ure may be determined directly and
accurately by finding the dew-point and applying the concave
(vapor-pressure) curves. This does away with the necessity
for the empirical convex curves and wet-and-dry-bulb
readings. To find the dew-point some form of apparatus,
consisting essentially of a thin glass vessel containing a
thermometer and a volatile liquid, such as ether, may be
used. The vessel is gradually cooled through the evaporation
of the liquid, accelerated by forcing air through a tube
until a haze or dimness, due to condensation from the
surrounding air, first appears upon the brighter outer
surface of the glass. The temperature at which the haze
first appears is the dew-point. Several trials should be
made for this temperature determination, using the average
temperature at which the haze appears and disappears.

To determine the relative humidity of the surrounding air by
means of the dew-point thus determined, find the concave
curve intersecting the top horizontal (100 per cent
relative humidity) line nearest the dew-point temperature.
Follow parallel with this curve till it intersects the
vertical line representing the temperature of the
surrounding air. The horizontal line passing through this
intersection will give the relative humidity.

Example: Temperature of surrounding air is 80; dew-point is
61; relative humidity is 53 per cent.

The dew-point determination is, however, not as convenient
to make as the wet-and-dry-bulb hygrometer readings.
Therefore, the hygrometer (convex) curves are ordinarily
more useful in determining relative humidities.

The Hygrodeik

In Figure 94 will be seen the Hygrodeik. This instrument is used to
determine the amount of moisture in the atmosphere. It is a very
useful instrument, as the readings may be taken direct with accuracy.

To find the relative humidity in the atmosphere, swing the index hand
to the left of the chart, and adjust the sliding pointer to that
degree of the wet-bulb thermometer scale at which the mercury stands.
Then swing the index hand to the right until the sliding pointer
intersects the curved line, which extends downwards to the left from
the degree of the dry-bulb thermometer scale, indicated by the top of
the mercury column in the dry-bulb tube.

At that intersection,