REGNAULT’S HYGROMETRICAL RESEARCHES. 645 
blished his formulze* ; I give them here somewhat in detail, be- 
cause the researches of M. August have not been hitherto pub- 
lished in any French work. 
M. August admits that the moist bulb of the psychrometer is 
always surrounded with a stratum of air, which moreover may 
be of any conceivable thickness, having the same temperature 
as this bulb, and saturated with moisture. This temperature is 
lower than that of the external air; M. August supposes that 
the strata of air which come thus successively in contact with 
the moist bulb take the temperature of this bulb and are satu- 
rated with moisture. These strata, arriving with a temperature 
higher than that of the bulb, impart to it a certain quantity of 
heat; but, on the other hand, they evaporate some water on its 
surface, and consequently take away from the bulb another 
quantity of heat. The stationary temperature of the moist bulb 
is established by the equality between these two quantities of 
heat. According to this, let 
w be the weight of the small layer of air supposed dry at 0° and 
under a pressure of 0™760 ; 
h, the height of the barometer ; 
t, the temperature of the ambient air indicated by the dry ther- 
mometer ; 
i’, the temperature indicated by the moist thermometer ; 
Jf and j', the elastic forces of the aqueous vapour in a state of 
saturation for the temperatures ¢ and 7’; 
z, the elastic force of the aqueous vapour actually existing in 
the air. 
In the stratum of air which surrounds the moist bulb, the 
aqueous vapour exerts an elastic force /’, and the air an elastic 
force h —f'. The weight of this dry air is 
Ir Var 
"Tb all 760 ° 
The aqueous vapour existing in this air is composed of the 
quantity found in it before the contact of the bulb, and which 
has « for its elastic force, and of the quantity formed by evapo- 
ration. 
The first quantity is represented by 
1 wa: 
w 6 a) Daeceial The 7 . 760 3 
* Poggendorft’s Annalen, 2nd series, vol. y. p. 69. 
222 
