REGNAULT’S HYGROMETRICAL RESEARCHES. 629 
1. Are the hygrometers, which are constructed with the same 
kind of hair, and which have been divested of grease in the same 
operation, strictly comparable ? 
2. Are the hygrometers which are constructed with different 
hairs, but divested of grease in one and the same operation, 
comparable ? 
3. Lastly, are the hygrometers which are constructed with 
hairs of different kinds, divested of grease by different opera- 
tions, or by different processes, still comparable ? 
To solve the first of these questions, I observed, under very 
varied circumstances, a great number of hair hygrometers, re- 
cently constructed, which M. Bunten kindly placed at my dis- 
posal for this purpose. All these hygrometers had hairs of the 
same kind, which had been deprived of grease in the same 
operation. 
A first series of observations was made, in the open air, on 
three of these hygrometers, and on a fourth very old hygro- 
meter, constructed by Paul of Geneva, and belonging to the 
Physical Cabinet of the Collége de France. These four instru- 
ments were regulated simultaneously in the same vessels for 
their fixed points. 
Table I. contains their simultaneous indications :-— 
TaBLe I. 
Hygrometers of Bunten. Hygrometer 
of Paul. 
1. 2. 
2y% 2 
