METEOROLOGY. | 207 
the index stands; this will be the point of greatest dryness, or the 0 of the scale. 
It is next placed in a receiver provided with a basin of distilled water, and the 
new position of the index marked : this will indicate the maximum of moisture, 
or the 100th degree of the scale. The interval between the two extremes 
is then to be divided into one hundred equal parts. Other substances 
employed in hygrometers are from the animal kingdom: 1. Catgut, as used 
in the Dutch hygrometers, consisting of a paper house in which is suspended 
a piece of gut. The lower end of the gut carries a disk of paper, on which 
are fixed two figures, one a man with an umbrella, the other a woman 
with a fan; the former comes to the door when the moisture causes the 
string to turn, and the latter when this is untwisted by the dry air. 
2. Quills, as proposed by Chiminello. 3. The skin of the frog, fish bladders, 
gold beaters’ skin, silk, ivory (employed by Delue, who afterwards substi- 
tuted whalebone), &c., &c. The vegetable kingdom has furnished hempen 
threads, thin boards of pine or mahogany, or box wood, paper (especially 
straw paper), grasses, awns of oats, and a species of erodium. From the 
mineral kingdom have been derived solutions of sal-ammoniac, in which a 
sponge is dipped, salt, sulphuric acid, &c., &c. All these, however, are 
inapplicable to the purposes of science, owing to their great variability 
and the readiness with which they lose their hygrometric properties. 
The hygrometer of Daniell is constructed on an entirely different principle 
from the preceding, namely, condensation and rarefaction. This consists 
of a curved exhausted glass tube, ending in two bulbs, the one having a thin 
coating of gold or platinum foil, the other covered with a fine linen rag. 
The first bulb is half filled with sulphuric ether, and contains a small ther- 
mometer, the graduated portion of which passes up into the tube. On 
dropping ether on the other bulb, its rapid evaporation produces a considerable 
degree of cold; the vapor of ether in the tube becomes condensed, thereby 
permitting a new evolution from the ether in the bulb. The formation of 
vapor in this bulb cools it by rendering some of its heat latent, thus causing 
it to become covered with a delicate dew. The dew point can be read off 
on the inclosed thermometer, that is, the temperature at which condensation 
or the deposit of vapor begins, this taking place when the air is saturated 
with moisture. The reason that the dew point can serve to determine the 
amount of watery vapor in the atmosphere is found in the following consi- 
derations: The air is said to be saturated with moisture when the vapor 
contained in it has attained the maximum of tension and density, corre- 
sponding to the existing temperature, which is not always the case, for the 
warmer the air, the greater may be the tension and density of the vapor 
contained in it. When moist air is cooled, it is incapable of containing as 
much vapor as before; a portion then becomes condensed in the form of 
minute vesicles. This condensation will take place sooner with a given 
reduction of temperature, the more vapor is contained in the air, or the 
nearer it is to the maximum of condensation corresponding to the existing 
temperature. Upon the same principle depends a more perfect instrument, 
‘nvented by Dobereiner, and recently improved by Regnault. 
All hygrometers depending on the principle just mentioned, are subject to 
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