436 
tion (1), and writing in it the values just found for m’ 
and m, we arrive at the final equation 
48 — —f’ 
fap SS) yet (1) 
in which the force of vapour at the dew-point is expressed 
in terms of the force of vapour at ¢’, and of the difference 
of the temperatures of the wet and dry thermometers. 
‘** This formula is applicable for all vaiues of ¢’ above 32° ; 
but when the stationary temperature of the wet thermometer 
is lower than the freezing point, it will require modification. 
e , , P ; 
— grains of air, we have seen, in cooling through ¢ — ?’ 
a® ’ g § 
degrees, convert into vapour ¢ — ¢’ grains of moisture. But 
if ¢’ be less than 32°, a greater amount of air will be neces- 
sary for accomplishing this, inasmuch as the heat evolved 
has first to liquify ice, and then to convert the water jinto 
vapour. The additional quantity is obviously represented 
; 135 : F ; Te as 
by the fraction lg? 135 being the caloric of liquidity 
of water, and 1179 — ?¢’ the latent heat of aqueous vapour at 
i’. But this fraction, if we substitute 32° for ¢’, (which may 
be always done without sensible error) is equal to 0°118. 
Hence for values of ¢’ below 32°, < + 0-118 < res 1-118 = 
is, in grains, the weight of air which, in cooling through 
t — t’ degrees, vaporizes ¢—?’ grains of moisture. When 
this correction is applied, the final equation, applicable to 
observations in which the wet thermometer indicates lower 
temperatures than 32°, becomes 
f= a 43 a (¢ — t’) xP se i 
e 30 
** Assuming, as before, the specific heat of air, a, to be 
(IID 
-267, the value assigned to it by Delaroche and Berard, 
and taking for e the value it would have at 50°, upon the hy- 
pothesis that 967° is the latent heat of vapour at 212°, and 
