After he had, by such researches, acquired cele brity in the scientific world, Galileo accepted an in vitation, with a very handsome appointment, from Cosmo de Medici ; and devoting himself intensely to astronomical observations, aided by the telescope, which, &omen obscure hint, he had recently construct ed, 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 plane tary phases--and discovering new worlds—he was for a moment recalled to his early studies, by an inci dent destined to form an epoch in the history of physical science. Some artisans, in the service of the Grand Duke, having been employed to construct a lifting or sucking pump for a very deep well, found, with equal surprise and vexation, that, in spite of all the pains they had taken in fitting the piston and valves, the water could by no effort be made to rise higher in the barrel than eighteen palms, or thirty-two feet. In this dilemma, they applied to Galileo for an explication of the cause of a failure so unexpected and perplexing. But the philosopher was not yet pre pared to encounter such a discordant fact. The Aristotelian tenet of the impossibility of the exist ence of a void, was, at this period, universally re ceived 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,theftga vacui,ornatutv'shor ror of a void. To create a vacuum, they gravely main tained, would require the hand of Omnipotence, trans cending the utmost power of men' or even devils. But Galileo, though borne along by the current of opi nion, saw the necessity of at least modifying the ge neral principle. Without questioning nature's ab horrence of a vacuum, he supposed the influence of o this horror to be confined within certain limits, not exceeding the pressure of a column of water eighteen palms in height. This was evidently evading, rather than meeting, the difficulty proposed for his solution. Yet, in the last of his Dialogues, te actually mentions an experiment to ascertain this power or virtu, as he calls it, of a vacuum. A piston, exactly fitted into a smooth hollow cylinder, was rammed quite to the end, and this carefully shut up ; then placing the cy linder 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 advan cing 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 supposed to reach to the moon, and the pressure of columns of such enormous height seemed to mock all calculation, and overwhelm the imagination.• Yet, on reconsidering the subject, Galileo began to sus pect the solidity of the explication which he had given ; but it was now too late for him, in his ad vanced age, loaded with bodily infirmities, and di spirited by clerical persecution, to attempt any far. ther innovation in science. Recommending it ear nestly to his friend and pupil Torricelli to resume the investigation, this illustrious precursor of New ton expired in 1642, the very year in which the English philosopher was born. His uniform kind ness 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 contract ed 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 him self. The method which he took brought very neatly under one view all the circumstances affecting the question. Having selected a tube about a quar ter 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 bacon likewise containing mercury, though covered with a portion of water. The mercury instantly sunk to nearly thirty inches above the lower surface ; but on raising the tube, till its orifice communicated with the layer of water, the mercury run all out, and the water now sprung up to the top, and occupied the whole. of the cavity. It was thus proved, that the water and mercury are each supported by the same equipoise, which Toni celli, after some hesitation, at last concluded to be the pressure of the external atmosphere. He next converted the mercurial column into a form adapted for observation, by bending the lower end of the tube, and constructing what has since received the name of the syphon barometer. (See fig. 1. Plate XXXII.)
Thus armed with a commodious instrument, he soon detected the variation of atmospheric pressure, which depends on the change of weather. These import ant 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 spe culate on the cause of such an unexpected fact. Des cartes,- with his usual rapidity and boldness of con ception, did not hesitate, in his correspondence with Mersenne, to refer the suspension of the mercury in the tube at once to the pressure of the external atmosphere. But this influence appears not very consistent with his system, which assumed the ex istence of an absolute plenum, and only supplied the place of a void by the diffusion of subtile abraded particles of matter. He suspected also the accuracy of Galileo's estimate of the weight of the air, which he thought was scarcely appreciable by experiment.
But, in the same country, the subject was now : pursued with deliberate caution, and through all its I 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 extra ordinary talents, which were encouraged by his fa ther, a man of learning, who lived in habits of inti macy with the literati of Paris. The young philoso pher happened to be residing at Rouen, in 1646, when he was informed of the famous Italian experi ment. 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 va- • cuum," and thought that the condensation and rare . faction of the air point to a different, or at least a ' modified conclusion. With a view to clear up this subject, Pascal performed a number of satisfactory experiments, of which we shall cite a few of the more striking, nearly in his own language, tinctured evident ly with the prevailing opinions of the age : 1. Having fitted a piston to an open glass tube, and rammed it quite down, he applied his finger close to the lower end, and plunged the whele under water; then draw ing back the piston, which was done with ease, the finger felt strongly and rather painfully attracted, while an apparent vacuity was formed above it, and continued to enlarge : but instantly on removing the finger, the water, contrary to its nature, darted up and filled the whole of the cavity. 2. A glass tube, about fifty feet long, sealed hermetically at one end, and filled with water, or rather red wine, as a more visible fluid, was inverted perpendicularly in a bason of the same. The liquid immediately subsided, leav ing a vacant space of thirty-five feet; but, on gradually reclining the tube, the liquid rose again, and con tinued to mount, till it struck a sharp blow against the top of the glass. S. A syphon, havingone leg fifty five feet high, and the other only fifty, being filled with water, and planted in two bacons containing the same, such that the shorter branch bad a perpendi cular position, the water sunk in both to the same level, without being attracted, as usual in syphons, to the longer branch ; but, on leaning the syphon back, the columns rose till they united at the top, awl then the water began to flow towards the lower The same experiment was also performed with mercury, the syphon having one leg ten feet, and the other only nine feet and a half in length, the mercury being found to divide itself into two columns, which con tinued 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 immer sed this in a bason of mercury, and held the tube in a vertical position ; on gently drawing up the piston, the mercury closely followed it to the height of twenty nine inches, but then stopt, 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.
1 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 separa tion, 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 ex ceeds not the pressure of a column of water thirty three feet in height. He next ventured one step far ther, and concluded, that this apparent vacuity was not filled by air lodged in the pores of the glass, or de rived 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 vacu um had been formed.