An instrument for determining the moisture in the air. One form consists
of a pair of thermometers, one of which has its bulb wrapped in cloth
which is kept moist during the observation. The evaporation is more or
less rapid according to the dryness or moisture of the air, and as the
temperature varies with this evaporation the relative readings of the
two thermometers give the basis for calculating the hygrometric state of
the air. Another form determines the temperature at which dew is
deposited on a silver surface, whence the calculations are made.