only make the interior
hotter than it would otherwise be, but rob it of much of the moisture
which it should receive. The winter storms coming in from the ocean
find the cool mountains lying across their path and quickly part
with a large proportion of their moisture. Where the coast mountains
are low, as is the case with a great part of California and of
Oregon, more of the moisture passes on to the next line of mountains,
the Sierra Nevada-Cascade Range, the western slope of which is
well watered. In the region of the Columbia the Cascade Range is
also low, and the storms, which often follow one another in quick
succession, sweep across the Columbia plateau and over the Rocky
Mountains. Farther south, not only are the storms fewer in number,
but the mountains are very much higher, so that the desert basins
of the lower Colorado and Death Valley region are extremely dry.
One can in imagination stand upon the summit of the Sierra Nevada
mountains, and upon the one hand look down upon barren valleys of
vast extent, broken by mountains almost as barren, where nothing
can be grown except by means of irrigation; and upon the other
side, toward the coast, see a country plentifully visited by rain,
and either covered with forests or given over to farming and
fruit-raising.

The Rocky Mountains form the eastern barrier which the storms encounter.
Their summits are very high and are covered with deep snow during
the winter. East of these mountains lie the Great Plains, where the
precipitation is light until we go far enough toward the Mississippi
Valley to reach the influence of the moist air currents from the
Gulf of Mexico. Many storms originate over the region of the Gulf
of California, particularly in the late summer, and supplement to
some extent the light winter storms of Arizona and New Mexico.

The storms of which we have been speaking are known as cyclones.
This term does not refer to the local storms which occur in the
Mississippi Valley and are frequently so destructive, but to great
disturbances of the air. Sometimes the column of whirling air is
more than a thousand miles in diameter. The air in a cyclone is
circling and at the same time rising, so that the motion is spiral.
If you will study an eddy in a stream of water, you will get an
idea of the nature of the motion, except that in the case of the
water eddy the movement is downward. The motion of the particles
in the dust-whirls which all have seen moving a