On the Hygrometer. 333 



ter = 1, will have its temperature raised .J-l ?^ degrees by 



the heat required to convert into vapour a weight of water 



f^—f^ ^ ^^ supposition, 5^ = ^('^f'^fed . cons6- 

 48 > ^ t'f '30 48 



quently, when^. is the only unknown quantity, it may be found 

 by the equation 



•^^""30 X -7- =-^'" ^^'^ 



As Sir James Ivory considered the above formula chiefly in 

 its application under the ordinary pressure, he substituted 1 for 



B 



— , and the specific heat of air under a pressure of 30 inches for 



a'. But supposing that B differed considerably from 30, it 

 would be necessary to retain it in its original form ; or, instead 

 of a', to substitute a = the specific heat under a pressure of 30 

 inches, and to multiply it by a coefficient varying with B. 



This being a subject upon which there is considerable di- 

 versity of opinion, the author is anxious to state the grounds on 

 which he proceeds in drawing any conclusion. With this view, 

 he takes the liberty of making the following quotation from a 

 paper by La Place *. " On peut deduire des rapports prece- 

 dens divers theoremes sur les gaz: tel est le suivant, qui s'accorde 

 avec les experiences faites sur cet objet, autant qu''on doit Tat- 

 tendre d'experiences aussi delicates. La quantite de chaleur 

 degagee par un volume de gaz, en passant, sous une pression 

 determinee, d'une temperature a une autre inferieure, est pro- 

 portionelle a la racine carree de cette pression.'' According, 

 then, to the theorem here enunciated, if c be the specific heat 

 of air under a constant volume, when the pressure is 30 inches, 

 and (f the specific heat when the pressure is B, it will follow 



that c Y ( oA ) = ^* ^"*> denoting the specific gravity of air 



under the pressure of 30 inches by s, and that under the pres- 



sure B by 5 X — = / ; and having reference to the values of a 



* Annales de Chimie et de Physique, torn, xviii. 185. 



VOL. XVII. NO. XXXIV. OCTOBER 1834. Z 



