o the temperature and to the dryness of the air where
it rests upon the ocean. It probably amounts on the average to
somewhere about three feet per annum; in regions favourably situated
for the process, as on the west coast of northern Africa, it may be
three or four times as much, while in the cold and humid air about the
poles it may be as little as one foot. When contributed to the air,
the water enters on the state of vapour, in which state it tends to
diffuse itself freely through the atmosphere by virtue of the motion
which is developed in particles when in the vaporous or gaseous state.

The greater part of the water evaporated from the seas probably finds
its way as rain at once back into the deep, yet a considerable portion
is borne away horizontally until it encounters the land. The
precipitation of the water from the air is primarily due to the
cooling to which it is subjected as it rises in the atmosphere. Over
the sea the ascent is accomplished by the simple diffusion of the
vapour or by the uprise through the aerial shaft, such as that near
the equator or over the centres of the whirling storms. It is when the
air strikes the slopes of the land that we find it brought into a
condition which most decidedly tends to precipitate its moisture.
Lifted upward, the air as it ascends the slopes is brought into cooler
and more rarefied conditions. Losing temperature and expanding, it
parts with its water for the same reason that it does in the ascending
current in the equatorial belt or in the chimneys of the whirl storms.
A general consequence of this is that wherever moisture-laden winds
from the sea impinge upon a continent they lay down a considerable
part of the water which they contain.

If all the lands were of the same height, the rain would generally
come in largest proportion upon their coastal belt, or those portions
of the shore-line districts over which the sea winds swept. But as
these winds vary in the amount of the watery vapour which they
contain, and as the surface of the land is very irregular, the
rainfall is the most variable feature in the climatal conditions of
our sphere. Near the coasts it ranges from two or three inches in arid
regions--such as the western part of the Sahara and portions of the
coast regions of Chili and Peru--to eight hundred inches about the
head waters of the Brahmapootra River in northern India, where the
high mountains are swept over by the moisture-laden airs from t