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HYGROMETER.--This is an instrument never to be found, I believe,
in the rooms of our operators, although it would be of much use to them,
for ascertaining the quantity of moisture floating about the room;
and as it is necessary to have the atmosphere as dry as possible to prevent
an undue absorption of this watery vapor by the iodine &c., and to
procure good pictures,--its detection becomes a matter of importance.
Mason's hygrometer, manufactured by Mr. Roach and sold by Mr. Anthony,
205 Broadway, New York is the best in use.

It consists of two thermometre tubes placed, side by side,
on a metalic scale, which is graduated equally to both tubes.
The bulb of one of these tubes communicates, by means of a
net-work of cotton, with a glass reservoir of water attached
to the back of the scale. Fig. 17 and 18 represent a front
and back view of this instrument.

Fig. 17 is the front view, showing the tubes with their respective scales;
the bulb b being covered with the network of cotton communicating with
the reservoir c fig. 18, at d.
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The evaporation of the water from this bulb decreases the temperature
of the mercury in the tube b in proportion to the dryness of the atmosphere,
and the number of degrees the tube b indicates below that of the other,
shows the real state of the atmosphere in the room; for instance,
if b stands at forty and a at sixty-one the room is in a state
of extreme dryness, the difference of twenty-one degrees between
the thermometers--let a stand at any one point--gives this result.