walls, being cooler than the atmosphere, cause its moisture to be
deposited in the manner described. By leaving a space, however, between
the walls and the plaster, this moisture is prevented from being
troublesome.

[How does this principle affect the soil?

Explain the experiment with the two boxes of soil.]

Nearly every night in the summer season, the cold earth receives
moisture from the atmosphere in the form of dew.

A cabbage, which at night is very cold, condenses water to the amount of
a gill or more.

The same operation takes place in the soil. When the air is allowed to
circulate among its lower and _cooler_ particles, they receive moisture
from the same process of condensation. Therefore, when, by the aid of
under-drains, the lower soil becomes sufficiently open to admit of a
circulation of air, the deposit of atmospheric moisture will keep the
soil supplied with water at a point easily accessible to the roots of
plants.

If we wish to satisfy ourselves that this is _practically_ correct, we
have only to prepare two boxes of finely pulverized soil, one, five or
six inches deep, and the other fifteen or twenty inches deep, and place
them in the sun at mid-day in summer. The thinner soil will be
completely dried, while the deeper one, though it may have been
perfectly dry at first, will soon accumulate a large amount of water on
those particles which, being lower and more sheltered from the sun's
heat than the particles of the thin soil, are made cooler.

With an open condition of subsoil, then, such as may be secured by
under-draining, we entirely overcome drought.

[How does under-draining supply to the soil an increased
amount of atmospheric fertilizers?

How does it warm the lower parts of the soil?]

2. Under-draining _furnishes an increased supply of atmospheric
fertilizers_, because it secures a change of air in the soil. This
change is produced whenever the soil becomes filled with water, and then
dried; when the air above the earth is in rapid motion, and when the
comparative temperature of the upper and lower soils changes. It causes
new quantities of the ammonia and carbonic acid which it contains to be
presented to the absorbent parts of the soil.

3. Under-draining _warms the lower parts of the soil_, because the
deposit of moisture (1) is necessarily accompanied by an abstraction of
heat from the atmospheric vapor, and because heat is withdrawn from the
whole amount of air circul