626 REGNAULT’S HYGROMETRICAL RESEARCHES. 
it by aspiration into the drying tubes. A very delicate thermo- 
meter is placed in this same spot, which is observed every five 
minutes from a distance with a telescope. The variations of 
temperature are, in general, greater and more rapid than those 
observed in the experiments made with the cylindrical tube, 
page 620, and it is often found necessary to increase the velo- 
city of the current of the aspirator. 
The chemical method does not give the quantity of humidity 
which exists in the air at a determinate moment, but the mean 
quantity which the air contained during the experiment. This 
method is in other respects perfectly accurate, and it is very 
useful for studying the course of the other hygrometers. I 
shall give some examples when treating of the psychrometer. 
But it is too tedious, and it requires too long a manipulation to 
be often employed in meteorological observations. 
2. Hygrometers formed of Organic Substances or Absorbing 
Hygrometers. 
A great number of organic substances are lengthened very 
perceptibly when the quantity of humidity increases in the air, 
and become shortened when the moisture diminishes. This 
property has been taken advantage of to construct instruments 
which immediately indicate the degree of humidity in the air. 
For this object the most various substances have been proposed ; 
but these instruments were rather hygroscopes than hygrome- 
ters; they were devoid of one essential property, that of bemg 
capable of comparison; they were therefore soon abandoned. 
The hair hygrometer alone escaped the general neglect, which 
is owing to the perseverance of its inventor, who made nume- 
rous experiments to render this instrument capable of compari- 
son, and to give it a graduation by which the fraction of satura- 
tion of the air might be calculated. 
Saussure’s hygrometer was for many years in great favour, 
but objections were afterwards made to it. Among these were 
its extreme fragility, and the disturbances in its graduation oc- 
casioned by time. Some pretended even that the hair entirely 
lost its sensibility in a short time. After the death of Saussure 
artists soon lost the traditions of that able physicist, and often 
neglected his most important directions. Nevertheless the hy- 
grometer of Saussure offers, for meteorological observations, 
such great advantages over the other hygrometric methods, that, 
