REGNAULT’S HYGROMETRICAL RESEARCHES. 627 
notwithstanding the strong prejudices I had against this instru- 
ment, I have not hesitated to make numerous experiments to 
ascertain how far it would furnish precise indications. 
_I shall recall in a few words the directions of Saussure for the 
construction of the hair hygrometer. Fine and soft hairs should 
be selected, cut from a living and healthy head, and not curled. 
To divest them of grease, a small parcel of the size of a quill is 
placed in a small piece of linen, which is sown up, and they are 
boiled in a long-necked flask with 1 litre of water and 10 grms. 
of crystallized carbonate of soda. The boiling is continued for 
thirty minutes ; then the bag which contains the hairs is taken 
out, and they are washed, boiling them twice at intervals, for 
several minutes, in pure water. The cloth is unstitched, and, 
after having taken out the hairs, they are shaken in various di- 
rections in a large vessel filled with cold water, to complete their 
washing and to separate them from one another; lastly, they 
are hung up and left to dry in the air. The hairs well-washed 
in ley are clean, soft, brilliant, transparent, and separated from 
one another. The weight which stretches the hair should not 
exceed O°2 grm. Saussure observes that a hair which is only 
weighted with 0°6 grm. proceeds at first very regularly, but that 
it stretches after some time, and that the instrument becomes 
irregular. 
The length of the hair, in the ordinary portabie hygrometers, 
is 24 centimetres. ihe diameter of the pulley on which the 
hair is wound should be about 5 millimetres. The point of ex- 
treme humidity is ascertained by placing the instrument under a 
bell-glass whose sides are moistened. To obtain the extreme 
point of dryness, Saussure recommends to powder with salt of 
tartar (bitartrate of potass) a plate of sheet-iron bent into a 
cylinder and heated red-hot; the plate of sheet-iron is then 
coated with a layer of carbonate of potass which readily absorbs 
moisture. This cylinder is placed under a very dry glass bell, 
and the hygrometer is suspended in the middle. ‘The interval 
between the point of extreme dryness and that of extreme 
humidity is divided into 100 degrees. 
The present makers follow exactly the directions of Saussure 
for the dimensions of the various parts of the apparatus; but 
they weight the hair much more. Thus the weight often 
amounts to 1°8 grm., which is more than three times the maxi- 
muni weight indicated by Saussure. This circumstance is very 
VOL. IV. PART XVI. 2Y 
