ATM 



mcnt at each term. Let L=the entire decrement or differ- 

 cnce between the heat at the furface and 32°; D=the 

 diltance of the lower line of congelation, in feet; n=the 



D L . 



number of terms= > t/=the firft decrements — ; and 



100 n 



R=:thc rank of any given term, whofe decrement is required. 

 Tlien the decrement at any given term is = R(/; and, fiib- 

 trafting this from the heat at the fui face, we have the heat 

 at that given height. Tlie temperature at the upper term 

 of congelation may be inveftigatcd in the fame manner, or 

 that of any other height in the atmofphere, except over 

 mountains; for the air over mountains is generally warmer 

 than air of the fame height over the fea or over plains. 



Sometimes the temperature of the upper air is higher 

 than that of the lower, particularly when a large mafs of 

 ■vapour is condenfed by eleftrical agency; for no part ot the 

 heat given out by that caufe being loll by communication 

 with air much colder, that which furrounds the condenfed 

 vapour mull be heated to a confiderahle degree. Air, 

 rendered opake by clouds, tranfmits Icfs, and ablorbs 

 more light, and is therefore more heated than clear air. 

 Sometimes winds, in oppolite direftions and different tem- 

 peratures, flow at different heights, the appermoll being 

 often the warmefl:; all which circumllances, efpecially in 

 cloudy weather, render all calculations of the height of the 

 terms of congelation on any particular day precarious, 

 though when iliey regard a particular month or feafon, they 

 may be fufficiently exaft. 



With regard to the effeft of elevation on the temperature 

 of the atmofphere, we may obferve, that as heat is propa- 

 gated througli the atmofphere, chiefly by contact and com- 

 munication with the earth, lofty mountains of limited furlace 

 cannot warm it to any confiderable degree, as they receive 

 the fun's rays more obliquely, and communicating lefs with 

 the common mafs of the earth, are lefs heated than plains. 

 Hence it happens that the fteepefl mountains are always 

 the coldeft. Indeed, the coldnefs of the atmofphere on the 

 tops of mountains has been afcribed, by M. Lambert and 

 M. De Luc, to the greater rarity of the igneous fluid, or ele- 

 mentary fire, in fuch elevated lituations, than on the plains. 

 M. Lambert is of opinion, that it is rarefied above by the 

 aflion of the air, and that it is condenfed below by its own 

 weight. Without abfolutely deciding the quellion, he 

 feems inchned to admit the identity of fire and light. 

 M. De Luc compares elementary fire to a continuous fluid, 

 whofe parts are condenfed by being mutually compreired: 

 and though he denies that fife and light are the fame, 

 yet he fuppofes that light puts into motion the igneous 

 fluid contained in bodies, and that it atls with greater 

 force near the eartli than at a diltance from its furface, 

 by means of this fluid, which he calls an heavy and elaflic 

 one, by being more condenfed there than at a greater height. 

 M. Bouguer has demonflratei^, by fimplc and obvious prin- 

 ciples and fafts, that in order to account for the diminu- 

 tion of heat on mountains, it is unneceffary to recur 

 to dubious hypothefis. In his account of what was ex- 

 perienced on the mountains of Peru, he fays, " it was 

 proper, in order to explain tliis fubjcft, to infill on the fhort 

 duration of the fun's rays, which cannot ftrike the different 

 fides of mountains but for a few hours, and even this not 

 always. A horizontal plain, when the fun is clear, is ex- 

 pofed at mid-day to the perpendicular and undiminiihed 

 adlion of thefe rays, while they f^U but obliquely on a plain 

 not much inclined, or on the fides of a high pile of tleep 

 rocks. But let us conceive for a moment an infulated 

 point, half the height of the atmofphere, at a dillance from 

 all mountains as well as from the clouds which float in tlie 



ATM 



air. The more a medium is tranfparent, the lefs heat it 

 ought to receive by the immediate aftion of the fun. The 

 free paffage which a very tranfjiarcnt body allows to the 

 rays of light, fliews that its fmall particles are hardly 

 touched by them. . Indeed what imprefTion could they 

 make on it, when they pafs through almoft without ob- 

 flruclion? Light, when it confitls of parallel rays, does 

 not, by pafPnig through a foot of free atmofpheric air, near 

 the earth, lol'e an hundred thoufandth part of its force. 

 From this we may judge how few rays are weakened, or 

 can aft on this fluid, in their palfage through a flratum of 

 the diameter, not of an inch or line, but of a particle. 

 Yet the fubtilty and tranfparency are ftill greater at great 

 heights, as was obvious on the Cordilleras, when we looked 

 at diftant objects. Laflly, the grofler air is heated below 

 by the contaft or neighbourhood of bodies of greater den- 

 fity than itfelf, which it furronnds, and on which it refls; 

 and the heat may be communicated by little and httle to a 

 certain dillance. The inferior parts of the atmofphere by 

 this means contrail daily a verj' confiderable degree of heat, 

 and may receive it in proportion to its denfity or bulk. 

 But it is evident that the fame thing cannot happen at the 

 dillance of a league and an half or two leagues above the 

 furface of the earth, although the light there may be fome- 

 thing more aftive. The air and the wind therefore mull 

 at this height be extremely cold, and irt>lder in proportion to 

 the elevation." 



This theory is adopted by Sauffure, who has fuperadded 

 the following faft to prove, that the force of the fun's rays, 

 confidered abllrafledly and independently of any extrinfic 

 fource of cold, is no lefs powerful on mountains than on 

 plains; viz. that the power of burning lenfes and mirrors is 

 the fame at all heights. For afcertaining this faft, he pro- 

 c\ired a burning glafs, fo weak in its effcft, that at Geneva 

 it would juft let fire to tinder. This glafs was carried to 

 the fummit of mount Saleve, 3000 feet high, and it there 

 produced the. fame effeft, and even with greater eafe. 

 Hence he concluded, that the principal fource of cold on 

 the tops of moinitains is their being perpetually furrounded 

 by an atmofphere, Vvhich cannot be much heated by the 

 rays of the fun, on account of its tranfparency, or by their 

 refleftion from the earth, by reafon of its dillance; but he 

 wifhed alfo to know, whether the direft folar rays had the fame 

 power on the top of a high mountain as on the plain below, 

 whilll the body on which they acted was placed in fuch a 

 manner as to he unaftedled by the fuiTOunding air. With 

 this view he inilituted a fct of experiments, from which he 

 deduced the following conclufions ; viz. that a difference of 

 777 toifes in height diminiflies the heat which the rays of 

 the fun are able to communicate to a body expofed to the 

 external air, 14° of the thermometer; th;'.t it dlm.inifhes the 

 heat of a body partially expofed, only 6° ; and that it aug- 

 ments by I ° the heat of a third body completely defended 

 from the air. Hence it appears, that the atmofphere coun- 

 teradls the operation of die folar rays in producing heat, 

 by a power vvhich is exerted at all dillances, from the fur- 

 face to the higher regions. From the experiments of M. 

 Piiflet, to this purpofe, it is inferred, that even in places ex- 

 pofed to the rays of the fun, the heat at five feet from the 

 ground is greater only by 1° or 2' than at fifty feet above 

 the furface, although the ground was at that time 15° or 20° 

 warmer than the air immediately in contaft with it. This 

 difference, however, fmall as it is, does not obtain in higher 

 regions; for if it did, the cold on the top of the mountain of 

 Saleve, 3000 feet above the lake of Geneva, would be 60° 

 greater than at the foot of it; whereas it really is only 10°. 

 la tht night the. cafe is reverfed; for the ftratum of air, at 



five 



