HYGROMETER. 
tag inaccurate, and not being of long ser- 
vice. To deduce from hygroinetry real ad- 
vantages, it must be put in a state of ri- 
valry with the thermometer, by presenting 
a series of exact observations, such as may 
be comparable in the different hygrome- 
ters. The celebrated Saussure, to whom 
we are indebted for a very estimable work 
on hygrometry, has attained the accom- 
plishment of tliis object by a process of 
which we shall attempt to give some idea. 
The principal piece in this hygrometer is a 
hair, v£hich Saussure first causes to undergo 
a preparation, the design of which is to di- 
vest it of a kind of oiliness that is natural 
to it, and that secures it to a certain point, 
from the action of humidity. This prepa- 
ration is made at the same time upon a cer- 
tain number of hairs forming a tuft, the 
thickness of which need not exceed that of 
a writing pen, and. contained in a fine cloth 
serving them for a case. The hairs thus en- 
veloped are immersed in a long-necked 
phial full of water, which holds in solution 
nearly a hundredth part of its weight of sul- 
phate of soda, making this water boil nearly 
thirty minutes,; the hairs are then passed 
through two vessels of pure water, while 
they are boiling ; afterwards they are drawn ' 
from their wrapper, and separated; then 
they are suspended to dry in the air ; after 
which there only remains to make choice 
of those which are the cleanest, softest, 
most brilliant, and most transparent. It is 
known that humidity lengthens the hair, 
and that the process- of drying shortens it. 
To render both these effects more percepti- 
ble, Saussure attached one of the two ends 
of the hair to a fixed point, and the other to 
the circumference of a moveable cylinder, 
that carries at one of its extremities" a light 
index or hand. The hair is bound by a 
Counter weight of about three grains, sus- 
pended by a delicate silk, which is rolled in 
a contrary way about the same cylinder. In 
proportion as the hair lengthens or shortens, 
it causes the cylinder to turn in one or the 
other direction, and by a necessary conse- 
quence, the little index turns likewise, the 
motions of which are measured on the cir- 
cumference of a graduated circle, about 
which the index performs its revolution as 
in common clocks. In this manner a very 
small variation in the length of the hair be- 
comes perceptible, by the much more con- 
siderable motion that it occasions in the ex- 
tremity of the index ; and it will be easily 
conceived, that equal degrees of expansion, 
of contracti on in the hair, answer to equal 
VOL. Ill, 
arcs described by the extremity of the in- 
dex. To give to the scale such a basis as 
may establish a relation between all the hy- 
grometers that are constructed upon the 
same principles, Saussure assumes two fixed 
terms, one of which is the extreme of hu- 
midity, and the other that of dryness : he 
determines the first by placing the hygro- 
meter under a glass receiver, the whole in- 
terior surface of which he had completely 
moistened with water ; the air being satu- 
rated by this water, acts by its humidity 
upon the hair to lengthen it. He moisten- 
ed anew the interior of the reeeiver, as of- 
ten as it was necessary; and he knew that 
the term of extreme humidity was attained, 
when, by a longer continuance under the 
receiver, the hair ceased to extend itself. 
To obtain the contrary limit of extreme 
dryness, the same philosopher made use of 
a hot and well-dried receiver, under which 
he included the hygrometer, with a piece of 
iron plate, likewise heated and covered 
with a fixed alkali. This salt, by exercising 
its absorbent faculty upon the remaining 
humidity in the surrounding air, causes the 
hair to contract itself, until it has attained 
the ultimate limit of its contraction. The 
scale of the instrument is divided into a hun- 
dred degrees. The zero indicates the limit 
of extreme dryness, and the number one 
hundred that of extreme humidity. The 
effects of moistqre and of dryness upon the 
hair, are modified by those of heat, which 
act upon it, sometimes in the same sense, 
and sometimes in a contrary one; so that, 
if it be supposed, for example, that the air 
is heated about the hygrometer, on one 
part, this air, whose dissolving faculty with 
regai d to the water will be augmented, will 
take away from the hair a portion of the 
water which it had imbibed, thus tending 
to shorten the hair ; while, on the other 
part, the heat, by penetrating it, wijl tend, / 
though much more feebly, to lengthen it ; 
and hence the total effect will be" found to 
consist of two partial and contrary effects, 
the one hygrometrie, the other pyrometric. 
In observations which require a certain pre- 
cision, it is therefore necessary to consult 
the thermometer at the same time with the 
hygrometer; and on this account, the in- 
ventor has constructed, from observation, 
a table of correction, which will put it in 
the power of philosophers always to ascer- 
tain the degree of humidity of the air, from 
the effect produced by the heat. 
De Luc, who devoted his attention to 
the same object, has followed a different 
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