648 REGNAULT’S HYGROMETRICAL RESEARCHES. 
If we substitute in the place of f” the function 9 (¢’), which 
expresses the elastic force of the aqueous vapour in the state of 
saturation relatively to the temperature, we shall have an equa- 
tion in ¢' which, resolyed by relation to that quantity, will give 
the temperature to which a thermometer will fall whose bulb is 
constantly moistened, when this thermometer is placed in a cur- 
rent of dry air of a temperature ¢. But the function ¢ (#') is too 
complicated for us to be able to resolve the equation in 7’; we 
must reverse it, suppose successively 
PSO SS o ay 2s 
and resolve the equation by relation to ¢. We thus obtain the 
temperatures ¢ and 7?’ of two thermometers, the first dry, the 
second moist, placed in the same current of dry air. The inter- 
mediate numbers may be calculated by a simple proportional 
interpolation. The equation (3), resolved by relation to ¢, gives 
‘f 
PPE i ee ee 
ete PTs DT fax tates T tee 
ey | ! 
or pae4 (610 — 7) f 
0:429 (h —f') 
The following table contains some values of ¢ calculated in 
this manner, supposing 4 = 760 millimetres: 
—~3 | oes | ses || 44 | 15-42 | 11-42 
—4 | 218 | 618 5 | 17-22 | 12:22 
—3 | 370 | 670 6 | 19:08 | 13:08 
—2 | 535 | 7:35 7 | 2099 | 13-99 
—1 | 695 | 7-95 8 | 22-96 | 14-96 
0 | 865 | 865 9 | 24-97 | 15-97 
+1 | 1028 | 9-28 10 | 27-05 | 17-05 
2 | 1195 | 9-95 11 | 29-21 | 18-21 
3 | 13-67 | 10°67 | 
The numbers in this table do not differ much from the results 
observed by M. Gay-Lussac in direct experiments. 
The formula (3.) takes no account of the velocity of the cur- 
rent of air ; according to that formula the difference of tempera- 
ture should be the same whatever be that velocity. This result 
appears @ priori impossible. I have sought to determine, by 
direct experiments, the influence of this velocity, and to ascer- 
tain whether, starting from a certain value of the velocity, the 
differences of temperature of the dry and moist thermometers 
