100 Mr. Herapath on the Quantity/ of Vapour [Ave. 



T-f- (4) 



is the elasticity of the vapour in the atmosphere, and of course 



P l» +m ~ p + m P ^^ 



is the forc« of the dry air. Dividing the preceding hyp, gives 



p+m 

 for the absolute volume of vapour in a unity of the atmosphere. 

 And if instead of hyp we divide by t, the result 



p + m 



expresses the humidity of the air. 



The formulas in this method ought to give the same results as 

 the corresponding of the preceding method. Equating therefore 



-^— with -, we shall find 



p + m p' 



I' — P 

 T p + m 



the left hand member of which is the formula for the humidity 

 of the air as given by the first method, and the right hand mem- 

 ber the formula for the same thing given by the second method. 

 In the same way we may deduce other expressions in either 

 method, by only knowing its expression in the other method, 



and having: given the equation = -. 



* *3 ^ p+m p 



This second method has a considerable advantage over the 

 first, inasmuch as it is free from the uncertainty of the tenta- 

 tive process. It may also be applied to almost any portion of 

 any compound air however small. Another advantage is its 

 not requiring attention to regularity in the caliber of any part of 

 the tube, which may be regular or irregular in any degree. 



By opening the cock D, and allowing the mercury to force 

 out the confined air, and then turning the cock C so as to let 

 off the mercury to any extent, we may draw into C D a fresh 



Eortion of air, and commence the experiment anew. It will, 

 owever, be advisable in this case not to allow the mercury to 

 ascend quite to the top of C D, as it will be extremely difficult 

 to see the incipient deposition wherever the mercury has touched 

 the glass, unless it be cleaned before re-commencing ; which 

 may throw some doubts on the real temperature of the tube, a 

 point of the utmost consequence to the success of the experi- 

 ment. 



If instead of atmospheric air we experiment on a mixture of 

 any gas and vapour at the temperature of the air f, and it be 

 required to find the specific gravity of the gas ; then replacing 

 p in (3) by (4) and taking five-eighths of the result from the 



