,Chap. 8.] Cold in toe higbsr Rgglons of Air. 413 



and other great elevations are ufually covered with, 

 ihow. There is indeed always a certain height of the 

 atmofphere where water will be found at the freezing 

 point, and this has been called by philofophers the 

 line of perpetual fhow. The line, however, varies 

 according to -climate and circumftances. On the peak 

 of Tenerif it commences at the height of about two 

 miles and a half, and in England it is generally found 

 at the height of a mile, or a mile and a half. Some 

 botanifts have afferted, that the variation of climate in 

 afcending mountains, was difcernible from- the vege- 

 tables found upon them, the plants which required a 

 mild temperature being commonly found near the 

 bottoms, and the hardier and more northern vegetables 

 towards the fummit. 



Different opinions have been entertained concerning 

 the cold in elevated filiations. It was for a confider- 

 able time imagined, that it depended altogether on the 

 rarity of the atmofphere in thofe regions, which is very 

 confiderable ; but it has been remarked, on the autho- 

 rity of Count Rumford, that the rarity or denfity of the 

 air appears to have little effect on its conducting power. 

 Some have fuppofcd, that as the air is (b much rarer 

 in the upper regions, lefs fire or caloric is required to 

 keep it in a ftate of fttii'.uty, and confequently that there 

 is a real deficiency of that element. The hypothefls 

 of M. Bouguer *, however, comes recommended by 

 its fimpficity, and by its agreement with mod of the 

 other phenomena of heat, and I mail therefore adopt 

 it with only ibrne fli'ght variations. t 



Without entering into the controverfy concerning 

 the identity of fire and light, it is only neceffary to 

 aflume as a principle the well known circumftance, 



Reafons for the cold on the top of the Andes. 



that 



