VII
185
HYGROMETER
only require the average humidity over a lengthened period
(days, weeks, or months), as in meteorological investigations.
An instrument of the first kind was exhibited to the Society.
The hygroscopic substance was a piece of tissue-paper of 1
sq. cm. surface, saturated with calcium chloride, and attached
to one arm of a lever (glass fibre) about 10 cm. long. The
latter was supported on a very thin silver wire stretched
horizontally, so that the whole formed a very delicate torsion
balance. The hygrometer was calibrated by means of a series
of mixtures of sulphuric acid and water by Regnault's method.
In dry air the fibre stood about 45° above the horizontal. In
air of relative humidity 10, 20, . . . 90 per cent it sank
downwards through 18, 31, 40, 47, 55, 62, 72, 86, 112 de-
grees. In saturated water-vapour it naturally stood vertically
downwards. The only thing ascertained as to the effect of
temperature was that it is very small. For equal relative
humidities the pointer stood 1 to 2 degrees lower at 0° than
at 25°. When brought into a room of different humidity, the
instrument attained its position of equilibrium so rapidly that
it could be read off after 10 to 15 minutes. The instrument
has the disadvantage that when the humidity is very great
(85 per cent and upwards), visible drops are formed on the
paper, and if it be carelessly handled these may be wiped or
even shaken off.
In instruments of the second kind the calcium chloride
would be contained in glass vessels of a size adapted to the
interval of time for which the mean humidity is required.
These vessels would be weighed from time to time, or placed
on a self-registering balance.