435 
iy Detaag m’ 
fl =f (1 es (I) 
in which m’ represents the amount of vapour formed by the 
caloric’extricated from a given volume of air, in cooling 
through ¢ — /’ degrees; and m the maximum amount of va- 
pour, which the same volume of air could contain até’. In 
this expression /’ may be considered as known, the corres- 
ponding temperature ¢’ 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 
ager By : 
that 7 Stains of air, in cooling through ¢ — ¢’ degrees, evolve 
sufficient heat to vaporize exactly ¢ — i’ grains of mois- 
ture. For m’, therefore, in the formula just given, ¢— ¢’ 
may be substituted. Again, m may obviously be replaced 
by the maximum amount of moisture capable of being con- 
pale a gS Ae | : 
tained in = grains of air at the temperature ¢’ and pressure p. 
ee ee whites BLL 
But to obtain this, it is only necessary to reduce 5 erains of 
air to cubic inches; to multiply the resulting volume by 
poy ,* in order to get the expansive effect of moisture ; 
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 J, . Reverting now to equa- 
pron lliela ins Siena FSS NAMES SS er Tey LY 
* 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. 
