[ 2 82 ] 



any degree of heat, and under any preffure ; and this he attri- 

 butes tp the action of air upon it. And fince the removal of 

 the greater part of tlic preffure promotes evaporation, the dimi- 

 nution of a fmall part of the preifure muft promote it in a" 

 degree proportionably fmaller, let the temperature be vvhat it 

 may. Hence vapours arife more copioufly and readiiy in rare- 

 fied air than in air of greater denfity, if the temperature of both 

 forts of air be equal. 



The confequence deduced from thefe experiments is farther 

 flrengthencd by the obfervations of Sauffure on evaporation on the 

 Colde Geant compared with that which took place at the fame time 

 at Geneva. On the mountain the barometer flood at 19,88 Englifli 

 inches, and at Geneva at 29,04. The thermometer on the mountain 

 was at 45^,3 of Fahr. at a mean, and at Geneva at ^o';, yet on the 

 mountain 39,5 grains w^ere evaporated, while at Geneva only 14,5 

 grains difappeared. It is true that the air at Geneva wanted only 2,38 

 grains to faturate it, while that on the mountain wanted 4,03 ; 

 but fo fmall a difference cannot account for fo confiderable a 

 difparity in the effed. See the ingenious calculations and the 

 feries of CGnfcquences deduced by this excellent philofopher, 

 Voyages aux AJpes, vol. vii. Svoi § 2059 and 2060. His genera! 

 conclufion is, that a diminution of the denfiiy of the atmof- 

 phere amounting to one-third, every thing elfe being equal, doubles 

 the quantity of evaporation, § 2062. 



Hence 



