I 348 ] 



riment is made at, namely in this cafe, as it is found at baro- 

 meter 28,77 ii^ches ; but fince in reality air at great heights is ge- 

 nerally much colder than below, to afcertain the real proportion 

 of vapour at thofe heights it will be neceffary to find the quan- 

 tity of vapour which a cubic foot of air is capable of holding at 

 that temperature barometer 28,77, and the ratio which the quan- 

 tity or weight of vapour adually found bears to the complement 

 at that temperature. Then, 2do. to find the complement of a 

 cubic foot of air at the temperature which prevails at the given 

 barometrical height, and diminifh it in the fame ratio in which it 

 was found diminifhed below, and finally diminifh it ftill farther 

 in tlie ratio which that barometrical height demands. An exam- 

 ple will fully explain this rule. 



Thus Sauflure found, barometer 28,77 and thermometer Sa*' of 

 Fahr. a cubic foot of air contained about 10 grains of moifture 

 at Geneva. Now the complement of 82° is nearly 15 grains, 

 and the ratio of 10 to 15 is ^. Then at Mount Blanc, on the 

 fame hour, the barometer flood at 16° and the thermometer at 

 26*^,8 ; the complement of a cubic foot of air at this temperature 

 is 5>3 grains, which diminifhed in the ratio of 2 to 3 becomes ^,^, 

 and this, farther dimipithed by the ratio which the barometrical 

 htight of 16 inches demands, namely ,78 = 35 x ,78 = 2,7 

 grains, by obfervation it was found to be 1,7; the difterence is 



only 



r 



