One of the most active members of the Royal So-1 ciety was Mr Boyle, who, having become ed with experimental researches in the course of his travels, devoted, after his return home, his time and his fortune to such calm but engaging pursuits. In this occupation, he derived the most essential aid from Dr Hook, whom he had the discernment to engage as his assistant,---:the most skilful mechanician, and the best practical philosopher, of the age. The same ingenious person was likewise employed as operator to the society, and undertook to produce at each meet ing some new experiments for the instruction and entertainment of the members. One of the favourite sub jects was to exhibit the properties of the atmosphere. Dr Hook, at the instance of Mr Boyle, had given a more convenient form to the air-pump, and had ma terially 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 Gilricke had obtained ; and the Eng lish philosophers were thus enabled to perform a va riety of delicate and interesting experiments, which extended the influence of the original 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, se it was tidied, was (regionally 'repeated and varied in the presence of a few of the more assiduous members: who,. though delighted with the exhibition, continued to reason and to doubt con cerning the cause of the phenomenon. These doubts acquired new force from a singular experiment which the celebrated Huygens some years afterwards com municated, during a visit he made to Londeo.' Waving filled a glass tube eighty inches long with mercu ry, and carefully expelled whatever air was lurking about the sides, he gently inverted it, as usual, in a boson ; when the mercury notwithstanding remained, still banging from the top of the tube, and did not side to the proper height, till it was struck with a slight blow. This anomalous fact appeared then extremely puzzling. The experiment, indeed, requires great nice ty and address on the part of the operator, and evident ly depends on a concurrence of circumstances which have not yet been sufficiently explained. There pan, at present, exist no doubt that this extraordinary sus pension of the mercury is occasioned by its obstinate adhesion to the inside of the tube, which, in the cess of purging the air, becomes probably lined with a very thin film of mercurial osyd. But Huygens,. who had embraced the leading principles of the Car tesian philosophy, was inclined to draw a very differ - est conclusion. He thought that the fact proved the existence of another fluid, besides, the atmosphere, arch one possessed of such extreme satiety and.power as to be capable of permeating the grosser bodies. Im ordinary cases, this fine ethereal• substance might be supposed to escape through, the pores of the glass, and: leave the mercurial column to the mere pressure of the atmosphere. Such was- the unfortunate intro duction of that ideal being—an ether-lint° experi. mental 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.
It was soon perceived, that the syphon-barometer of Torricelli has a disadvantageous form. Both branches of the tube being supposed of the Same width, the mercury must evidently sink as much in the one as It will rise in the other ; so that the varia times in the height of the column are thence reduced to half the true quantity. A small boson, or semi circular wooden box, to hold the surplus mercury, was therefore attached to the frame of the instru ment ; and this construction, with very little change, was adopted, during the course of a century, by the makers of tbe ordinary barometer. But the syphon
barometer itself was afterwards materially improved by having its lower branch blown into a wide bulb for holding the charge of mercury. (See fig. Q. Plate XXXII.) This form of the barometer is not quite ac curate, owing to the smallness and unequal shape of the round bulb; but being very convenient for car rage, it has grown into general use, at least for the cheaper and more common sore of instruments.
As soon as the barometer came to be regarded as mreather-glase,ingemitywas set at work to devise•the means of enlarging•its scale of variations. Descartes first proposed a sitxple method for efaethist that eh. jeer, by combisthig amersurislwith &waft bareahettpr; which arrangement, though subject to impeefeetion,htrs led to many of the subsequent improvements. He directed two short barometric tubes to be ce mented, the onehato the bottom, other to the neck of &phial ; or, still better, that the tubes should be joined, by the. *um 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 intandueed purrme ter, 'teaching to the top of the upper tube, and there sealed close, When this compound tube was inverts ed in a bacon of mercury, it is evident that the co lumns both of mercury and of water would ink, till their joint pressure became just equal tot the super incumbent weight of the external atmosphere. But the variation of this weight would afterwards be in. dicated chiefly by the large motion of the' water; since the mercurial column, spreading. out above into a broad surface, must, in any • case, experi ence a very slight difference of altitude. Thula suppose the cylinder to have eight times the diameter of the upper tube; or a section sixty-four times greater, mercury being 13.6 times denser than water : For each inch of increase of altitude which the ordinary mercurial column gains, the top of the water would be raised in the tube 11.4 inches, its own rise being 11.18 inches, and that of the wide mercurial cylinder only .1B of an inch, yet equal in pressure to 2.4 inches of water. But Deseartery geurelly satisfied with mere theory and speculation, did not live to see his construction of the hereunto* earned into effect; and Chanuc the Fruch,reaident at Stockholm, to whom be• had+ impute& hie views, coot with such diffiadv in the execution of the ject, that, after 801118 fruitless attempts, absadoahed it altogether.
Huygens was more fortunate and succeeded, by I dint of peneveranoe and- skill, in conseructing the 1 Cartesian barometer. But he Ind. the.mortificatioa to find that, in spite of all the pains he amid take" the water, after it was relieved from the pressure- of the atmosphere by the•sealing of the tube> tat• study discharged a portion of air, which collected, at the top, and by its elasticity depressed the mom pound column below its due altitude. Conrtineed, that this source of imperfection is he sought to rectify the construction of the and produced his Doulik Baroeseter alorm ofeoahbi nation frequently used, especially when the eldest is rather to make the variations very sensible titian oho tab) delicate results. (sSeefig. 3.) He joined ahem. metric tube of the usual length by the flame e ablarn-, pipe, to two wide cylinders, the one sealed•at the top, and the other annexed likewise hermetioslipup a tall and narrow tube,. open at its eatronapt: he then bent the thicker tube a little shove the lower qlinder, and brought the two branches to be parallels The instrument being thus formed, he filled the bet branch with mercury, and introduced above, itt the second branch, some liquid of comparative lightness Alcohol would, in this. respect, answer estewaely well, if it -were not so liable to waste by erapeUdeth Bereeteme- An alkaline lye, or the geliquiate salt of tartar, • *bleb also readily admits of being coloured, was, therefore, on the whole, preferred.