we climb a mountain in Mexico, we leave the sultry valley, where the
heat is almost unbearable, and very soon notice a change. For every
three hundred feet of altitude we gain there is a fall of one degree in
the temperature. Before we are half way up the slope we have left behind
the tropical vegetation, and come into a temperate zone, where the
plants are entirely different from those in the lower valley. As we
climb, the vegetation becomes stunted, and the thermometer drops still
lower. At last we come to the region of perpetual snow, where the
climate is like that of the frozen north.

So we see that the air becomes gradually colder as we go north or south
from the Equator, and the same change is met as we rise higher and
higher from the level of the sea.

It is only when air is in motion that we can feel and hear it, and there
are very few moments of the day, and days of the year, when there is not
a breeze. On a still day fanning sets the air in motion, and creates a
miniature breeze, the sound of which we hear in the swishing of the fan.
The great blanket of air that covers the earth is in a state of almost
constant disturbance, because of the lightness of warm air and the
heaviness of cold air. These two different bodies are constantly
changing places. For instance, the heated air at the Equator is
constantly being crowded upward by cold air which settles to the level
of the earth. Cold streams of air flow to the Tropics from north and
south of the Equator, and push upward the air heated by the sun.

This constant inrush of air from north and south forms a double belt of
constant winds. If the earth stood still, no doubt the direction would
be due north and due south for these winds; but the earth rotates
rapidly from west to east upon its axis, carrying with it everything
that is securely fastened to the surface: the trees, the houses, etc.
But the air is not a part of the earth, not even so much as the seas,
the waters of which must stay in their proper basins, and be whirled
around with other fixed objects. The earth whirls so rapidly that the
winds from north and south of the Equator lag behind, and thus take a
constantly diagonal direction. Instead of due south the northern belt of
cold air drifts south-west and the southern belt drifts northwest. These
are called the Trade Winds. Near the Equator they are practically east
winds.

The belt of trade winds is about fifty degrees wide. It swings northward