ATM 



five feet from the ground, was found by M. Piclet to be 

 colder than that at 50^ Bcfidts, different ftrata are found 

 to poffefs very different and variable degrees of cold, without 

 anv regard to the altitude or dcprcffion of their fituation. 

 In lySo, Dr. Wilfon of Glafgow (Phil. Tranf. for 1780, 

 p. 467.; and for 1781, p. 368.) found a remarkable cold ciofe 

 to the furface of the ground; fo that the thermometer, when 

 laid on thefurface of the fnow and hoar-froil, funk, many de- 

 grees lower than one fufpended twenty-four feet :ibove it. 

 Hence it has b^en concluded, that fnow, falling from the 

 higher regions of the atmofphere, is generally colder than the 

 lower air. 



With refped to the precife effect of elevation, Mr. Kiiwan 

 found it to be oearly as follows: when the elevation is mo- 

 derate and gradual, fuch as that of the interior parts of 

 moft countries very diilant from the fea, its effects are fo 

 blended with tliofe of diftancc from the llandard ocean, that 

 the fame allowance in the diminution of temperature is to 

 be made for both. By a gradual elevation, he means fuch 

 as rifes at a lefs rate than fix feet per mile, counting from 

 the nearell confiderable fea. If the elev?.tion proceeds at a 

 greater rate, then for every 200 feet of elevation, the annual 

 temperature of the itandard mull be diminillied in that lati- 

 tude, as follows: 



If the elevation be at the rate of 



6 feet per mile ... | of a degree. 



7 feet - ... I 

 13 feet - . . . ^ 

 15 feet, or upwards - - 2 



For every 50 miles diftance from the ftandard ocean, the 

 mean annual temperature in different latitudes mult be de- 

 prelfed or raifed, nearly at the following rate : 



From lat. 70'' to lat 35° cooled, i of a degree. 



35° - - i 



30= - - o 



25° warmed - f 



20° . . I 



10"^ - - i^ 



See on this fubjeft Kirwan's Eftimate of the Temperature of 

 different Latitudes, 1787, paffim. 



It has been obferved, that in clear weather, though the 

 furface of the earth be then moil liable to be heated by the 

 fun, yet after fun-fet, and during the night, the air is coldeft 

 near the ground, and particularly in the vallies. The expe- 

 riments made on this fuhjeft for a whole year, by Mr. James 

 Six, may be feen in thefe venty-eighth volume of the Philo- 

 fophical Tranfaftions, but our limits will not allow our re- 

 citing them. The conclufions deduced from them are 

 thele: that a greater diminution of heat frequently takes 

 place near the earth in the night time, than at any altitude 

 in the atmofphere within the limits of the writer's inquiry; 

 that is, 220 feet from the ground; and that at luch times the 

 greatell degrees of cold are always met with nearell the fur- 

 face of the earth. This is a . conftant operation of nature, 

 under certain circumftances .of the atmofphere, and occurs at 

 all feafcns of the year; and this difference never happens in 

 any confiderable degree, except when the air is ftill, and the 

 Jky perfectly unclouded. The refrigeration was not at all 

 impeded, but rather promoted, by the moifteft. vapour, as 

 dews and fogs. In very fevere frolls, when the air fre- 

 quently depofits a quantity of frozen vapour, it is commonly 

 found greatell; but the excefs of heat, which in the day was 

 found in the lowed ilation in fumnier, was diminiihed in 

 winter almoft to nothing. The fad of the mcrcur)'s fink- 

 ing in a thei-momcter, mcUided in a receiver, vshen the air 

 begins to be rarefied, has been ufually attributed, not to any 

 degree of cold thus produced, but io'the fudden expaufion of 



ATM 



the bulb of the thermometer, in confeq.ier.ce of the removal 

 of the atmofpheric preffure ; but from fomc experiments of 

 Dr. Darwin (fee Philof. Tranf. vol. 78. p.43. &c.) it appears, 

 that tlie atmofphere always becomes warm by comprcflion, 

 and cold by dilatation from a comprtffcd Hate. This in- 

 gcnious author mentions a curious phenomenon obferved in 

 the fountain of Hiero, conftructed on a ver)- large fcale in 

 the Chemnicenfian mines in Hungary. In this machine the 

 air, in a large veffel, is comprcfftd by a column of water 260 

 feet high ; a ftop-cock is then opened ; and as the air iffucs 

 out with great vehemence, and in confcqucnce of its previous 

 condenfation becomes immediately much expanded, the 

 moiftiire contained in it is not only precipitated, as in tlie ex- 

 hauftcdrectivcr, butfallsdown in a ihower of fnow, with icicles 

 adhering to the nofel of the cof k. See Phil. Tranf. for 1 761, 

 vol 52. From this phenomer.on, as well as from his experi- 

 ments. Dr. Darwin infers, that there is good rcafon for 

 concluding, that in all circumftances where air is mechani- 

 caily expanded, it becomes capable of attracting the fluid 

 matter of heat from othsrr br.dies in contaA with it. (Sec 

 Caloric) Now (fays he), as the vail region of. air 

 which furroimds our globe is perpetually moving along its 

 furface, climbing up the fides of mountains, and dcfecnding 

 hito the vallies; as it paffcs along, it mull be perpetually 

 vaiying its degree of heat, according to the elevation of the 

 country it traverfes; for, in rifing to the fummits of moun- . 

 tains, it becomes expanded, having fo much of the preffure 

 of the fuperincumbent air taken away; and when thus 

 expanded, it attracts or abfoibs heat from the mountains in 

 contiguity with it; and, v.ben it deicends into the valleys, and 

 1? again compreffed into Icfs compafs, it again gives out the 

 heat it has acquired to the bodies it becomes in cantad with. 

 The fame thing mull happen in refpectto the higher regions 

 of the atmofphere, which are regions of perpetual fn.ft, 

 as has lately been difcovered by the aerial navigators. When 

 large dillrids of air, from the lower parts of the atmofpheic, 

 are raifed two or three miles high, they become fo much ex- 

 panded by the great diminution of the preffure over them, 

 and thence become fo cold, that hail orfiiow is produced by 

 the precipitation of the vapour: and as there is, in thel'c 

 high regions of the atmofphere, nothing elfe for the ex- 

 panded air to acquire heat from after it has parted uith its 

 vapour, the fame degree of cold continues, till [he air, on 

 del'cending to the earth, acquires its former Hate of con- 

 denfation and of wannth. 4 



The Andes, almoll under the line, refts its bafe on burn- 

 ing fands; about its middle height is a mod plcafant and 

 temperate climate covering an extenfive plain, on which is 

 built the city of Quito; while its forehead is encircled with 

 eternal fnow, peihaps coeval with-the mountain. Yet, ac- 

 cording to the accounts of Don UUoa, thefe three dif- 

 cordant chmates feldom encroach much on each other's 

 territories. The hot winds below, if they afcend, become 

 cooled by their expanllon; and hence they cannot affeft the 

 fnow upon the fummit; and the cold wjnds, that fweep 

 the fummit, become condenfed as they defcend, and of 

 temperate warmth before they reach the fertile plains ot 

 Quito. 



The temperature of the atmofphere, and the vicifQtudcs 

 of its heat and cold, are I'ubjedt to a variety of irregularities, 

 which no theory that has yet been propofed is alif'-clhcr 

 fufficient to explain. For other obfervations on this fubjeCl, 

 fee the articles Clouds, Cold, Congelation, Evapora- 

 tion, Hail, Heat, Hygrometer, Meteorology, 

 Rai^:, Snow, Weathex, Wind, &c. 



ATMOSPHtRE, l/jh of thi. Thefe are fo numerous and 

 various, Uiat il would req^iure a vcrj- minute and extended 



^ detail 



