the rain water that falls upon it.

In connection with the question of absorption and filtration, it is
interesting to investigate the movements of water in the ground. The
natural tendency of water, in the soil as well as out of it, is to descend
perpendicularly toward the center of the earth. If it meet a flat layer of
gravel lying upon clay, and having a free outlet, it will follow the
course of the gravel,--laterally,--and find the outlet; if it meet water
which is dammed up in the soil, and which has an outlet at a certain
elevation, as at the floor of a drain, it will raise the general level of
the water, and force it out through the drain; if it meet water which has
no outlet, it will raise its level until the soil is filled, or until it
accumulates sufficient pressure, (head,) to force its way through the
adjoining lands, or until it finds an outlet at the surface.

The first two cases named represent the condition which it is desirable to
obtain, by either natural or artificial drainage; the third case is the
only one which makes drainage necessary. It is a fixed rule that water,
descending in the soil, will find the _lowest_ outlet to which there
exists a channel through which it can flow, and that if, after heavy
rains, it rise too near the surface of the ground, the proper remedy is to
tap it at a lower level, and thus remove the water table to the proper
distance from the surface. This subject will be more fully treated in a
future chapter, in considering the question of the depth, and the
intervals, at which drains should be placed.

*Evaporation.*--By evaporation is meant the process by which a liquid
assumes the form of a gas or vapor, or "dries up." Water, exposed to the
air, is constantly undergoing this change. It is changed from the liquid
form, and becomes a vapor in the air. Water in the form of vapor occupies
nearly 2000 times the space that it filled as a liquid. As the vapor at
the time of its formation is of the same temperature with the water, and,
from its highly expanded condition, requires a great _amount_ of heat to
maintain it as vapor, it follows that a given quantity of water contains,
in the vapory form, many times as much heat as in the liquid form. This
heat is taken from surrounding substances,--from the ground and from the
air,--which are thereby made much cooler. For instance, if a shower moisten
the ground, on a hot summer day, the drying up of the water will cool both
the groun