A T M 



thelefs, as the fame force which contributes to eleva'e the 

 air, diminifhes the prefflire on the furface of the earth, higher 

 columns of it at or near the equator, all other circuiiiftanc 



ances 



being the fame, may npt be heavier than thofe that are 

 lower at or near the poles. 



Mr. Kirwan (Iridi Tranf. for 1788. p. 61.), flr.ting the 

 height of the mercuiy in the barometer on the level of the 

 fea, indicating the natural ftate of the atmofphere, to be 

 thirty inches under the equator a*nd under the poles, ob- 

 fei-ves, that in order to produce this ftate, the weight of the 

 atmofphere muft be every where equal at the furface of the 

 fea ; and as the weight of the atmofphere proceeds from 

 its denfity and height, this eq-jalily of weight requires that 

 the atmofphere fliould be lowell where its denfity is o-reateft, 

 and highell where its deniity is leaft. Thefe extremes of 

 flenfity take place in the equatorial and polar regions. 

 Under the equator, the centrifugal force, the dillance from 

 the centre of the earth, and the heat are all at their maximum ; 

 in the vicinity of the poles, on the contrary, they are at 

 their minimum. From, this reafoning it follows, that the 

 atmofphere muft be h'ghefc under the equator, and loweft 

 under the poles, with feverai intermediate gradations. Kir- 

 wan fuppofes the rar-rfaclion of the atmofphere in the polar 

 regions to proceed from the aurora: borealis and aullralis, 

 which he takes to be a con'.buftion of inflammable air, caufed 

 by eleclricity ; and as this air is lighter than any other, it 

 conftquently occupies the higheft regions of the atmofphere. 

 See Aurora, and Barometer. 



Atmosphere, H'eight and Prejfme of ihe. The weight 

 of the atmofphere, depending partly upon its height, and 

 partly upon its denfity, and its confequcnt prelTure, are 

 properties that have been long afcertained by means of the 

 afcent of mercury i;i the barometer, and of water in pumps, 

 fyphons, and other fimilar engines. (See Air, Wiigbt of.) 

 The quantity oi this prcfTure may be eafily eftimatcd by 

 comparing the weight of a column of atmofpherical air with 

 that of a correfpoudii^g cclun-.n of quickf.iver, or of waier, 

 by which it is counterbalanced. Upon this principle it I'.as 

 been found, that the preifure of the atmofj)here fu'lains a 

 column of quickfilver, in the tube of the barometer, of the 

 height of about thirty inches ; and hence it follows, that 

 the whole prefTure of the atmofphere is equal to the we'ght 

 of a colun.n of quickfilver, having an equal bafe, and about 

 thirty inclies in height. But as a cubical inch of quickfilver 

 weighs about 8 oz. 1.49 drams, avoirdupoife, the weight of 

 30 cubical inches will be 15 pounds, nearly. Such, there- 

 fore, is the weight of the atmofphere on eveiy fquare inch 

 of furface. It has been alio found, by pumps and other 

 hydraulical engines, that the preflure of the atmofphere 

 fuftains a column of water from 34 to 55 feet, fay 34' feet, 

 high ; and as a cubical inch of water weighs 9.25 drams, 

 and a cubical foot 1728x9.25 drams, nearly loco ounces 

 avoirdupoife, or 62j pounds, the amount of the prelTure of 

 the atmofphere on a fqaare foot will be 342 X 62I, or 2 1 56 J 

 pounds ; and a (quare foot, containing 144 iquare inches, 



— —— , or nearly 15 pounds, will be its preDTure on a fquare 



144 

 inch. Hence it follows, that if a man's body contain (5 

 fquare feet, which is near the truth, he will fuftain a weight 

 equal to 2156^ X 15 = 32343^ pounds, or about 145 tons, 

 when the quickfilver in the barometer ftands at 30 inches. 



This preifure is fo great, that it would be abfolutely in- 

 fupportable, and even fatal to us, if it were not equal m 

 every part, and counterbalanced by the fpring of fome other 

 elaftic fluid within us, which is diffufed through the whole 

 body, and reatts with an equal force againd the outward 

 preifure. The nature of this internal elaftic fluid is not 



Vol. IIL 



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clearly underrtood, nor, indeed, is its cxiftence pofitivcly 

 afcertained. But whatever it be, it is fuch as to counteraft 

 the weight of the atmofphere. However, if any conQd,.rable 

 preflure be fupera Jdcd to that of the air, as e. g. by defcend- 

 ing into deep water, it is always felt in a greater or lefi dc- 

 gree (fee Diving), more efpecially when the change it 

 fudden ; and, on the other hand, if the preflure of the at- 

 mofphere be taken off Turn any pait of the human bod)-, 

 as from the hand placed over the exliaufted receiver of an 

 a-r-pump, the weight of the fuperincumbent atmofphere is 

 felt, and the flcfli of the hand is tliruft down, as it were by 

 fuclion, into the glafs. We might add, that the heat of 

 our bodies rarefus th; air contiguous to their furfaces, and 

 therefore a living animal does not fnilain an equal atmofphe- 

 rical prcffure with that of inanimate and cold fubftances. 

 Moreover, as tiie earth's furface contains, in round num- 

 bers, 200,000,000 fquare miles, and every fq-.iarc mile 

 27,87S,40ofquare fett, there mull be 5,5 75,680,000,000,000 

 fquare feet on the earth's furface ; which, multiplied by 

 2156+ pounds, will give 12,022,560,000,0-00,000,000 

 pounds for the preffure or weight of the whole atmo- 

 fphere. 



Mr. Cotes (Hydroflaticiil and Pnciirr.atical Left. p. 1 1 2.) 

 mentions the refult of a computation which he made of the 

 weight of all the air, which preffes upon the whole furface 

 of the earth ; and he obftrves, that it is equal to the prcf- 

 fure of a globe of lead, nearly 60 miles in diameter. The 

 computation proceeds upon thefe principles ; that the 

 we'ght of a column of air, reaching to the top of the at- 

 mofphere, is moil commonly equal to a column of wat-.r, 

 having the fame bafis, and the altitude of 34 feet ; that the 

 femidiameter of the earth is equal to 20949655 feet ; and 

 that the fpeeific gravity of water is to that of lead as icoo 

 to 1 1,325. 



The dilference of the weight of the atmofphere^ and of 

 its confcqucnt prcffure, at different times, and in different 

 fituations, is a circum'.lance that deferves our particular 

 notice. This difference in the fame fituation aiifes from 

 changes in the ftate of the atmofphere ; and it chiefly occurs 

 in places at fome diftance from the equator. It is indicated, 

 and of courfe eafily eftimatcd, by the different height to 

 which the mercury is raifed in the barometer. As the 

 greateft variation of the height of the mercury occupies a 

 rangeof about 3 inches, or from 28 to 31 inches, being ^J^th 

 of the whole range, a column of air of any affignable bafe, 

 equal to the v%-cight of a cylinder of mcrcurj- of the fame 

 bafe, and of the altitude of 3 inches, will be taken off from 

 the preflure upon a body of an equal bafe, at fuch times as 

 the mercury is three inches lower in the barometer : ^nd 

 therefore every fquare inch of the furface of our bodies is 

 preffed upon at one time more than another, by a weight of 

 air equal to that of three cubical inches of mercury. As this 

 is about •j^'jth of the whole quantity, the difference of the 

 prcffure, which the human body fuftair.s at one time more 

 than another, amounts to about i4 ten. The reafon why wc 

 are not fenfible of this preflure is explained in the following 

 manner by Borellus, de Mot. nat. a grav. fac. prop. 29, &c. 

 After faying that fand, perfedlly rammed in a haid velfel, is 

 not capable, by any means, of being penetrated or parted, 

 not even by a wedge; and likewife that water, contained in 

 a bladder coinprcfled equally on all fid.";:, cannot yield or 

 give way in any part : he proceeds; " In like manner, within 

 the fl<in of an animal is contained a diverlity of parts, fome 

 hard, as bones; others foft, as mufeles, nerves, membranes, 

 &c.; others fluid, as blood, fat, &:c. Now it is not poITiblc 

 the bones (hould be broke or difplaced in the body, uiilefs 

 the weight lay heavier on one part thau on another, as we 

 i i runic timet 



