from the Indications of the Wet-bulb Hygrometer. 185 



Berard, (whose views, if not rigorously exact, are sufficiently 

 so for my present purpose,) for small variations of pressure, 

 such as occur to the natural atmosphere, the differences of 

 specific heats under a constant volume are proportional to 

 the differences of pressure. And the same philosophers 

 have shown that for pressures in the ratio of 1 to 1*3583, 

 the corresponding capacities are 1 and 1*2396. Hence, as 



•3583 : '2396: : ~- — 1 : 1, c being the specific heat 



under a constant volume at 30, and x that at /; ; a propor- 

 tion from which we deduce x = ('0223 p 4- '33 12) c. 



But the specific heats under a constant volume, divided by 

 the densities, give the specific heats of equal weights. And 

 as the densities vary as the pressures direclly, and as the tem- 

 peratures + 44?8 inversely, and are therefore to each other 



in the present case as — -r to 77—— r/» we shall have 



^ 508 4j4'8 -f t 



^x c: (-0223 i> + -3312) ex i^^±^ : : -267 : ^ ; 



so that x'^ or the specific heat of air at temperature /' and 



4,40 I /^ so 

 pressure 'p = -~~^ x '-— x ('0223 /? + -3312) x '267. 



The value, therefore, of y^' already given, when corrected 

 for the influence of pressure on specific heat, will become 



f— -—x ^ X {'022S p -h '3312); an expression which 



is obviously, as it ought, reduced to f— — when t' = 60°, 



and p = 30. 



The theoretical justness of the fourth objection must also 

 be conceded. The medium which is in contact with the bulb 

 of the hygrometer is not dry air, but air charged with the 

 amount of vapour which belongs to the existing dew-point ; 

 and, as the specific heats of air and vapour are different, this 

 mixed atmosphere in cooling through t—t' degrees will evi- 

 dently not give out the same quantity of caloric, and can 

 therefore not convert into vapour the same quantity of water 

 that would be evolved and vaporized by the same weight of 

 dry air alone. In fact, for -267 the specific heat of air, we 

 should in strictness substitute the specific heat of the mixture 

 of air and vapour; or, what will answer the same purpose, 



Third Series. Vol. 6. No. 33. March 1835. 2 B 



