What is thermometrically called the dew-point is that degree at which
the moisture present in the atmosphere, on being subjected to a
decrease of temperature, begins to be precipitated or condensed. It is
the same as the point of saturation. Daniell calls it "the constituent
temperature of atmospheric vapor." It is our criterion for
ascertaining how much moisture there is in the air, and at what degree
of heat or cold it would be precipitated. When the air is saturated, a
dry bulb and a wet bulb will read alike.

The dew-point has been a puzzle to most persons. Very few treatises
explain it satisfactorily. The definition just given, though explicit,
is not quite enough. For it will be perceived that an ordinary
subtraction of the degrees of temperature on a wet thermometer, which
had cooled down by evaporation, from the actual temperature indicated
by a dry thermometer, will not give us the dew-point.

For example,--if a free or dry thermometer indicates 63°, and the one
with the wet bulb has by evaporation cooled down to 54°, the
difference would be 9°. The dew-point would not be 54°, but that
degree to which the mercury would fall in the free thermometer, for
the atmosphere to become saturated with the quantity of moisture then
actually existing in it. It would be 46.8°.

This dew-point, which figures so largely in all well-kept
meteorological reports, is the key to many important conditions of the
atmosphere, affecting health, vegetation, and climate.

It is found that the air at different degrees of heat has different
degrees of elasticity, different degrees of tension, and different