supplies from that part of the rainfall which soaks into the
ground, and the ground-water surface rises.

In moist climates the ground-water surface (Fig. 24) lies, as a
rule, within a few feet of the land surface and conforms to it in
a general way, although with slopes of less inclination than those
of the hills and valleys. In dry climates permanent ground water
may be found only at depths of hundreds of feet. Ground water is
held at its height by the fact that its circulation is constantly
impeded by capillarity and friction. If it were as free to drain
away as are surface streams, it would sink soon after a rain to
the level of the deepest valleys of the region.

WELLS AND SPRINGS. Excavations made in permeable rocks below the
ground-water surface fill to its level and are known as wells.
Where valleys cut this surface permanent streams are formed, the
water either oozing forth along ill-defined areas or issuing at
definite points called springs, where it is concentrated by the
structure of the rocks. A level tract where the ground-water
surface coincides with the surface of the ground is a swamp or
marsh.

By studying a spring one may learn much of the ways and work of
ground water. Spring water differs from that of the stream into
which it flows in several respects. If we test the spring with a
thermometer during successive months, we shall find that its
temperature remains much the same the year round. In summer it is
markedly cooler than the stream; in winter it is warmer and
remains unfrozen while the latter perhaps is locked in ice. This
means that its underground path must lie at such a distance from