REGNAULT’S HYGROMETRICAL RESEARCHES, 647 
it to be equal to 0°6235, according to the experiments of M. 
Gay-Lussac ; 
4. The latent heat 4 of the aqueous vapour between the tem- 
peratures ¢ and ¢’. M. August at first admitted Southern’s law 
for this datum, and he supposed 4 = 550; subsequently he 
adopted Watt’s law, that is to say, he supposed that this quan- 
tity is represented by 640—/’. 
Substituting these numbers and neglecting some very small 
quantities, M. August obtains the numerical formula* 
_ y  0:558 (¢— #/) 
xv i i — aT = ae AV(P ae ae lake (A.) 
We shall modify some of the preceding numerical data. Let 
us suppose the density $ of the aqueous vapour equal to 0°622, 
that is to say, equal to the theoretical density, and the latent 
heat of the steam represented by 610-7’, on substituting these 
numbers in the formula (2.), and supposing ¢ = & = 0°2669, we 
shall have 
or, neglecting the minute quantities, 
cornea ALON tenth) 
@ == ips 5 BIO L. Allie Sogn, eh, 8 (B.) 
M. August has sought to verify the accuracy of his formula 
by comparative experiments which he has made with the psy- 
chrometer and Daniell’s hygrometer. He cites similar experi- 
ments made by other persons, and he finds in all the cases a 
sufficient agreement between the elastic force of the vapour de- 
ducted from the observation of the temperature of the dew-point 
and that which he determines by means of the formula (A.) from 
observation of the psychrometer. 
M. August finds also a complete verification of his formula 
in the experiments formerly made by M. Gay-Lussac on the 
cold produced by the evaporation of water on the surface of the 
bulb of a thermometer placed in a current of dry airt. 
To obtain the formula which applies to this last case, we must 
Suppose w = O in the equation (1.); the latter then becomes 
sf a Se ee pee ena 
* Ueber die Fortschritte der Hygrometrie; August, p. 30. 
t Ann. de Chimie et de Phys. 2nd series, vol, xxi. p. 82. 
