orbed by an acre of land
in a dry season, by considering that the clay loam, in the above
statement, absorbed in 12 hours a fortieth part of its own weight.

OF DEW.

Dew is one of the most ordinary forms in which moisture is deposited in
and upon the soil, in its natural conditions. The absorbent power of
artificially-dried soils, as has been seen, seems to depend much upon
their chemical constitution; and that topic has been considered, without
special reference to the comparative temperature of the soil and
atmosphere. The soil, as we have seen, absorbs moisture from the air,
when both are of the same temperature, the amount absorbed depending
also upon the physical condition of the soil, and upon the comparative
moisture of the soil and atmosphere.

The deposition of dew results from a different law. All bodies throw
off, at all times, heat, by radiation, as it is termed. In the day-time,
the sun's rays warm the earth, and the air is heated by it, and that
nearest the surface is heated most. Evaporation is constantly going on
from the earth and water, and loads the air with vapor, and the warmer
the air, the more vapor it will hold.

When the sun goes down, the earth still continues to throw off heat by
radiation, and soon becomes cooler than the air, unless the same amount
of heat be returned, by radiation from other surfaces. Becoming cooler
than the air, the soil or plants cool the air which comes in contact
with them; and thus cooled to a certain point, the air cannot hold all
the vapor which it absorbed while warmer, and part of it is deposited
upon the soil, plant, or other cool surface. This is dew; and the
temperature at which the air is saturated with vapor, is called the
dew-point. If saturated at a given temperature with vapor, the air, when
cooled below this point, must part with a portion of the vapor, in some
way; in the form of rain or mist, if in the air; in the form of dew, if
on the surface of the earth.

If, however, other surfaces, at night, radiate as much heat back to the
earth as it throws off, the surface of the earth is not thus cooled, and
there is no dew. Clouds radiate heat to the earth, and, therefore, there
is less dew in cloudy than in clear nights. If the temperature of the
earth sinks below the freezing-point, the aqueous vapor is frozen, and
is then called _frost_.

To radiate back a portion of the heat thus thrown off by the soil and
plants, gardeners cover their tender pl