A 1 R 



A I R 



it. That tin's is the cafi.', is evident ; becaufe, if the 

 ^vholc apparatus be iiichulwl in tlie receiver of an air pump, 

 tlie mcreiiry will fall in proportion as the air is exhaulled ; 

 and on grad\ially letting in the air again, the mercury re- 

 afcends to its termer height. Tliis makes what is iifiially 

 called the Torrici- llian fx/ierimi-n/. 



To fay no moie, we can adnalh- weigh air ; for a vcflTel, 

 full even of common air, is found, hy a veiy nice hidanee, 

 to weigh more than when the air is exl;au(lcd ; a quart of 

 air weighing about 17 grains; and the efl'etl is propor- 

 tionablv more fentible, if the fame vcfFel be weighed full 

 of condenfed air, and more efpeeially in a receiver void of 

 air. 



The weight of air is continually varying, according to 

 the different degree of heat and cold, and the concurrence 

 of other cauics. Palehal oblerved it in France ; and Dcs 

 Cartes in Sweden, in 1650. Mr. I'loyle, and others, ob- 

 ferved it in England, in 1656. Some obfervers noticed, 

 that it was generally greattft in the night and in winter ; 

 and that its variations were moil cenfiderable during winter, 

 and in the northern regions. Hence arofe the application 

 of the BAROMETER to the ufes of a weather-glass. 

 Ricciolus ellimates the weight of air to that of water, to 

 be as I to 1000; Merfennus as i to 1300, or 1 to 

 1356; Lana, as l to 640; Galileo only makes it as i 

 to 400. Mr. I'joyle, by a more accurate experiment, f nmd 

 it about Loudon, as i to 938 ; and thinks, all things 

 coniidered, the proportion of 1 to 1000 may be taken as 

 a medium ; for there is no fixing any precife ratio, finee not 

 only the air, but the water itfelf, is continually varying. 

 Befides, experiments made in different places necelfarily 

 vary, on account of the different heights of the places, the 

 feafons of making the experiment, and the different denfitics 

 of air correfponding to thefe circumftances. It niuil be 

 added, however, that by experiments made fince, before 

 the Royal Society, the proportion of air to water was, 

 firft, found as i to S40 ; then, as i to 852; and a 

 third time, as i to 860. Phil. Tranf. N° 181. And 

 lartly, by a very fimple and accurate experiment of Mr. 

 Haukfbee, the proportion was fettled, as i to '885. 

 Phyf. Mechan. Exper. But thefe experiments being all 

 made in the iumnier months, when the barometer was 29I 

 inches high, Dr. Jurin thinks, that at a medium between 

 heat and cold, when the barometer is 30 inches high, the 

 proportion between the two fluids may be taken as i to 

 800 ; which agrees with the obfervation of the honourable 

 Mr. Cavendlfh, the thermometer being at 50°, and the 

 barometer at 29J inches. Pliil. Tranf. vol. Ivi. p. 152. 



Sir George Shuckburgh, (Phil. Tranf. vol. Ixvii. p. 560.) 

 by a very accurate experiment, found it as 1 to 836 ; the 

 barometer being at 29,27 inches, and the thermometer at 

 53° ; and the comparative gravity of quickfdver to air, as 

 1 1364,6 to I. The medium of all thefe is about one to 832 

 or 833, when reduced to the preffure of 30 inches of the 

 barometer, and the mean temperature 55° of the thermo- 

 meter. Upon the whole, it may be concluded, that when 

 the barometer is at 30 inches, and the thennometer at the 

 mean temperature of 55°, the denfity or gravity of water 



is to that of air as 833 | to i ; that is, as — — to i, or as 



2500 to 3 ; and for any changes in the height of the baro- 

 meter, the ratio varies proportionally ; and alfo that the 

 denllty of the air is altered by the x^"" part, for every de- 

 gree of the thermometer above or below tempt rate. This 

 number, which is a wrf good medium, having the fraftion 

 i, gives exaftly i^"" of an ounce for the mean weight of a 

 cubic foot of air; the weight of the cubic foot of water 



l^eing juft loco ounces avoirdupois, and that cf quiek- 

 fdver equal to 13600 ounces. 



Air, then, being heavy and fluid, the laws of its gnvi- 

 tation, or preffure, may be inferred to be tiic fame as tliofe 

 of otlier fluids ; and confequcntly its prcfVurc muft be pro- 

 portional to its perpendicular altitude. This is alfo con- 

 firmed by txperir.ient. For reinoving the Tonicellian tube 

 to a more elevated place, where the incumberit column of 

 air is lliorter, a proportionably Ihorter column of mereuiy 

 is fullained ; ar.d that nearly at the rate of 100 feet for ^'^ 

 of an inch of quickhlvcr. On this principle depend the llruc- 

 ture and office of the baromf.tkr. 



From hence, alfo, it follows, that the air, like all other 

 fluids, muft prefs equally eveiy way. This is confirmed by 

 obferving, that loft bodies fuffain this preffure without any 

 change of figure, and brittle bodies witliout breaking ; 

 though the preffure upon them be equal to that of a column 

 of mercury thirty inches high, or a column of water of 

 thirty-two or fomewhat more feet. It is obvious, tiiat no 

 other caufe can preferve fuch bodies unchanged, but tlie 

 eqtiable preffure on all fides, wliich refills as much as it is 

 refilted. And hence, upon removing or diminifliing the 

 preffure on one fide only, the effeft of the preffure is foon 

 perceived on the other. For the quantity and effeit of 

 this pred'ure of the atmofphere on the human body, and 

 on the furface of the earth, and the laws of different 

 heights, fee Atmos there. 



From the gravily of the air, confidercd in connexion with 

 its JIuk'i/y, feveral of its ufes and effeifls may be eafily deduced. 



1. By means of its weight, &c. it clofely inverts the 

 earth, with all the bodies upon it ; and conftringes and 

 binds them down with a force amounting, according to the 

 computation of M. Pafchal, to 2232 pounds weight upon 

 every fquare foot, or upwards of 15 pounds upon every 

 fquare inch. Hence it prevents, e. gr. the arterial veffcls 

 of plants and animals from being too much diilcnded by the 

 impetus of the circulating juices, or by the elallic force of 

 the air, fo plentifully contained in the blood. — Thus we fee, 

 in the operation of cupping, that, upon a diminution of 

 the preffure of the air, the parts of the body grow tumid ; 

 which neceffarily alters the manner of the circulation through 

 the capillaries, &c. 



The fame caufe hinder the iuices from oufing and cfcap- 

 ing through the pores of their containing vedels : this is 

 experienced by fuch as travel up high mountains, who, in 

 proportion as they afcend, find themfelvcs more and more 

 relaxed ; and at length become fubjeil to a ("pitting of blood, 

 and other haemorrhages ; becaufe the air doth not fuffi- 

 ciently conftringe the veffels of the lungs. Similar cffefls 

 are obfervcd in animals that are cnclofed under the receiver 

 of the air-pump, who, as the air is taken from them, pant, 

 fwell, vomit, and dilchargc their urine and excrementi. 

 See Vacuum. 



2. The weight of the air |)romotes the mixture of conti- 

 guous fluid bodies. Hence many liquids, as oils and falls, 

 which readily and fpontaneoufly mix in air, remain, on the 

 removal of it, in a Rate of ieparation. 



3. Tliis gravity of air does in fome cafes determine the 

 aftion of one body above another. 



4. To the fame principle are chiefly owing our winds, 

 wliich are only air put in motion by fome alteration in its 

 equilibrium. It is the weight of the air that caufes the 

 clouds and vapours to float in it. 



ni. Eliijlicily — or a power of yielding to an impref- 

 fion by contrafting its dimenfions ; and upon removing or 

 din.ipifhing the impreflive caufe, of returning to its former 

 fpace or figure, is another quality of air. Tliis clailic 



force 



