A T M 



tending from 38 to 3 1 inches, and ftewina-, b)- its various aki- 

 tudes, conx-fpondiiig variations in the weather. Tlie caufes 

 that influence tlie variations of the one, produce alio a fimi- 

 hr effcft on the other; and if the former were known, tlie 

 latter might be afcertaiiied. I'lie immediate caufes may 

 probably be reduced to the two following ; viz. an emiffion 

 of latent heat from thc'vapouvs of the atmofphere, or of 

 eleftric fluid from thcfe or from the earth. Both thcfe 

 caufes are obfervtd to produce the fame elTedl with the folar 

 lieat in the tropical climates, which is that of rarefying ihe 

 air by bler.dli-g with it, or fetting loofe a lighter fluid, which 

 did not previouily aft with fuch power in any particular 

 place. For a more particular account of different theories 

 on this fubjeft, fee Barometer, Hail, Mi>tlokology, 

 Rain, Snow, Weather, and Wind. Of the importance 

 and utility of this property of the atmofphere, many in- 

 ilances occur in the animal economy, ch.tmical proctfles, 

 and mechancal operations. See Cupping, Resriration, 

 Colour, Combustion, Vapour, Pump, and Svphon. 



With the gravity and preffure of the air are nearly con- 

 netted its other properties of denftty and elaft'uily. The 

 denfity of the atmofphere mull principally depend on its 

 gravity; and, in general, increafe and decreafe in the fame 

 proportion. In the lovver and intermediate ftrata of the 

 atmofpherical air, this ratio obtains; but it is not uniform 

 and conllant in all elevations. In the higher regions of the 

 atmofphere, where the eleftric fluid abounds, this fluid may 

 diminilh the gravity of the ;rtmofp!iere, without afftCting its 

 denfity. Beiides, the denfity of the atmofphere in the 

 torrid zone will not decreafe fo fall in proportion to the 

 height of the column, as in the temperate and frigid zones; 

 becaufe the column is longer, and becaufe a greater 

 portion of atmofpheric air occupies the higher parts 

 of this column. Confequenlly the denfity of the atmo- 

 fphere at the equator, which is lefs at the furface of 

 the earth, mull at a certain height equal, and at a greater 

 height exceed, the denfity of the atmofphere in the tem- 

 perate zones and at the poles. As a current of atmofpheric 

 air is continually afcending at the equator, and part of it oc- 

 cupies the higher regions of the atmofphere, and as its 

 fluidity will prevent its accumulation at the equator, it will 

 of courfe defcend tov/ards the poles: and during our 

 winter, a greater portion of the equatorial column \^ill flow 

 to the northern than to the fouthcrn hemifphere; but a lefs 

 portion will purfue this courfe during the fummer. The 

 mercurial column, therefore, will be al.vays highell with us 

 in winter, and the correfponding range of the barometer 

 more confiderable than in fummer; and vice verfa. The 

 denfity of the atmofphere' will be materially affed'ted by the 

 caloric or matter of heat which it contains, and of courfe it 

 will depend iu great meafiire on the degree of cold which 

 prevails. Where the cold is greateft, the denfity of the 

 atmofphere will alfo be greateft, and its height will be dimi- 

 iiilhed. In thofe countries which abound with higli moun- 

 tains that are generally covered with fnow, the cold will be 

 more intenfe than in others lefs elevated, though fituate in 

 the fame latitude; and of courfe the height of the atmo- 

 fpheric columns will be proportionally lower. Hence the 

 fupcrior air in its palfage to the poles will be retarded, and 

 accumulate over them. Such accumulations will take place 

 over the uorth-weftern parts of Afia, and over North Ame- 

 rica ; and on this account the barometer ufually Hands 

 higher, and its range is more uniform than in Europe. 

 Similar accumulations are alfo formed in the fouthern 

 parts of the old continent ; for inllancc, over the moun- 

 tainous trafts of Thibet, Tartary, Turkey^ in Europe, 

 Africa, and even in fome degree on the Pyrenees and Alps. 



ATM 



When thefe acctimulations have for any time prevailed, tlie 

 denfity of th-- atmofphere becomes too confiderable to be 

 balanced by the furrounding medium; and of courfe it will 

 defcend towards the regions of the atmofphere that lie over 

 the adjacent countries, and produce cold winds, that will 

 raife the mercury in the barometer. Thus the north-cail 

 winds in Europe are occalioiially accompanied by a rife of 

 the barometer, becaufe they proceed from accumulations of 

 the atmofphere in the north-well parts of Afia, or about the 

 pole; and hence it is, that the north-well wind from the 

 mountains of Thibet, raifes the barometer at Calcutta. 



As the mean heat of our hemifphere is not permanent, 

 the denfity of the atmofphere, and conftquently the quaii- 

 tity of equatorial air, which flows towards the poles, mufl 

 be fubjecl to correfponding variations. The accumulations 

 of atmofpheric air on the mountainous parts of the fouth of 

 Europe and Afia, occafionally exceed their ufual hmits, 

 which is partly owing to earlier falls of fnow, or to the ex- 

 clufion of the folar rays by fogs of long continuance. In 

 this cafe the atmofphere in the polar regions will fuftain a 

 correfponding diminution of denfity. In the torrid zone 

 and equatorial regions the heat is uniform ; and the denfity 

 of the atmofphere, modified by it, as well as the height of 

 the atmosphere, will not be fubjecl to much variation. 

 Kirwan, Irifli Tranf. for 1788, p. 60. See Dcnsity. 

 See alfo Barometer, under wliich article the caufe of the 

 variations in the weight and prefl'ure of the atmofphere is 

 particularly difcufled. For the efix-cls of the removal of the 

 prefl'ure of the atmofphere, fee Air Pump, and Vacuim. 

 For the elafticity of the atmofphere, fee Air, and Elasti- 

 city of the jlit: 



Atmosphere, Heigh: of ihe. The height of the atmo- 

 fphere has been a fubjedl of particular inveftigation ; more 

 eipecially fince it was difcovered by the Torricellian tube, 

 that air is endued with weight and prefiTure. And, indeed, if 

 the air pofleiTed no elailic power, but were every where of the 

 fame denfity, from the furface of the earth to the extreme 

 limit of the atmofphere, hke water, which is equally denfe 

 at all depths, the whole height of the atmofphere ir.ight be 

 afcertained without difficulty. It has already been ob- 

 ferved, that the weight of a column of air, reaching to the 

 top of the atmofphere, is equal to the weight of the mer- 

 cury contained in the barometer, and counterbalancing it; 

 and the proportion of weight hkewife being known between 

 equal bulks of air and mercury; it will be eafy to find the 

 height of fuch a column, and confequently that of the atmo- 

 fphere itfelf. — For a column of air, one inch high, being to 

 an equal column of mercury as i to 1 1364.6; it is evident 

 that I 1364.6 fuch columns of air, that is, a column 947 

 feet high, would be equal in weight to one inch of mercury : 

 and confequently the 30 inches of mercury fuitaintd in tlie 

 barometer, require a column of air 2^410 feet higii ; 

 whence* the height of the atmofphere would only b^-* 

 28410 feet, or little more than five Englifli inilcs and a 

 quarter high. 



But the air, by its tlaftic property, e:;pands and cou- 

 trafts; and it being found by repeated experiments in Eng- 

 land, France, and Italy, that th.e fpaccs it takes up, whtu 

 comprefled by difierent weights, are reciprocally propor- 

 tional to thofe weigh.ts themielves; or, that the air takes up 

 tile lefs fpace, the ir.ore it is prefied; it follows, that thf 

 air in the upper regions of the atmoiphere, where the 

 weight is fo much lefs, mull be much rarer than near 

 the furface of the earth; and, confequently, that the 

 height of the atmofplKrc mull be much greater than is 

 above affigued. 



Mr. Cotes, in his Hydrollatical Leftures, iccl. ix. la's 

 I i 2 demon* 



