282 Observations on the Hygrometer. 



That neither of these opinions singly is correct, seems appa- 

 rent from the following considerations : — If, as Professor Leslie 

 supposes, the heat is derived entirely from the air, what reason 

 can be assigned for the temperature of the evaporating surface 

 continuing steadily so much below that of the surrounding me- 

 dium, from which it must necessarily be constantly receiving 

 heat ? surely this can only be caused by its imparting to the dis- 

 solved moisture as much heat as it receives. Again, if the moist 

 surface supplied the whole heat which becomes latent in the va- 

 pour, as maintained by Dr Anderson, then, when the evapora- 

 tion is greatly increased by a current of air, the degree of cold 

 should be proportionally increased, which is contrary to obser- 

 vation and experiment. 'Hence it seems necessarily to follow, 

 that the heat employed in converting the water into vapour is 

 derived both from the air and from the moist surface of the hy- 

 grometer. 



Further, when the pressure is diminished, the capacity of the 

 vapour for heat is augmented, and the increased quantity of ca- 

 loric required for its formation must be derived from the same 

 sources as before, namely, the moist bulb and the contiguous air, 

 and probably in the same relative proportions. But the capa- 

 city of the air being also augmented, it evolves, when its tem- 

 perature is reduced by contact with the wet surface, a quantity 

 of heat corresponding to the increased absorbing power of the 

 vapour, or at least to that part which is exerted on itself; so 

 that, had the vapour been supplied with all its heat by the air, 

 it is possible that the cold might not have varied with the pres- 

 sure. As has been shown, however, a portion of the heat which 

 becomes latent in the vapour is derived from the wet bulb of 

 the hygrometer ; and since, unlike the air and vapour, this un- 

 elastic body has not its capacity augmented by the diminution 

 of pressure, the whole additional efflux of heat from it must be 

 sensible, and be indicated by the descent of the liquid in the 

 stem. Such, perhaps, may be the cause of the variation of D 

 with the pressure. 



According to this theory, when t and t" are constant, ff _f>, 

 ought to be proportional to (i-^)B + n30 ^^ ^^ '^^~, since 

 n appears to be nearly = ,4. 



