435 



in which m' represents the amount of vapour formed by the 

 caloric extricated from a given volume of air, in cooling 

 through t — t' degrees ; and m the maximum amount of va- 

 pour, which the same volume of air could contain at t'. In 

 this expression f may be considered as known, the corres- 

 ponding temperature t' being the result of observation. In 

 order, therefore, to render the formula available, it is only 

 necessary to determine in known terms the values of m' 

 and m. 



" If a be the specific heat of air, and e the caloric of elasti- 

 city of aqueous vapour at the temperature t ', it is easy to see 



that - grains of air, in cooling through t — t' degrees, evolve 



a 

 sufficient heat to vaporize exactly t - t' grains of mois- 

 ture. For m', therefore, in the formula just given, t — t' 

 may be substituted. Again, m may obviously be replaced 

 by the maximum amount of moisture capable of being con- 

 tained in - grains of air at the temperature 2' and pressure p. 

 a 



But to obtain this, it is only necessary to reduce - grains of 



air to cubic inches ; to multiply the resulting volume by 



P * i n order to get the expansive effect of moisture ; 

 P —f ' 

 and finally multiply the volume thus obtained by the weight 



of a single cubic inch of aqueous vapour. When this is done, 



we find m - '625 X - X "' . Reverting now to equa- 

 a P J 



* In the investigation given in the Philosophical Magazine, this step is omitted. 

 The omission, however, does not sensibly affect the accuracy of the resulting 



formula. 



