rth
through the heat which it acquired from the sun is returned to the air
through which it falls or to the ocean surface on which it strikes. In
this case the circuit of the rain is short and without geological
consequence which it is worth while to consider, except to note that
the heat thus returned is likely to be delivered in another realm than
that in which the falling water acquired the store, thus in a small
way modifying the climate. When, however, the precipitation occurs on
the surface of the land, the drops of frozen or fluid water apply a
part of their energy in important geological work, the like of which
is not done where they return at once to the sea.

[Illustration: Fig. 10.--Showing the diverse action of rain on wooded
and cleared fields, _a_, wooded area; _b_, tilled ground.]

We shall first consider what takes place when the water in the form of
drops of rain comes to the surface of the land. Descending as they do
with a considerable speed, these raindrops apply a certain amount of
energy to the surface on which they fall. Although the beat of a
raindrop is proverbially light, the stroke is not ineffective.
Observing what happens where the action takes place on the surface of
bare rock, we may notice that the grains of sand or small pebbles
which generally abound on such surfaces, if they be not too steeply
inclined, dance about under the blows which they receive. If we could
cover hard plate glass, a much firmer material than ordinary stone,
with such bits, we should soon find that its surface would become
scratched all over by the friction. Moreover, the raindrops
perceptibly urge the small detached bits of stone down the slopes
toward the streams.

If all the earth's surface were bare rocks, the blow of the raindrops
would deserve to be reckoned among the important influences which lead
to the wearing of land. As it is, when a country is in a state of
Nature, only a small part of its surface is exposed to this kind of
wearing. Where there is rain enough to effect any damage, there is
sure to be sufficient vegetation to interpose a living and
self-renewed covering between the rocks and the rain. Even the lichens
which coat what at first sight often seems to be bare rock afford an
ample covering for this purpose. It is only where man bares the field
by stripping away and overturning this protecting vegetation that the
raindrops cut away the earth. The effect of their action can often be
noted b