86 On the Hygrometer by Evaporation. 



that is, in words, " The actual tension of the vapour in tlie 

 atmosphere is less than the maxinunii tension at the reduced 

 temperature t — It, by a quantity proportional to the observed 

 depression." 



If we were in possession of any different method of finding 

 the tension of the vapour in the atmosphere, we should be able 

 both to correct the coefficient of 8t in the foregoing formula, 

 which number is liable to some uncertainty, and likewise to 

 judge of the degree of accuracy of which Dr. Hutton's ex- 

 periment is susceptible. Now such a method of experiment- 

 ing has been employed by Dalton. It consists in exposing to 

 the air a clean and dry surface of glass cooled artificially till 

 it is observed that moisture begins to be deposited on it. 

 When this takes place, the air in contact with the cold surface 

 is in a state of saturation ; and, the temperature of the dewing 

 poi?tt being thus determined, it is easy to deduce from it the 

 tension of the vapour at the actual temperature of the atmo- 

 sphere. 



Let y denote the weight of vapour contained in a cubic foot 

 of the air at the general temperature x; and //, the weight of 

 vapour in a cubic foot at the reduced temperature t— Stj 

 then, according to what has already been shown, we have 



And hence, 



x= ■—- X (1 + ?ht) 



and, by substituting these values in the formula (2), we get 



7 /, c \ 240 OW AT 



y= h(\ — wSt) X — — ; 



and, by omitting insensible quantities, 



, 2400 w> ^ ,„. 



or, in words, *'The (juanlity of vajiour in a cubic foot of air 

 at the actual temperature t, is less than the maximum of hu- 

 midity which the air is capable of retaining at the reduced 

 temperature t — St, by a quantity proportional to the observed 

 depression." 



It we denote by H tlie maximum of humidity in a cubic 

 foot of the air at the temperature t; then 



H, •J'lO aw -x , , V 



_/, -|. _ X67, (4) 



ys\\\ 



