434 
“If ¢ and ¢’ be the temperatures shown bya dry and wet 
thermometer, encompassed by atmospherical air, ¢” the dew- 
point, f’ and f” the forces of aqueous vapour at ¢’ and ¢”, 
and p the existing pressure,—I have shown (Trans. R. I. 
Academy, vol. xvii., p. 285,) that 
pl ty Fea sins 
87 30 
“Tn investigating this expression, it is assumed that the 
specific heat of air, and the caloric of elasticity of aqueous 
vapour, are constant, and represented (within the ordinary 
variations of atmospheric temperature and pressure,) the 
former by . 267, the latter by 1115. In subsequently apply- 
ing this expression to the determination of the specific heats 
of the gases, (Trans. R. I. Academy, vol. xviii.) it was ne- 
cessary to give it its most general form, when it was found 
to become 
We 48 a (t—t) _ p—f! 
f'=fi- aa x — 
a being the specific heat of air, and e the latent heat of 
aqueous vapour, both being supposed at the temperature 
represented by ¢’, and under the pressure p. I shall here 
briefly indicate the steps which conduct to this result. 
They are given at length in the Philosophical Magazine, for 
October, 1838. 
“The two following propositions constitute the basis of 
the investigation : 
“ Ist. When, in the case of the wet thermometer, the 
stationary temperature is attained, the caloric which vapo- 
rizes the water, is necessarily equal to that which the sur- 
sounding gas evolyes in descending through ¢ — ¢’ degrees, 
2. e. from the proper temperature of the air to that of the 
moistened bulb. 
**2dly. The air so cooled, by successive contacts with 
the moistened bulb, is saturated with humidity. 
“From these propositions we easily deduce the equation 
