t.

Let us divide, then, the possible sources of water for domestic
consumption into two groups, those found under the surface of the soil
and those found on or above the surface. In the first group will come
wells and springs, and in the second group will come brooks, streams,
and lakes.

_Underground waters._

Springs result from a bursting out of underground waters from the
confined space in which they have been stored or through which they have
been running. Thus in Fig. 24 is seen how water falling on the pervious
area _a-b_ is received into the soil and gradually finds its way
downward between impervious strata which may be clay or dense rock. At
the point _B_, where the cover layer has, for any reason, been weakened,
the pressure of the water forces its way upward and a spring is
developed at the point _C_. Or, conditions may be as shown in Fig. 25,
where the confined water, instead of being forced upward by pressure,
flows slowly out from the side of a hill, making a spring at the point
_D_, while the water enters the pervious stratum at the point _a-b_ as
before.

[Illustration: FIG. 24.--Diagram of a spring.]

[Illustration: FIG. 25.--Water finding its way from a hillside.]

[Illustration: FIG. 26.--The sinking of wells.]

If the water is held in the ground as in the first case, it is possible
to develop the spring artificially; that is, to drill through or bore
through the overlying impervious strata so as to allow the escape of the
water. When this happens, the water bursts forth exactly as in a natural
spring except that under some conditions the pressure may be sufficient
to force the water rising in a pipe instead of through the ground to
flow above the surface of the ground as a fountain or jet, making what
is known as an "artesian well." A true well, on the other hand, may be
put down in the ground and through strata where springs could never
develop; that is, where no pressure exists in such a way as to bring the
water to the surface, as in Fig. 26. The well here is sunk until it
reaches the water, and it is safe to say that one can always reach a
layer of water in the ground by a well if the well is deep enough.

The flow of underground water is, however, always very uncertain and
confusing, and even in localities where water would naturally be
expected in quantity, as, for instance, in the bottom of a valley filled
with glacial drift, much disappointment is often experienced because the
exp