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II. JVf'ighl ovgrai'hy. Of this property of air the ar.cicuts 

 were not iJlogclucrdimppriiVd ; though their fcntimenta on 

 the fnbirft wtre confuftd ard unfetisfaftory. Ariftotle (de 

 Ca;lo, lib. iv. c. I. op. torn. i. p. 485.) obferves, that all the 

 tjements, lire excepted, have weight ; and he adds, that a 

 bladder inilati-d with air, weighs more than when it is quite 

 empty. Plutarch (de Phcitis. lib. i. c. 12. torn. ii. p. ^33.) 

 and Stob:"ii8 (Edog. Pbyf. lib. i. c. 17. p. 32. Ed. 1609.) 

 tjuote Arillotle as teaching, that the weight oi'air is between 

 that of iire and earth; and lichinifelf, treating of refpiration, 

 (cap. vii. oper. torn. i. p. 722.) reports the opinion of Em- 

 pedocles, who afcribes the eaule of it to the weight of the 

 air, which by its prefTuiv iniiimates itfelf with foi-ce into 

 the lungs. Plutarch (de Piaeit. lib. iv. c. xxii. torn. ii. 

 p. 90^.) cxprefTes, in iimilar terms, the opinion of Afcle- 

 piadej on this fubjeft ; and reprefents him as faying, thatthe 

 external air, by its weight, opened its way with torce into 

 the breafl. Heron of Alexandria, in his treatife intitlcd 

 Spir'Ualia, conftantly applies the elijll'ity of the air to pro- 

 duce fuch effects as are fuiUcient to convince us that he 

 well underftood tl at property of it : and Ctefibius, admit- 

 ting the principle of the air's elallicity, invented wind-guns, 

 wliich have been confidered as a modern contrivance. Philo 

 of Byzantium (in Veter. Mathem. p. 77. Ed. Paris.) de- 

 feribes thefe curious machines, conftrufted upon the prin- 

 ciple of the air's being capable of condenfation. Seneca 

 alio (Quasll. Nat. lib. v. c. v. and vi.) was acquainted with 

 the weight and elailic foixe of the air ; for he defcribes the 

 conltant effort by which it expands itfelf when it is com- 

 preffed, and affirms, that it has the property of condeniing 

 itfelf, and forcing its way through all obflacles that oppofe 

 its paflage. See Dutens's Inquiiy into the origin of the 

 Difcovcries attributed to the Moderns, p. 1 86. 1769. The 

 followers of Arillotle, however, abandoned the fentiments 

 of their mailer on tl is fubjetl ; and for many ages main- 

 tained a contrary' doftrine. The effects which are now 

 known to refult from the weight and elafticity of the air, 

 were for a long time attributed to the imaginaiy principle, 

 called/z/fa imcu'i, or nature's abhorrence of a vacuum ; and 

 Galileo himfelf admitted the principle, though he afTigned 

 a limit to it, correfponding to the weight of a column of 

 water 34 feet high. This diftinguidied philofopher, how- 

 ever, was well apprifed of the weight of the air as a body ; 

 and, in his Dialogues, he points out two methods of de- 

 monftrating it, by weighing it in bottles. But the preflure 

 of the air was difcovered by his difciple, Torricelli. In the 

 year 1643, it occurred to him, that whatever might be the 

 caufeby which a column of water, 34 feet high, is fullained 

 above its level, the fame force would fuilain a column of 

 any other fluid, which weighed as much as that column of 

 water, on the fame bafe ; and hence he concluded, that 

 quicklilver, being about 14 times as heavy as water, would 

 not be fullained at a greater height than that of 29 or 30 

 inches. He then made the experiment, called after his 

 name ; and inferred from it, that the weight of the air in- 

 cumbent on the furface of the external quickfilver, countei-- 

 balanced the fluid contained in the tube. By this experi- 

 ment he not only proved, as Galileo had before done, that 

 the air had weight, but that its weight was the caufe of the 

 fufpenfion of water and quickfilver in pumps and tubes, and 

 that the weight of the whole column of it was equal to 

 that of a like column of quickiilver, 30 inches high, or of 

 water 34 or 35 feet high ; but he did not afcertain the 

 weight of any particular quantity of it, as a gallon, or a 

 cubic foot ; nor its fpecific gravity to water, which had been 

 done, though inaccurately, by Galileo. Tonicelli's experi- 

 ment was publifhed at Warfaw, in Poland, by Valerianus 



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Magnus, as his own difcovciy ; but from the letters of Ro- 

 jtrval, it appears, that Torricelh's claim to priority is indil- 



Ma 



btrval, It appea 



putable ; and that neither Valerianus, nor Honoratus Fabri, 

 to whom it has been afcribed fo early as the year 1641, can 

 jufUy difpute it with him. The firfl difcovery of the 

 "weight and elafticity of the air has been lately afcribed to 

 Jean Rer, who wrote in 1 629, before Galileo, Torricelli, 

 Des Cartes, and Pafchal. His fourth and tenth effays have 

 been cited in favour of his claims ; but though he was ap- 

 prized that compretfion augmented the weight of the air, and 

 he feems to have believed, with Arillotle and others at a 

 very ancient period, that air was heavy, yet the proofs 

 which he allcdges were not fniHcicnt to convince the incredu- 

 lity of the peripatetics. The Tomcellian experlmeHt, by 

 wliich the fail was eftabliflied, and which father Merfenr.e 

 received" an account of in 1644, was immediately eumip.u- 

 nicated to the philofophers of France, and repeated m 

 various ways by Meffrs. Pafchal and Petit : and this gave 

 occalion to the ingenious treatife publillied by Pafchal, at 

 23 years of age, iutitled, " Experiences Nouvclles touchant 

 la Vuide." Having, after fome hefitation, adopted Torri- 

 celli's idea, and abandoned the principle of z fiiga vacui, he 

 devifed feveral experiments for confirming it. One of thefe 

 was to make a vacuum above the refervoir of quickfilver, 

 in which cafe he found that it funk to the common level : 

 and he then engaged M. Perrier, his brother-in-law, to 

 execute the famous experiment of Puy-de-Domme, who 

 found that the height of the quickfilver half way up the 

 mountain was lefs by fome inches than at the foot of it ; 

 and that it was flill lefs at the top. Thefe fafts incontef- 

 tibly proved, that it was the weight of the atmofphere 

 which counterpoifed the quickfilver. Des Cartes had alfo 

 jufl notions of the power of the air for fuflaining fluids 

 above their level, as appears by fome letters about this 

 time, and fome years before ; and in one of thefe he lays 

 claim to the idea of the Puy-de-Domme experiment. See 

 Cartefii opera, tom. ii. p. 243, 246. 



The experiment of Pafchal was repeated in various parts 

 of the world ; and particularly in 1653, by Dr. Power, in 

 England ; and in 166 1, by Mr. Sinclair, profefTor of philo- 

 fophy at Glafgow, in Scotland. 



That the air is heavy, follows from its being a body ; 

 weight being an efiential property of matter. And that it 

 is a body, is evident froiti its excluding all other bodies out 

 of the fpace it poflelTes ; for if a glafs jar be inverted into 

 a veffel of ^^■ater, the air, of which it is full, will allow but 

 little water to enter into it. But we have many argum.ents 

 to the fame purpofe from fenfe and experiment : thus, the 

 hand, applied on the orifice of a veflel empty of air, foon 

 feels the load of the incumbent atmofphere. Thus, glafs 

 veflels, exhaufled of their air, are eaiily crufhed to pieces 

 by the weight of the air without. So, two fmall hollow 

 fegments of a fphere, four inches in diameter, exaftly fit- 

 ting each other, being emptied of air, are preffed together 

 with a force equal to iSS pounds, by the weight of the 

 ambient air ; and that they are kept together by the preffure 

 of the air is evident, by fufpending them in an exhaufted 

 receiver, where they will feparate of themfelves. Farther, 

 if a tube, clofe at one end, he filled with mercury, and the 

 other end immerged in a bafon of the fame fluid, and thus 

 eredled, the mercurv in the tube will be fufpended at the 

 height of about 30 inches above the furface of that in the 

 bafon. The reafon of which fufpenfion is, that tb,e mercury 

 in the tube cannot fall lower without raifing that in the 

 bafon ; which being prefled down by the weight of the 

 incumbent atmofphere cannot give way, unlefs the weight 

 of the mercury in the tube exceeds that of the air out of 



it. 



