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 particularly 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 congenial 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, especially 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, forms a new era in the annals of philosophy. He was the first who 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 Ctesibius 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 ball 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; an error probably arising from some imperfection of the valve that confined the air within the ball.

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, Incidental having been employed to construct a lifting or sucking failure of a 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 then 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 themselves, as Leibnitz did afterwards in proposing his principle of sufficient reason, by the glimmer of a metaphorical expression, the *figura 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 abhor-Timid reverence of a vacuum, he supposed the influence of this hor-imperfect ror to be confined within certain limits, not exceeding the explication 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 *virtù*, as he calls it, of a vacuum. A piston with a valve, 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 stopt 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 believed to reach to the moon, and the pressure of columns of such enormous altitude 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

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1 From this remarkable experiment it is easy to perceive that Galileo was really the inventor of our pneumatic apparatus, though his title has been so long overlooked by chemists.

2 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 inserting unworthy motives to those patriarchs of science who could not open their eyes all at once to the bright effulgence of dry, deserves severe reprehension. We may remark, in passing, that M. Biot, who ranks among the first mathematicians and philosophers in France, does not hesitate to make elaborate disquisitions, to allege that Galileo merely 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 suffer it to die with him. Such conduct would certainly have been a reproach to Galileo's acknowledged candour. Barometer.

Barometer him, in his advanced age, loaded with bodily infirmities and dispirited by clerical persecution, to attempt any further 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 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 adopted brought very nearly 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 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 basin 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 ran 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 equispise, 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 siphon barometer. (See fig. I. Plate CIV.) Thus provided 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 Mersenme, to refer the suspension of the mercury in the tube at once to the pressure of the external atmosphere. But this inference 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 subtile abraded particles of matter. He suspected, besides, the accuracy of Galileo's estimate of the weight of the air, which he thought could be scarcely appreciable by experiment.

But, in the same country, the subject was now pursued with deliberate caution, and through all its 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 rarefaction of the air point to a different, or at least Barometer, 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, tinctured evidently with the prevailing opinions of the age.—I. 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 basin 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 siphon, having one leg fifty-five feet high, and the other only fifty, being filled with water, and planted in two basins 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 siphons, to the longer branch; but, on leaning the siphon back, the columns rose till they united at the top, and then the water began to flow towards the lower basin. The same experiment was also performed with mercury, the siphon 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 basin 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 stoop, 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 subtile 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 by Niel thought proper, in the mean time, to publish a short abstract of it, which appeared in 1647, and involved him in a wretched controversy. Father Niel, 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 seve- detached from it, and violently forced through the pores of the glass, to occupy the deserted space. To enforce these peculiar arguments, the reverend prelate did not scruple to employ the poisoned weapon which his order has often wielded with deadly effect, namely, hinting an oblique charge of heresy. This rude attack only roused Pascal, and disposed him boldly to throw off the fetters of inveterate 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 disencumbered 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 held 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 conical 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 September 1648, a few curious friends joined him in the garden of a monastery, situated 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 38 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 twenty 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 altitudes 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 Limus, plunging still deeper in mysticism and absurdity, gravely proposed the funicular hypothesis, which attributes the suspension of the mercurial 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 force against that insidious and formidable order of priesthood. He composed in 1653, though they were not published till after his death, 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 by some considered as a new invention.

A similar discovery, which was made about the same time in Germany, came seasonably to support the triumph of the air of innovation. Otto Gürické, a wealthy 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 Barometer 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 further 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 since been 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, with the help of a nice balance, the loss which a large bottle sustained on being exhausted, and found that the air is 970 times lighter than water: a very near approximation, if allowance be made for the residuum of air still left in the bottle. He was the first who proposed the Statistical 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 basin in 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-maumilkin, as he was called, made its appearance only when raised up into view in fine weather. This whimsical contrivance, which received the name of an Anemoscope, or semper eiron, excited among the populace vast scope of admiration; and the worthy magistrate was in consequence shrewdly suspected by his townsmen of being too familiar with the powers of darkness.

Before the taste for experimental science was imported from the Continent into England, the great struggle for the security of public rights had called forth the national experimental 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 in 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 being the first to extend 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 was the humble beginning of that illustrious body, and such all the countenance it received from a needy and profligate government. The institution, however, proved at first eminently useful, by its influence in directing 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 undermining of bigotry.

One of the most active members of the Royal Society was the honourable Mr Boyle, who having become acquainted with experimental researches in the course of

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It deserves perhaps to be mentioned, that the theatre of the natural philosophy class, in the University of Edinburgh, has for several years been furnished with a water barometer, constructed by Mr Adie. It consists of a fine drawn tube of tin, of half an inch bore, rising thirty feet from a copper basin inclosed under the benches of a class below, and cemented at the top to a glass cylinder two inches wide and about six feet high, exposed to view, but terminating in a small basin containing water. Both ends of this compound column are fitted with stop-cocks, which are opened or shut at pleasure by means of concealed wires. By this large apparatus the Torricellian experiment is likewise exhibited in a very striking manner. Barometer.

Baremeter, 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 Hooke, whom he had the discernment to engage as his assistant,—the most skilful mechanician, and one of the best practical philosophers, 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. Hooke, at the instance of 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 Guericke 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 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, still continued to argue 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 basin; 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 oxide. 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 pressure merely 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 siphon barometer of Torricelli has a disadvantageous form. Both branches of the tube being assumed to be 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 thus reduced to half the true quantity. A small basin, or semicircular 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 siphon 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 CIV.) 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.

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 basin 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 11·8 inches, and that of the wide mercurial cylinder only 1·8 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 Chamut, 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 Huygens's perseverance and skill, in constructing the Cartesian double 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 li- Barometer.

An alkaline lye, or the deliquescent 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 variations, 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 Hooke 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 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 163 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 Wheel barometer was invented by the same fertile genius, which has the advantage of uniting great simplicity with tolerable accuracy. (See fig. 7.) Resuming the siphon 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 column 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 method of enlarging the divisions of the Inclined barometer is commonly ascribed to Sir Samuel Moreland, the same speculative adventurer who invented, or rather introduced from abroad, the Speaking Trumpet. (See fig. 8.) It consisted in merely bending the upper part of the tube into a very oblique position. By this deflexion, however, the scale which depends on the perpendicular altitude cannot be augmented beyond three or four times without incurring evident risk of inaccuracy. The 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 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 when, from its motion and exposure, it becomes covered with dust and partial oxidation.

The simplest of all the barometers with an enlarged Conical scale, and at the same time one of the most ingenious, is the Conical or Pendulum 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 CIV.) 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 twentieths parts of an inch, while near the Barometer-top it may be contracted to about one tenth. A column of about 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 air unless it be shaken; but, overpowering the atmospheric pressure, it descends till it has contracted into the equiponderant altitude, by passing into a wider part of the tube. To obtain equal divisions on the scale, it is necessary that the tube should have a uniform taper. The most accurate construction of a barometer of this kind is, therefore, attained by joining 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 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 indissolubly 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 thus 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 skilful 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, Effect of and to expel any portions of air or moisture adhering to moisture the inside of the tube. The influence of aqueous vapour within the 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 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 vacancy, 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, of 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 Barometer 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 refer our readers with the utmost confidence to a valuable paper communicated to the Royal Society of London, by Mr Ivory, late 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 anywise 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 oxidated; 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 Cavendish, 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 siphon 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 resumed, and the inconvenience arising from the large variation of the lower lever entirely obviated by an ingenious contrivance introduced above fifty years ago. This consists in the application of a leathern bag, instead of a wooden or ivory cistern, to hold the surplus mercury. Besides the barometric tube, there is placed adjacent to it another short one of the same Barometer 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 Effect of to estimate the effect of heat in dilating the mercury, and heat on consequently increasing the altitude of the equiponderant barometer 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 invention statistical 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 Drebbel, a very clever and scheming Dutch artist, who visited London in the reign of James I., and imported 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 contained 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 blurred 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 Florence the thermometer, began its improvement. After the glass 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 atmospheric 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 ar- Barometer, 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 thermoscope, appears never to have obtained an established reputation.

The great object proposed was to bring thermometers to an exact correspondence. It was expedient, therefore, not only to select a proper fluid, but to adopt a uniform and consistent scale. Alcohol, linseed oil, and mercury, had been successively tried. The graduation was at first derived 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 thermometric 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 boiling of water; and he seemed contented with framing a rude standard, by 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 dividing 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 viscous 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.

Roemer, 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 Petersburg 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, a distinguished Italian mathematician, 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 Dantzig, 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 sixty-four of them as the expansion between freezing and blood-heat, and thirty-two as the contraction from the same point to what he considered as extreme cold, or that produced by the mixture of one part of salt with three parts of 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 a 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 affected by 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 would 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 Hooke 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 there- Barometer—fore 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 never 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.

Blondeau's An ingenious and very substantial kind of marine barometer was, above twenty years ago, recommended by Blondeau, one of the professors of the naval academy at Brest. (See fig. 11, Plate CIV.) It consisted of an iron tube, bent below into a siphon, 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 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. Passemant, 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 Barometer, (see fig. 12, Plate CIV.), 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 India. To facilitate the keeping of a register of barometrical observations, the barometer's meritorious and indefatigable Mr Horsburgh, hydrographer to the East India Company, 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 officer, Captain Basil Hall, then 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.

A very convenient substitute for the marine barometer is the sympiezometer constructed by Mr Adie, optician in Edinburgh. It is merely an improved manometer, partly filled with a fixed oil, and indicating the conjoined effect of heat and atmospheric pressure on the inclosed air; but the influence of temperature is corrected by means of a sliding scale, regulated by an attached mercurial thermometer. The sympiezometer occupies little room, is easily rectified, and comparatively cheap.

To explain the cause of the variations of the barometer, Difficulty 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 re- Barometer. curial column generally falls before rain, seemed at complete variance with the intimation of the senses, a notion having become universally prevalent, that the air is heavier when the sky appears lowering and overcast; another proof, were it 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 adds its own weight to the pressure of a fluid in which it is suspended, yet it will cease to be ponderous during 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, by 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 further 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 more than two inches in both the barometers.

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.

Similar 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 siphon 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 the 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. See the articles Climate and Meteorology. (J.L.)

**Supplement**

**Aneroid Barometer.** A very ingenious instrument for indicating atmospheric pressure is the invention of M. Vidi of France, which is called the *Aneroid Barometer* (from *anépoual*, I inquire). The appearance and construction will be best understood from the annexed diagrams.

The action of the aneroid depends on the effect produced by the pressure of the atmosphere on a circular metallic chamber exhausted of air and hermetically sealed; thus the chamber is a substitute for the Torricellian tube, and the vacuum for the column of mercury. The mechanical arrangements of the instrument will be understood by reference to the woodcuts.

Figure 1 represents the external appearance of the instrument. It is four inches and three-quarters in diameter across the face, and one inch and three-quarters in thickness. The pressure of the atmosphere is indicated by a hand pointing to a scale which is graduated to forty divisions to the inch; one or two thermometers are fixed on the face, as shown in the drawing, but these are not essential.

Figure 2 represents the internal construction, as seen when the face is removed, but with the hand still attached. \(a\) is a flat circular metallic box, about two inches and three-quarters in diameter, and a quarter of an inch in depth, having its upper and under surfaces corrugated in concentric circles. This box or chamber being exhausted of air, through the short tube \(b\), which is subsequently made airtight by soldering, constitutes a spring, which is affected by every variation of pressure in the external atmosphere, the corrugations on its surface increasing its elasticity. At the centre of the upper surface of the exhausted chamber there is a solid cylindrical projection \(x\), about half an inch high, to the top of which the principal lever, \(c, d, e\), is attached, as shown in the drawing. This lever rests partly on a spiral spring at \(d\); it is also supported by two vertical pins, with perfect freedom of motion. The end \(e\) of the large or principal lever is attached to a second or small lever \(f\), from which a chain \(g\) extends to \(h\), where it works on a drum attached to the arbor or axis of the hand, connected with a hair spring at \(k\), changing the motion from vertical to horizontal, and regulating the hand, the attachments of which are made to the metallic plate \(i\). The motion originates in the corrugated elastic box \(a\), the surface of which is depressed or elevated as the weight of the atmosphere is increased or diminished, and this motion is communicated through the levers to the axis of the hand at \(h\). The spiral spring on which the lever rests at \(d\) is intended to compensate for the effects of alterations of temperature. The actual movement at the centre of the exhausted box, from whence the indications emanate, is very slight, but by the action of the levers this is multiplied 657 times at the point of the hand, so that a movement of the twenty-second part of the tenth of an inch in the box carries the point of the hand through three inches on the dial.

The effect of this combination is to multiply the smallest degrees of atmospheric pressure, so as to render them sensible on the index.

The aneroid is a very delicate instrument, and seems to answer well at moderate elevations above the sea.

Metallic Barometer. M. Bourdon constructed in 1850 a modification of the aneroid, and, like it, a very portable instrument, in which the movement is produced by the inequalities of atmospheric pressure on the convex and concave sides of a thin box in the form of a segment of a circle. The idea is ingenious. It is very similar to a contrivance of Messrs Bryson of Edinburgh in 1849, which diminished the instrument to the size of a small snuff-box. Both appear to be original, but are perhaps inferior to the instrument of Vidi.

Register Barometers. Barometers have been invented which indicate their own extremes of altitude during the absence of the meteorologist. These instruments render his task less irksome; and if generally adopted, would increase our knowledge of atmospheric phenomena.

Dr Traill exhibited to the meeting of German philosophers at Hamburg in 1830, a register-instrument which he had constructed for showing the maxima and minima of barometric pressure during the absence of the observer. It is cheap and simple in construction, and consists of two diagonal barometers, one of which is inverted, and resembles in principle the rectangular barometer. Both are attached to the same frame. Before bending and filling the upright one, and after filling the inverted barometer, a piece of smooth steel wire is introduced into each, to serve as indices to the instrument.

When the upright barometer rises, the mercurial column pushes before it the index, which remains there by its own gravity on the sinking of the mercury; so that its extremity next the mercury will indicate the extreme rise of the barometer. When the mercurial column descends in the inverted instrument, which has a small aperture to admit atmospheric pressure at its lower extremity, that index is pushed before it, and its end nearest the mercury will indicate the extreme of the barometric depression, should the increased atmospheric pressure cause that end of the column to recede.

In the first instrument of this kind which Dr Traill constructed the inverted one was the common rectangular barometer; but he conceived that it would be preferable to give the same inclination to the angles of each tube, that the gravitation of both indices might be equal, as represented in the woodcut.

To adjust the instrument for observation, the indices are... drawn into contact with the end of each mercurial column by a small magnet. One of the instruments was used for several years without requiring any fresh adjustment. It is not, however, very portable.

A thermometer should be attached to the frame, to indicate the temperature of the instrument, for the same reason as in other barometers. If the cisterns be of considerable size in proportion to the bore of the tubes, we may for common purposes neglect the adjustment for inequalities in the level of the mercury in the cisterns; to which, indeed, it would not be easy to adapt an accurate indicator.

The general appearance of the instrument will be seen in the woodcut; and the construction, it is hoped, will be understood from this short description. This instrument was first published in a German journal, Oken's Isis; and again described in the article PHYSICAL GEOGRAPHY, in the seventh edition of this work.

Keith's Siphon Barometer. The late Alexander Keith, Esq. of Ravelston, invented one of the first of such instruments, which was described in the 4th volume of the Transactions of the Royal Society of Edinburgh. It is a siphon barometer, in the shorter limb of which an ivory float rests on the surface of the mercury, and carries a wire bent at the top at right angles. This bended extremity is placed between two lists of oiled silk, which it moves up and down as the float rises and falls with the mercury, on a wire stretched along the scale of the instrument. The objection made to this contrivance was, that it is subject to derangement from slight accidental causes, as the collection of dust, and the wearing of the apertures in the silk induces. But Mr Keith improved his barometer farther, by adapting to it a clock, which turned a vertical cylinder, covered with paper ruled into 31 columns, for the days of the month, and a pencil attached to the kneed wire traced the oscillations of the column through its whole monthly course. At the commencement of each monthly period, a new paper was substituted for the old, which was preserved in the register of the weather.

This very ingenious instrument never came into much use, both on account of the expense, and because it was alleged that the continual friction of the pencil rendered slight changes of atmospheric pressure inappreciable.

Bryson's Improved Siphon Barometer. More recently a great improvement on the instrument of Mr Keith was contrived and executed by the late Mr Bryson, an eminent watchmaker of Edinburgh.

This is also a siphon barometer, furnished with an ivory float, and kneed wire, ending in a small knife-edge, intended to mark the oscillations of the mercurial column. A vertical cylinder of japanned tinned iron, about three inches in diameter, moves round its axis once in every 24 hours. At one of its ends the hours from 1 to 12 A.M. are in red, and from 1 to 12 P.M. in black characters. Seven such cylinders are made, one for each day of the week. The cylinder for each day's observations has its surface thinly coated with a mixture of fine chalk and water, laid on with a camel-hair pencil. The cylinder fits accurately on a square arbor or spindle, which is steadily moved round by clockwork attached to the barometer. The knife-edge, instead of constantly pressing against the cylinder, is made to touch it only for a moment every hour, by means of a series of pins on the wheel connecting the clock with the cylinder. This slight touch is sufficient to cut through the thin film of chalk, and show a black line on a white ground. There is therefore on the cylinder one such mark for every hour; and their position indicates the successive oscillations of the barometer.

When the day is completed, the cylinder is removed, and a fresh one substituted, and the first is fixed horizontally between two pivots, on which it is made to revolve steadily in what is termed the reader. On the front of the reader is a scale of inches from 27 to 32, divided into hundredths by means of a vernier, moved by a firm tangent screw, and ending upwards in a point. The precise value of each mark on the cylinder is thus ascertained.

This scale is first adjusted by simultaneous observations on a standard barometer. Should the scale be too high, the fixed pivot of the reader is loosened, and screwed out, until the point of the vernier corresponds to the indications of the standard barometer; after which the fixed pivot is permanently secured in its place for future observations.

The simple mechanism by which he has secured the steady movements of the cylinder in its horary revolution, and when transferred to the reader, is creditable to the skill of the inventor; and the barometer of Mr Bryson may be considered as probably the best register barometer hitherto constructed.

Specimen of Bryson's Hourly Barometric Register.

| | A.M. | | |-------|---------------|-------| | 1843 | | | | June 22 | ... | ... | | 23 | 29-93 | 29-91 | | 24 | 29-95 | 29-95 | | 25 | 29-97 | 29-97 | | 26 | 29-85 | 29-85 | | 27 | 29-80 | 29-79 | | 28 | 29-71 | 29-69 | | 29 | 29-67 | 29-67 | | 30 | 29-65 | 29-65 |

| | P.M. | | |-------|---------------|-------| | June 22 | ... | ... | | 23 | 29-96 | 29-96 | | 24 | 29-95 | 29-95 | | 25 | 29-93 | 29-93 | | 26 | 29-84 | 29-84 | | 27 | 29-74 | 29-74 | | 28 | 29-63 | 29-63 | | 29 | 29-62 | 29-62 | | 30 | 29-61 | 29-61 |