at pleasure, or even obstructed entirely. The cylindrical appendix was tapered at the top to a narrow orifice, through which an iron wire, attached to a small ivory float, had been introduced. To prepare this instrument for action, the mercury was first boiled in the tube; then the arch, filled with hot mercury, was screwed to the end, the cock opened, and the surplus mercury allowed to flow over; next the vertical piece, with its float, was screwed on, and a little mercury added, to give it due play. The origin of the scale was to be determined from the comparison with another good barometer of the ordinary construction; but, owing to the equality of the bores of the opposite tubes, the divisions were only half the usual size, or the inches were exhibited by half-inches.

This species of barometer is certainly free from all sort of risk, while the facility which, by means of turning the arch, it affords in checking the ascent and descent of the mercury, prevents in a great measure the oscillations of that fluid. If the instrument were properly suspended, therefore, its indications would be tolerably steady and regular. The chief objection to it consists in the diminutive range of its scale.

In every marine barometer, the main object is to give steadiness to the mercurial column, by retarding its motion in the tube; in short, to imitate the equalizing effect of the fly in mechanics. One form of construction was, instead of the cistern below, to annex a spiral tube composed of a number of horizontal convolutions. Passement, an ingenious Parisian artist, about the year 1758, improved on this idea. He twisted the barometer tube near the middle, at least twice round, and joined to its upper end a wide cylinder. But more effectually to prevent all irregular oscillations, he took a tube with a very narrow or capillary bore, and contracted it below, while he annexed a wide cylindrical piece at its other extremity. The only thing wanted now to the perfection of this instrument, was to devise a mode of suspending it that should soften the jerks, and allow it generally to maintain a vertical position. Our English artists have, by repeated trials, at last succeeded in surmounting all the difficulties. The marine barometer, manufactured by Mr Cary of London, (See fig. 12, Plate XXXII.) is one of the most approved kind. It consists of a capillary tube, about twenty-seven inches long, with a bore scarcely exceeding the thirtieth part of an inch, but terminated by a cylinder four or five inches high, and nearly three-tenths of an inch in diameter. It is suspended by a spring and jimbols, near the top at a certain point, which in each case is discovered by actual trial. By making the suspension lower, it is found, that the agitation of the barometer will cause the mercury to rise a little; while, by placing the suspension higher, the mercurial column suffers always some depression. The reason of this curious observation is not well explained. It probably results from the different centrifugal tendencies communicated to the opposite portions of the columns. The swinging of the instrument would evidently augment the pressure of the upper portion of the column, while it diminished that of the under portion. But this lower portion, being longer than the other, its tendency to descend would be proportionally so much greater. About the point of suspension, however, the opposite effects of the centrifugal tendencies are balanced, since the superior force being employed to set in motion a narrower column, the reflux and efflux of the mercury in the upper cylinder must be preserved nearly equal.

Marine barometers, thus improved, are now very generally used, and with great benefit to the public service, on board ships of war and Indiamen. To facilitate the keeping of a register of barometrical observations, the meritorious and indefatigable Mr Horsburgh, hydrographer to the East India Company, has lately published a set of engraved ruled sheets, adapted for the convenience of navigators. In these plates, the height of the mercury, from twenty-seven to thirty-one inches, is represented, in inches and tenth parts, by horizontal lines, while each successive day has a space apportioned to it by vertical bars. The state of the barometer at every observation is marked with a dot, and these dots being afterwards connected together, exhibit an irregular waved line, stretching across the sheet, and indicating the series of the changes of the weather. At the lowest points, from which the curve again returns, a gale generally follows. From the observations made off the Cape of Good Hope, during the month of May 1815, by an ingenious and active young officer, Captain Basil Hall, of his Majesty's sloop Victor, it appears that whenever the mercury fell to 29.60 inches, a storm ensued; the column always rose when the gale abated, and when it reached near thirty inches, the weather became fair. Those gales often came on suddenly, without any visible change in the aspect of the sky, but the marine barometer never failed to give warning of their approach.

To explain the cause of the variations of the barometer, has long perplexed philosophers. Many hypotheses have at different times been advanced on the subject; but it would be a mere waste of time, to make any detailed recital of such crude and unsatisfactory attempts. The various and often imaginary effects of vapours of heat and winds have been employed in framing an explication of the changes of the atmosphere. The fact that the mercurial column generally falls before rain, seemed at complete variance with the intimation of the senses, it being a notion universally prevalent, that the air is heavier when the sky appears lowering and overcast; another proof, if it were wanted, how fallacious are all current opinions in matters of science.

Leibnitz, fancying he had discovered a new principle in hydrostatics, endeavoured, by a sort of metaphysical argument, to demonstrate that, though a body Barometer adds its own weight to the pressure of a fluid in which it is suspended, yet it will cease to be ponderous in the act of falling. This alleged principle will not, in the actual state of science, be thought to require any serious refutation; nor, were it even admitted, would it be found at all adequate to the explication of the phenomenon, since the weight of moisture precipitated from the whole body of atmosphere could never, by the absence of its pressure, occasion a diminution of the tenth part of an inch in the altitude of the mercurial column.

Dr Halley and Mairan sought to account for the depression of the barometer before a storm, to the withdrawing of the vertical pressure of the atmosphere, when borne swiftly along the surface of the globe by a horizontal motion. This hypothesis at first sight appears very plausible, and might seem farther confirmed by a noted experiment which most authors have admitted without due examination.

Hauksbee, a skilful and ingenious experimental philosopher, about the year 1704, placed two barometers, about three feet asunder, with their naked cisterns in two close square wooden boxes, connected by a horizontal brass pipe; one of these boxes had, inserted at right angles, an open pipe on the one side, and a second pipe terminating in a screw, on the other side; to this end he adapted a strong globular receiver of about a foot in diameter, which had been charged, by injection from a syringe, with three or four atmospheres; then suddenly opening the stop-cock, and giving vent for the escape of the air through the box and over the surface of the included cistern, the mercury sunk equally in both the barometers more than two inches.

This elegant experiment might be deemed entirely conclusive, if a minute circumstance, on which the success really depends, had not unfortunately been overlooked. It will be perceived from the inspection of the figure which Hauksbee has given, that the exit pipe of the box was considerably wider than the pipe which conveyed into it the stream of air. This fluid, escaping from compression, would, therefore, be carried by its elasticity as much beyond the state of equilibrium; while the width of the orifice, by facilitating its emission, would allow the portion occupying the box and the connected reservoir to preserve its acquired expansion. If the pipe of discharge from the box had been much narrower than the other, an opposite effect must have taken place; for the air accumulated over the cistern, not finding a ready vent, would remain in a state of condensation. This curious fact is another of the many instances which might be cited to show the great delicacy and circumspection required in performing philosophical experiments.

The same results, however, can be exhibited by a very simple apparatus. Let a small box, or rather a glass ball, have a short narrow tube inserted in the one side, and another wide tube opposite to this, with a cross slider of brass, for contracting the orifice at pleasure; and, to the under part of the ball, join a long perpendicular tube, bent back like a syphon to more than half its height and containing a double column of water. Now, blow through the narrow tube into the cavity of the ball, while the orifice of emission is quite opened, and the liquid will rise several inches in the long stem; but, still continuing the blast, let the orifice be gradually contracted, and the column will first descend to its ordinary level, and then sink considerably below it.

The fall and rise of the mercury in the barometer must evidently be occasioned by some corresponding reduction or accumulation of the atmosphere at the place of observation. Whatever augments the elasticity of the air will cause part of the incumbent fluid to evade and leave for the time a diminished vertical pressure. The efflux of wind might also produce a temporary reduction of the atmospheric column. But the real difficulty consists in explaining why the variations of the barometer should be greater in the high latitudes than between the tropics, and why they so much exceed in all cases the quantities which calculation might assign.

The influence of heat will account for the semi-diurnal variations of the barometer which are observed, especially within the torrid regions. From ten o'clock in the morning till four in the afternoon, the mercury generally falls; but, after that hour, it rises again, till ten o'clock at night, when it drops till four in the morning, and then mounts till ten o'clock in the forenoon. These regular changes, which amount to about the five-hundredth part of the whole atmospheric pressure, depend on the prevalence of the alternating land and sea breezes, occasioned by the diversified action of the sun's rays upon the earth and water. The accumulation of air is greatest at four o'clock in the morning and evening, and the mercury then attains its highest point; but it sinks lowest at ten o'clock in the morning and evening, when the incumbent mass has been the most reduced.

A similar reason will explain the effects of the northerly and easterly winds, in elevating the mercury of the barometer in our climate. A chill air, with enfeebled elasticity, is thus accumulated, and exerts a predominant pressure.

The augmented elasticity communicated to the air by the action of heat or the presence of humidity, and the reduction of the incumbent mass by the efflux of winds, have each their distinct influence, in disturbing the equilibrium of atmospheric ocean. But the effects, particularly in the high latitudes, much surpass the regular operation of those causes. The only mode, perhaps, of removing the difficulty, is to take into consideration the comparative slowness with which any force is propagated through the vast body of atmosphere. An inequality may continue to accumulate in one spot, before the counterbalancing influence of the distant portions of the aerial fluid can arrive to modify the result. In the higher latitudes, the narrow circle of air may be considered as, in some measure, insulated from the expanded ocean of atmosphere, and hence, perhaps, the variations of the barometer are concentrated there, and swelled beyond the due proportion. We content ourselves with throwing out this hint at present but hope to be able to resume and discuss the subject at some length under the article CLIMATE. BAROMETRICAL MEASUREMENTS.

It was remarked in the preceding article, that the decisive experiment by which Pascal established the reality of atmospheric pressure, had likewise suggested to this ingenious philosopher the method of determining the elevations of distant points on the surface of the globe. But the first attempts were very rude, proceeding on the inaccurate supposition that the lower mass of air is a fluid of uniform density. Different authors estimated variously from eighty to ninety feet as the altitude, which corresponds to a variation of the tenth part of an inch in the mercurial column. The Torricellian tube or cane, as it was then called, was, on its first introduction to England, carried accordingly to the tops of mountains, or conveyed to the bottom of pits and mines, or even let down to great depths in the sea.

Among those experimentalists who laboured most assiduously in the study and application of the barometer in this part of the island, we should mention George Sinclair. This ingenious person had been Professor of Philosophy in the University of Glasgow, but seems to have conscientiously resigned his office soon after the Restoration, rather than comply with that hated episcopacy which the minions of Charles II. had forced upon the people of Scotland. He then retired to the village of Tranent, not far from Edinburgh, and was employed as a practical engineer, in tracing the levels of coal-pits, in directing the machinery employed in the mines at Leadhills, and afterwards in the great undertaking of conducting water from the heights of the Pentlands to supply the northern metropolis. Though not a profound mathematician, he was skilled in mechanics and hydrostatics, and possessed no small share of invention. Sinclair is said to be the first who applied to the mercurial tube the name of baroscope, or indicator of weight, the more definite appellation of barometer, or measurer of weight, not having been appropriated till many years afterwards. During his excursions in 1668 and 1670, he employed that instrument to measure the heights of Arthur's Seat, Leadhills, and Tinto, above the adjacent plains. He followed the original mode of using a tube sealed at the top, with a paper scale pasted against the side, which he carried to the top of the mountain, where he filled it with mercury; and, inverting it in a bason, he noted the altitude of the suspended column, and repeated the same experiment below; a very rude method certainly,—but no better was practised in England during the succeeding thirty years.

In a small scarce tract, printed in 1688, and bearing the quaint title of Proteus bound with Chains, Sinclair gives some judicious remarks on the variations of the barometer, considered as a weather-glass, and delivers very sound opinions, on the whole, respecting the causes of the chief meteorological phenomena. In a postscript to that piece, he proposes a most efficient and ingenious method of weighing up wrecks from the bottom of the sea. It consisted in employing two large arks, or square wooden boxes, fastened to the sides of the ship, and charged with air carried down to them by a succession of inverted casks, open at the lower end. An ark of a cubical shape, and twenty feet in every dimension, the smallest which he mentions, would, as he computes, have a buoyancy equivalent to 448,000 pounds Troy. It is remarkable that the celebrated Mr Watt always employs this very mode, using a large gazometer, floating in a pond dug in the court of his manufactory, and charged gradually by the action of bellows, for raising the ponderous engines constructed at Soho, and lifting them over his walls into the boats, which are stationed to receive them in the adjacent canal.*

In all the computations hitherto made from different altitudes of the barometer, the air was considered as an uniform fluid; no regard being had to the gradual diminution of density which must evidently take place in ascending the atmosphere. To estimate the effect of that gradation, it became requisite previously to determine the actual relation subsisting between the density of the fluid and its elasticity. This was first ascertained in England by Townley, who inferred from some experiments of Boyle, that the elastic force which the air exerts is exactly propor-

* Sinclair was author of a well-known little book, entitled, Satan's Invisible World Discovered, which, at a former period, was sold at all the public fairs in the country, and devoured with eagerness and dismay by the Scottish peasantry. In a quarto volume, on Hydrostatics, and the Working of Coal-mines, printed in Holland, and published by subscription in 1672, he digressed so widely from his subject, as to insert A True Relation of the Witches of Glenluce. But this was the folly of the age, which several of the most learned men had not been able to escape. It is painful to observe, that James Gregory, the inventor of the reflecting telescope, who, although endowed with talents of the highest order, yet appears to have had a keen temper, and to have imbibed an hereditary attachment to royalism and episcopacy, should have stooped to attack an unoffending and less fortunate rival. He wrote a little tract against Sinclair's Hydrostatics, with the title of the Art of Weighing Vanity, and under the thin disguise of Patrick Mather, archbeadle to the University of St Andrew's. It is a piece full of low scurrility, and memorable only for a very short Latin paper appended to it, containing the series first given to represent the motion of a pendulum in a circular arc. In the British Museum, there is a letter of Gregory to Collins, the secretary of the Royal Society, boasting of his project, and soliciting information, with which to overwhelm the poor author. But with all his eagerness to hunt down Sinclair, he never touches on the strange episode of the witches of Glenluce. What a picture of times approaching so near our own! tional to its density. A similar conclusion was about the same time drawn by Mariotte, a French philosopher, from a still better series of experiments. Following out this very simple law, he thought of computing heights from barometrical observations, by the rules usually employed in constructing tables of logarithms; and had, therefore, obtained some glimpse, no doubt by a sort of conjectural process, of the remarkable result, that the density of the atmosphere decreases in a geometrical progression, corresponding to the elevations taken after an arithmetical one. But seemingly not aware of the importance of the principle at which he was pointing, Mariotte immediately deserted it; and calculating from a repeated bisection of the column of air between the two stations into successive horizontal strata, he contented himself with interpolating the densities according to a harmonic division, which he next abandoned for the simplicity of a series with equal differences. This able experimenter hence only sketched out a mode of investigating the problem of barometrical measurements, without arriving at any very definite or consistent rule of solution.

In 1686, the ingenious and active philosopher Dr Halley resumed the subject, and discovered the law that connects the elevation of the atmosphere with its density; of which he gave a clear demonstration, derived from the well known properties of the hyperbola referred to its asymptotes. Since the height of the mercury indicates the pressure, and consequently the elasticity of the external air, it must be proportioned likewise to the density. Wherefore the breadth of a given mass of air, or the thickness of a stratum which corresponds to a certain portion of the mercurial column, will be inversely as this altitude. Let O be the centre of a rectangular hyperbola, of which OA and OB are the asymptotes; and conceive the distances OA and OB to represent the heights of the mercury at two stations. The perpendiculars AC and BD, which are reciprocally as OA and OB, must hence express the relative thickness of strata corresponding to equal portions of the barometric scale. Divide AB into a multitude of equal segments, and erect the perpendiculars EM, FL, GK, and HI. The included spaces, from AC to BD, will denote the successive thickness of the series of strata into which the whole mass of air between the two stations is subdivided. Consequently the aggregate or mixtilineal space DBAC, which is proportional to the logarithm of the ratio of OB to OA, will express the difference of atmospheric elevation when the mercurial column mounts from B to A. Taking equal ascents, therefore, in the atmosphere, the corresponding densities must form a decreasing geometrical series.

To apply this elegant theorem, Dr Halley availed himself of the best experiments which had been deduced, performed to determine the relative densities of air, water, and mercury. In different trials made near the earth's surface, it was found, when the barometer stood at 29\( \frac{3}{4} \) inches, that the air is 840. 852, or even 860 times lighter than water. Taking round numbers, therefore, and assuming the specific gravity of mercury to be 13\( \frac{1}{2} \), he reckoned \( 800 \times 13\frac{1}{2} \times 30 = 10,800 \) inches, or 90 feet, as the altitude of an atmospheric column which, near the surface, would exert a pressure equivalent to that of an inch of mercury. For the coefficient, which answers to the actual constitution of the atmosphere, Halley should have taken the thirtieth part of .4342945, the modulus of the common system of logarithms, or .0144748. But he proceeded less directly, having satisfied himself with taking the arithmetical mean between the differences of the logarithms of 29 and 30, and of those of 30 and 31; a compensation of errors, which gives .0144765, hardly deviating from the former. Hence he gave this simple analogy for computing the heights of mountains by the barometer; as the constant number .0144765 is to the difference between the logarithms of the barometric columns at the two stations, so is 900 feet to the elevation required. The result of this operation is evidently the same as if the logarithmic difference had been multiplied by the number 62170; a very tolerable approximation at all seasons for a northern climate, and quite accurate, indeed, if the mass of intervening air had a medium temperature of 46° by Fahrenheit's scale. Dr Halley supposed that the observations themselves might, from the influence of heat, differ about the fifteenth part between summer and winter. But the thermometer was still so imperfect an instrument, that it could not be applied with confidence in correcting such variations.

The principle which Halley thus investigated otherwise might be derived from a simpler process. Conceive the atmosphere to be divided into a multitude of equally thin horizontal strata, it is obvious that each successive stratum would, to the pressure of the superincumbent stratum, add its own weight, which being as its density or elasticity, is therefore proportioned to the collective pressure; and, consequently, those densities will continually increase in going downwards, exactly in the same way, and after a like progression, as money accumulates at compound interest, where a constant portion of the aggregate fluid is regularly joined to the capital. Such, in fact, is the distinguishing character of a geometrical progression, that the increase or decrease of each succeeding term is always proportioned to the term itself. The logarithmic curve is hence the best adapted for exhibiting the relations which connect the densities with the elevations in the atmosphere; the axis of the curve expressing the elevation, while each ordinate represents the corresponding density of the stratum of air. It being a fundamental property of the logarithmic curve, that every sub tangent applied to it has the same length, the exact determination of this in the case of our atmosphere, is the only thing wanted for the final solution of the general problem.

Eleven years after Dr Halley had given his rule for barometrical measurements, this philosopher had an opportunity of applying it to discover the height of Snowdon in North Wales. He found that the barometer which stood at 29.9 inches on the sea-shore near Caernarvon, fell a few hours after, when planted on the summit of the mountain, to 26.1 inches, the altitude having been ascertained previously by a trigonometrical observation to be 1240 yards.

The year 1687 is memorable as the date of the first publication of the Principia, which was drawn up chiefly at the urgent request of Halley, from disjointed materials that had lain a considerable time in the author's hands. In that immortal work, Newton resumed the problem of the gradation of atmospheric density, and solved it in that general way which suited his penetrating genius. He demonstrated that, supposing the particles of air, like other bodies, to have their weight or gravitating tendency diminished as the squares of their distances from the centre of the earth, if those distances be taken in harmonic progression, the corresponding densities of the atmosphere will form a geometrical one. But since the diminution of attraction at the greatest height we are able to reach, amounts only to the two thousandth part of the whole; this difference is too minute to be admitted into practice; and the simpler law first established by the sagacity of Halley may be deemed sufficiently accurate for every real purpose.

Newton has given a sort of geometrical solution of the problem. But a more precise, and, in this case, a clearer investigation, is obtained by help of the symbols of the integral calculus. Let x and x' express the altitudes of two strata of atmosphere, and y and y' the corresponding densities, the radius of the earth; suppose farther, that e represents the altitude of the equiponderant column which measures the elasticity of the air. Since the density of the air depends on the incumbent pressure, its decrement must evidently be proportional to the weight of each superadded minute stratum, or to the density of this stratum multiplied into its thickness and power of gravitation. Whence \( -edy = ydx \left( \frac{r}{r+x} \right)^2 \),

or \( \frac{edy}{y} = \frac{r^2 dx}{(r+x)^2} \), of which the complete integral is \( e\ H\ \mathrm{Log.}\ \frac{y'}{y} = \frac{r^2}{r+x} - \frac{r^2}{r+x'} = \frac{r^2}{r+x} \cdot \frac{x'-x}{r+x'} \).

If r be regarded as indefinitely great in comparison of x, the expression will pass into \( e\ H\ \mathrm{Log.}\ \frac{y'}{y} = x'-x \), which is only the common formula.

Little seemed wanting, therefore, to complete the practice of barometrical measurements, but the application of the thermometer, to correct the results. This instrument, however, advanced slowly to perfection, and more than forty years yet elapsed before it came into current use. Some of the continental philosophers likewise, biased, perhaps, by a secret jealousy of the superiority which England had acquired in science, began to throw out doubts respecting the reality or accuracy of the law of geometrical progression in the atmosphere. Daniel Bernoulli, a man of candour on the whole as well as ingenuity, but who, with some proneness to speculative reasoning, had imbibed unfortunately many of the prejudices of the Cartesian and Leibnitzian schools, proposed in his capital work, the Hydrodynamica, which came out in 1736, certain vague hypotheses regarding the constitution of the atmosphere, as deduced from certain internal motions attributed to its component strata. The specious results of those calculations led him hastily to deviate from the principle of the geometrical progression of density in the upper regions. In this departure he was followed by Cassini and Horrebow, who concluded from some partial observations they had made, that the barometer, in its indications of atmospheric pressure, is subject to irregularity; and that, near the surface of the earth, it obeys a different law from what it obtains at great elevations. A strong light, however, was thrown upon the subject in 1753 by Bouguer, an able mathematician, and a very skilful and ingenious observer, who, with other academicians, had been employed for several years in measuring a degree of the meridian along the stupendous ridge of the Andes. From the comparison of more than thirty distinct observations, he deduced a simple and elegant rule for computing heights by means of the barometer. It is, that the difference between the logarithms of the mercurial columns at the two stations being diminished by one-thirtieth part, and the decimal point shifted four places to the right, will express the required elevation in toises. Since the English was to the French foot nearly as fifteen to sixteen, the rule would be accommodated to our measures, and the result expressed in feet, if the logarithmic difference were augmented by the thirtieth part, then multiplied by six, and the decimal point thrown back four places; or, what is the same thing, if that logarithmic difference were multiplied at once by 62,000. But Bouguer imagined, that this rule would not hold exactly in Europe, or in the lower regions of the torrid zone; and to explain the deviation, he had recourse to the forced supposition that the particles of air possess different degrees of elasticity. Lambert, a philosopher of great originality and penetration, afterwards published some excellent remarks on the comparison of barometrical measurements. But no material progress was made till 1755, when M. de Luc of Geneva resumed the subject, De Luc and carefully combined experiment with observation. For the space of upwards of fifteen years, he prosecuted his inquiries with diligence and perseverance, aided by the peculiar advantages of local situation, in a city abounding with skillful artists, and seated in the neighbourhood of lofty mountains. The discrepancies which had hitherto created so much embarrassment, proceeded mostly from the inattention of observers to the disturbing influence of heat, and particularly its effect in expanding the air, and consequently augmenting the elevation due to a given difference of atmospheric pressure. De Luc's first object was to improve the thermometer of Reaumur, which, though greatly inferior to that of Fahrenheit, had been adopted in France and the adjacent parts of the continent. Having ascertained that mercury has the valuable property of expanding equally with equal additions of heat, he substituted that metallic fluid for spirit of wine, but retained its arbitrary and inconvenient scale of 80 degrees between the points freezing and boiling water. He next examined the dilatation of air at different temperatures, and corrected those results by numerous observations made on the mountains of Savoy, and the mines of the Hartz, in which the barometer was combined with the thermometer. The formula which he thence deduced for the computation of barometrical measurements was, in 1772, published in his Recherches sur les Modifications de l'Atmosphere, and seemed to draw, especially in England, a very considerable degree of notice. Dr Maskelyne, the astronomer-royal, adapted it to our system of measures, and Dr Horsley made annotations and comments on it. But, what was of more importance, other accurate observers, incited by De Luc's example, entered the same field of inquiry, provided with instruments of greater delicacy and much better construction. In 1775, Sir George Shuckburgh Evelyn visited the Alps, and combined trigonometrical operations with corresponding observations by barometers and thermometers from the hands of Ramsden; and about this time likewise, General Roy not only measured, with instruments made by that excellent artist, some of the principal mountains in Scotland and Wales, but instituted a series of manometrical experiments. It resulted from all these researches that, for each degree on Fahrenheit's scale, mercury expands the 9700th part, and air the 435th part of their respective bulks. It further appeared that the atmosphere has its temperature almost uniformly diminished at equal ascents; and that the logarithmic difference, reckoning as integers the first four digits, expresses in English fathoms the height of an aerial column as cold as the point of congelation. General Roy proposed likewise another correction depending on the enfeebled gravity, and consequently the augmented altitude of the equiponderant column of atmosphere in the lower latitudes, occasioned by the influence of centrifugal force arising from the earth's rotation. Several years afterwards, Professor Playfair, in a learned paper, printed in the first volume of the Transactions of the Royal Society of Edinburgh, examined all the circumstances which can affect barometrical measurements, and discussed each question with the correctness and perspicuity that we might expect from his distinguished abilities. At nearly an equal interval of time, the celebrated Laplace resumed the subject in his Mecanique Celeste, and brought all the conditions together in a very complicated formula. Such an appearance of extreme accuracy, however, is perhaps to be regarded merely as a theoretical illusion, unsuited and inapplicable to any real state of practice. Biot has since attempted to arrive at a similar conclusion, by setting out à priori from some careful experiments on the relative density of air and mercury, performed by him in conjunction with Arago. He thence infers, that, in the latitude of Paris, and at the point of congelation, air, under a mercurial pressure of 76 metres, or 29,922 English inches, is 10,463 times lighter than mercury at the temperature of water at its lowest contraction. This would give 26,090 feet for the height of a column of homogeneous fluid, whose pressure is equivalent to the elasticity of the atmosphere. The coefficient adapted to common logarithms, and adjusted to the force of attraction at the level of the sea, would therefore be 60,148 feet, or 18,334 metres; scarcely differing sensibly from the quantity which Ramond had deduced from a very numerous set of experiments made by him on the Pyrenees. But Biot prefers, as the coefficient, the number 18,303, answering for an elevation of 1200 metres, or about 4000 feet above the sea, which is not far from the general level of such observations. The formula is hence, in English feet, \( 60,346 (1 + .002837 \cos. 2 \psi) \)

\[ \left( 1 + \frac{2(T+t)}{1000} \right) \log_e \frac{H}{h} \]

where \( \psi \) denotes the latitude of the place, T and t the temperatures of the air at the two stations, as indicated by the centesimal thermometer, and H and h the heights of mercurial columns corrected for the effects of heat.

This active writer has likewise given tables for expediting the calculation of barometrical measurements; in which he was anticipated, however, by Oltmans of Berlin, who published, in 1809, large Hypsometrical Tables, as they are called, accommodated to the complex formula of Laplace. Such tables might, no doubt, prove useful where very frequent computations are wanted, as in the case of the reduction of the numerous observations brought home by Baron Humboldt, for which, indeed, they were first designed. But still they contain a needless profusion of figures, and hold forth a show of extreme accuracy which the nature of the observations themselves can never justify. The mere calculation of barometrical measurements is a secondary object; the great difficulty is to procure good observations, and to combine tolerable accuracy with expedition. For this purpose, a very portable barometer is still wanted;—an instrument light and commodious, exempt from injury or derangement, and yet sensible to minute changes of atmospheric pressure. These properties, indeed, are seldom conjoined, and one advantage must generally be sacrificed to obtain another.

A barometer of the most improved construction is represented in fig. 19; a portion of the tube is shown in fig. 20; and a section of its cistern in fig. 21. By help of a screw pressing against the bottom of a leather bag, inclosed within a cylindrical ivory box, the mercury is always brought up through a tubular aperture to the same precise level; or till its convex surface appears to touch a very thin line of light, which is admitted through a slip of ivory applied against the side of the chink or separation of this tube from a wider one immediately over it. The lower end of the mercurial column being thus adjusted, the length is easily measured by drawing gradually down a hollow brass tube, divided at intervals by wide slits, covered on one side by thin bits of ivory, till by that softened light a contact is observed with the edge of a slit and the convex top of the column. The fine Vernier which the moveable tube carries gives the altitude of the mercury in thousandth parts of an inch. A thermometer is likewise constantly attached to the instrument, for the purpose of indicating the temperature of the mercury, which, from the heat of the hand in carrying, or the influence of the solar beams, is commonly warmer than the external air.

This mountain barometer is suspended for observation by jimbols from a tripod, as exhibited in fig. 18; but its several parts can be folded up together into a convenient compass, tolerably well fitted for carriage, as represented in fig. 19. The whole apparatus may not exceed the weight of ten pounds, yet even this, moderate as it might seem, would be felt a serious encumbrance by a traveller who is engaged, day after day, in the labour of climbing mountains. The risk which the instrument incurs, besides, in transporting it perhaps over rough precipices, imposes a perpetual constraint, while, to make correct observations with it, must always require time and patient attention. A lighter and more compact, though less accurate, barometer will generally be preferred by the geological traveller, whose object is rather to extend our acquaintance with the altitudes of mountains, than to aim at a superfluous and often illusory precision. The portable instrument, invented by Sir Henry Englefield, and represented in fig. 14, will, on the whole, answer those views. Its cistern is formed of box-wood, sufficiently tight to hold the mercury, without preventing the access and impression of the external air. When this barometer is inverted, the mercury, therefore, subsides very slowly in the tube, which must be firmly suspended in a vertical position. For greater security, the mercury is now put into a leathern bag introduced within the cistern.

A very simple and convenient sort of portable barometer was lately invented in France by that celebrated chemical philosopher M. Gay-Lussac. (See fig. 15 and 16.) It consists of rather a wide syphon tube, filled with mercury, and sealed hermetically at the inverted end, having a very fine capillary hole formed about an inch under this, by nicely directing the flame of a blow-pipe against the side of the glass, and drawing a melted spot of it out to a point. The lower portion of the principal branch has its bore contracted to less than the tenth part of an inch, to prevent the mercurial column from dividing in the act of inverting it. The mercury is boiled as usual, and the tube may be concealed in a walking stick, or lodged, like the complete mountain barometer, in a cylinder of brass, with moveable sliders bearing the divisions of a Vernier at both ends. (See fig. 17.) For greater simplicity, however, the larger divisions might be engraved on the tube itself. This kind of barometer is of ready use, and very little exposed to hazard in carriage. It is commonly held in a reclined or inverted position; but, in making an observation, it must be gently turned back, and kept perpendicular till the mercury descends through the contracted bore, and slowly rises again in the opposite short branch; the scale is noticed at both ends of the incurved column, and the difference of those indications gives its correct altitude.

A modification of the conical barometer, which, in travelling, we have ourselves employed with great ease and advantage, should likewise be mentioned. The principal part of it consists of a small stop-cock made of steel, and represented in fig. 13. A glass tube of 31 or 32 inches long, with a bore of the tenth part of an inch hermetically sealed at the top, and filled with quicksilver, is cemented into the one end of the stop-cock; and into the other end is cemented an open and wider tube, 16 inches or more in length, and having its diameter above the eighth part of an inch. This compound tube is lodged in a walking-stick, divided into inches and tenths through its whole extent, or only at the upper part, if uniform tubes be selected. In making an observation, the cock is turned, and the instrument inverted. The upper column then descends partly into the lower tube, till it becomes shortened to the proper altitude.

We have already stated the principles on which the calculation of barometrical measurements proceeds. But there still are some points, either assumed or overlooked, which may considerably modify the results. It is presumed, that, at equal successive heights, the temperature of the atmosphere decreases uniformly. This property, however, does not hold strictly, and it may be shown from a comparison of the best observations, that the decrements of heat follow a quicker progression in the higher regions. But we shall soon have another opportunity to examine this subject, and trace out its various consequences.

The humidity of the air also materially affects its elasticity, and the hygrometer should, therefore, be conjoined with the thermometer in correcting barometrical observations. But nothing satisfactory has yet been done with regard to that subject. The ordinary hygrometers, or rather hygroscopes, are mere toys, and their application to science is altogether hypothetical. A most philosophical course has lately been pursued, by multiplying calculations grounded on very loose data, instead of instituting a nice and elaborate train of original experiments.

In the actual state of physical science, it is preposterous, therefore, to affect any high refinement in the formula for computing barometrical measurements. The whole operation may be reduced to a very short and easy process. But the simplicity of the calculation would be still greater, if the centesimal thermometer were generally adopted. It will be sufficiently accurate, till better data are obtained, to assume the expansion of mercury by heat as equal to the 5000th part of its bulk for every centesimal degree, while that of air is twenty times greater, being an expansion for each degree of the 250th part of the bulk of this fluid.

Rule for Computing Barometrical Measurements.

1. Correct the length of the mercurial column at the upper station, adding to it the product of its multiplication into twice the difference between the degrees on the attached thermometers, the decimal point being shifted four places to the left. 2. Subtract the logarithm of this corrected length from that of the lower column, multiply by six, and move the decimal point four places to the right; the result is the approximate elevation expressed in English feet. 3. Correct this approximate elevation, by shifting the decimal point three places back to the right, and multiplying by twice the sum of the degrees on the detached thermometers; this product being now added, will give the true elevation.

If it were judged worth while to make any allowance for the effect of centrifugal force, this will be easily done, before the last multiplication takes place, by adding to twice the degrees on the detached thermometers, the fifth part of the mean temperature corresponding to the latitude. The mean temperature itself is formed by multiplying the square of the cosine of the latitude by 29. In illustration of these rules, we shall subjoin some real examples. General Roy, in the month of August 1775, observed the barometer on Caernarvon Quay, at 30,091 inches, the attached centesimal thermometer indicating 15.7; and the detached 15.6; while, on the peak of Snowdon, the barometer fell to 26,409 inches, and the attached and detached thermometers marked respectively 10°,0 and 8°,8. Here twice the difference of the attached thermometers is 11°,4, and twice the sum of the detached thermometer is 48°,8, which becomes 50,8, when augmented by the fifth part of the mean temperature on that parallel. Now, omitting the lower decimals, the first correction is .00264 × 11.4 = .030, to be added to 26.409. Therefore,

\[ \begin{align*} \text{Log. } 30.091 &= 1.4784366 \\ \text{Log. } 26.439 &= 1.4222450 \\ \text{Difference} &= .0561916 \\ \text{Constant multiplier} &= 60000 \\ \text{Approximate height} &= 3368.496 \end{align*} \]

And, for the true height, the correction is 3.87 × 50.8 = 171.2, which gives 3340 for the final result.

We shall take another example from the observations made by Sir George Shuckburgh Evelyn, at the same period, among the mountains of Savoy. This accurate philosopher found the barometer, placed in a cabin near the base of the Mole, and only 672 feet above the surface of the lake of Geneva, to stand at 28,152 inches, while the attached and detached thermometers indicated 16°,9 and 17°,4; but, another barometer carried to the summit of that lofty insulated mountain, the mercury sunk to 24,176 inches, the attached and detached thermometers marking 14°,4 and 13°,4. Therefore, twice the difference of the degrees on the attached was 3°,8, and twice the sum of the degrees on the detached thermometer was 61°,6. Consequently, the correction to be applied to the higher column was .0024 × 3.8 = .009, which makes it 4.185. Now,

\[ \begin{align*} \text{Log. } 28.152 &= 1.4495092 \\ \text{Log. } 24.185 &= 1.3885461 \\ \text{Difference} &= .0609631 \\ \text{Constant multiplier} &= 60000 \\ \text{Approximate elevation} &= 3957.786 \end{align*} \]

To correct this approximate elevation, remove the decimal point three places back, and multiply it by 61°,6, increased by 9°,9, the fifth part of the mean temperature, corresponding to the latitude; but \(3.96 \times 64.5 = 255.4\), and \(3957.8 + 255.4 = 4213\). Hence the summit of the Mole is 4882 feet above the lake of Geneva, or 6083 feet above the level of the Mediterranean Sea.

The last example we shall give is drawn from the observation which Baron Humboldt made among the Andes, near the summit of Chimborazo, the highest spot ever approached by man. This celebrated traveller found there, that the barometer fell to 14,850 English inches; the attached thermometer in the tent being at 10°, and the detached in open air being 1.6° under zero. But the same barometer, carried down to the shore of the Pacific Ocean, rose exactly to 30 inches, while both the attached and detached thermometers stood at 25°,3. Consequently the correction to be applied to the upper column is = .0015 × 30.6. = .045. Wherefore,

\[ \begin{align*} \text{Log. } 30,000 &= 1.4771213 \\ \text{Log. } 14.895 &= 1.1730405 \\ \text{Difference} &= .3040808 \\ \text{Constant multiplier} &= 60000 \\ \text{Approximate elevation} &= 18244.848 \end{align*} \]

Now, the difference of the detached thermometers or 26.9° being doubled and farther increased by 5.8°, the fifth part of the mean temperature at the equator, makes 59°,6; the final correction to be applied is therefore = 18.24 × 59°,6 = 1087, which gives 19,332 feet for the true elevation observed, or 2140 feet below the summit of Chimborazo.

These calculations are performed by the help of logarithms. It is desirable, however, to approximate at least to barometrical measurements without logarithms. A very simple rule for this object has been given by Professor Leslie in his Elements of Geometry. Since \(\log \frac{a}{b} = 2M \left( \frac{a-b}{a+b} + \frac{1}{3} \left( \frac{a-b}{a+b} \right)^3 + \frac{1}{5} \left( \frac{a-b}{a+b} \right)^5 \right.\) &c., where M denotes the modulus of the logarithmic system. When a approaches to b, the lower terms may be rejected without sensible error, or \(\log \frac{a}{b} = 2M \left( \frac{a-b}{a+b} \right)\), very nearly. Wherefore, in reference to our atmosphere, the modulus is expressed by the equiponderant column of homogeneous fluid, or 60,000 × .4342945 = 26,058 feet, or only 26,000 in round numbers; whence, as the sum of the mercurial columns is to their difference, so is the constant number 52,000 feet to the approximate height. Let General Roy's observation on Snowdon be resumed as an example: The analogy is 30.091 + 26.439 : 30.091—26.439, or 56.530 : 3.652 :: 52000 : 3,359, the approximate elevation, differing very little from the logarithmic result.

This mode of calculation may be deemed sufficiently accurate for determining any altitude that exceeds not 5000 feet. But it will extend to greater elevations, if the second term of the series be likewise taken; which is done by striking off three figures, and cubing the half of this number. Thus, resuming the mensuration of Chimborazo; 44.895:15.105 :: 52,000:17,496, and (8.75)^3 = 670, making together 18,166 for a nearer approximation.

The calculation of barometrical measurements, including the corrections required, is rendered most easy and expeditious by means of a sliding rule made by Mr Cary, optician in London. This small instrument should always go along with mountain barometers, and it will be found a very agreeable companion to every geological traveller.

But portable barometers, in spite of every precaution, are yet so liable to be broken or deranged, that other auxiliary methods are desirable for ascertaining distant elevations. In this view, the variation of the boiling point of water was proposed by Fahrenheit, as far back as the year 1724, the idea having occurred to him, as it had done before to Amontons, while engaged with experiments to perfect his thermometer. Little regard, however, seems to have been paid to the sugges- tion, till DeLuc and Saussure made a series of observations on the heat of ebullition at different elevations above the surface. About thirty years since, Cavallo attempted to revive the scheme of Fahrenheit, but experienced much difficulty in preventing the irregular starts of the thermometer plunged in boiling water. The best and surest way of examining the heat of ebullition, is to suspend the bulb of the thermometer in the confined steam, as it rises from the water; and this mode, we understand, has very lately been resumed, with great prospect of success, by the Reverend Mr Wollaston.

The heat at which water boils, or passes into the form of steam, depends on the weight of the superincumbent atmosphere. By diminishing this pressure, the point of ebullition is always lowered. It appears that, while the boiling heat sinks by equal differences, the corresponding atmospheric pressure decreases exactly, or at least extremely nearly, in a geometrical progression; it being found that every time such pressure is reduced to one half, the temperature of boiling water suffers a regular diminution of about eighteen centesimal degrees. This beautiful relation assimilates with the law which connects the density and elevation of the successive strata of the atmosphere. The interval noticed between the boiling points at two distinct stations must be proportional to their difference of altitude above the level of the sea. We have, therefore, only to determine the coefficient or constant multiplier; which may be discovered either from an experiment under the rarefied receiver of an air-pump, or from an actual observation performed at the bottom and on the top of some lofty mountain. We shall prefer at present the observation made by Saussure on the summit of Mont Blanc. This diligent philosopher found, by means of a very delicate thermometer constructed on purpose, that water which boiled at 101°.62 in the plain below when the barometer stood at 30,534 English inches, boiled at 86°.24 on the top of that mountain, while the barometer had sunk to 17,136. Wherefore the distance between the points of ebullition, or 15.38 centesimal degrees, must correspond to an approximate elevation of 15,050 feet; which gives 978\( \frac{1}{2} \) feet of ascent for each degree, supposing the mean temperature of the atmospheric column to be that of congelation. But it will be more convenient to assume 1000 for the constant multiplier, which corresponds to the temperature of 5\( \frac{1}{2} \)°.

To reduce this very simple result into practice, it would be requisite to have a thermometer with a fine capillary bore, and nicely constructed, the stem six or eight inches long, and bearing ten or a few more degrees from the boiling point; these degrees to be divided into twenty or perhaps fifty equal parts engraved on the tube, which should be rather thick, and terminating in a bulb of about half an inch diameter. This thermometer, being fitted with a brass ring two inches above the bulb, should screw into the narrow neck of a small copper flask, which holds some water, but has a hole perforated near the top for allowing the steam to escape. The water may be made to boil by the application of a lamp. The difference between the indications of the thermometers at the two stations being multiplied by a thousand feet, will give the elevation corresponding to a temperature of 5\( \frac{1}{2} \)°. The correction for the actual Barometrical mean temperature can easily be applied. If a more correct coefficient be afterwards determined, the same thousand, retained as a multiplier, may easily be adapted to another temperature.

This method of measuring elevations on the surface of the globe is, therefore, capable of great improvement, and might be employed with advantage in a variety of cases where observations with the barometer are not easily obtained. Its application would be most important to physical geography, in ascertaining the capital points for tracing the outline of the profile or vertical section of any country. The common maps, which exhibit mere superficial extension, are quite insufficient to represent the great features of nature, since the climate and productions of any place depend as much on its elevation above the sea as its latitude. Scientific travellers have accordingly turned their attention of late years to the framing of vertical sections. As a specimen, we give in fig. 22, from Humboldt's Geography of Plants, a section across the American Continent, one of the best and most interesting that has yet appeared. It consists, in fact, of four combined sections, traversing through an extent of 425 miles. The line begins at Acapulco on the shore of the Pacific Ocean, and runs 195 miles, about a point of the compass towards the East of North, to the city of Mexico; then 80 miles, a point to the South of East, to La Puebla de los Angeles; again it holds a North-East direction of 70 miles, to the Cruz Blanca; and finally bends 80 miles East by South, to Vera Cruz, on the coast of the Atlantic. A scale of altitudes is annexed, which shows the vast elevation of the table-land of Mexico. An attempt is likewise made in this profile to give some idea of the geological structure of the external crust. Limestone is represented by straight lines slightly inclined from the horizontal position: Basalt, by straight lines slightly reclined from the perpendicular: Porphyry, by waved lines somewhat reclined: Granite, by confused hatchets: Amygdaloid, by confused points.

By this mode of distant levelling, a very interesting discovery, in another quarter of our globe, has plan below been recently made by Engelhardt and Parrot, two the level of Prussian travellers. They proceeded, on the 13th July 1814, from the mouth of the Kuban, at the island of Taman, on the Black Sea; and, examining carefully every day the state of the barometer, they advanced with fifty-one observations, the distance of 990 wersts, or 711 English miles, to the mouth of the Terek, on the margin of the Caspian Sea. Similar observations were repeated and multiplied on their return. From a diligent comparison of the whole, it follows that the Caspian is 934 English feet below the level of the Black Sea. That the Caspian really occupies a lower level than the Ocean, had been suspected before, from a comparison of some registers of barometers kept at St Petersburg, and on the borders of that inland sea; but the last observation places the question beyond all doubt. It further appears, that within 250 wersts, or 189 miles, of the Caspian, the country is already depressed to the level of the Ocean, leaving, therefore, an immense basin, from which the waters are supposed to have retired by a subterranean percolation. (b.)

PLATE XXXII.

Fig.1. Fig.2. Fig.3. Fig.4. Fig.5. Fig.6. Fig.7. Fig.8. Fig.9. Fig.10. Fig.11. Fig.12. Fig.13. Fig.14. Fig.17. Fig.18. Fig.20. Fig.19. Fig.21. Fig.22.

Published by A. Constable & Co. Edin' 1816.