le as the
trade winds in the centre of the wide ocean. So steadfast and uniform
are they that it is said that the helm and sails of a ship may be set
near the west coast of South America and be left unchanged for a
voyage which will carry the navigator in their belt across the width
of the Pacific.

Rising up from the earth in the tropical belt, the air attains the
height of several thousand feet; it then begins to curve off toward
the north and south, and at the height of somewhere about three to
five miles above the surface is again moving horizontally toward
either pole; attaining a distance on that journey, it gradually
settles down to the surface of the earth, and ceases to move toward
higher latitudes. If the earth did not revolve upon its axis the
course of these winds along the surface toward the equator, and in the
upper air back toward the poles, would be made in what we may call a
square manner--that is, the particles of air would move toward the
point where they begin to rise upward in due north and south lines,
according as they came from the southern or northern hemisphere, and
the upper currents or counter trades would retrace their paths also
parallel with the meridians or longitude lines. But because the earth
revolves from west to east, the course of the trade winds is oblique
to the equator, those in the northern hemisphere blowing from
northeast to southwest, those in the southern from southeast to
northwest. The way in which the motion of the earth affects the
direction of these currents is not difficult to understand. It is as
follows:

Let us conceive a particle of air situated immediately over the
earth's polar axis. Such an atom would by the rotation of the sphere
accomplish no motion except, indeed, that it might turn round on its
own centre. It would acquire no velocity whatever by virtue of the
earth's movement. Then let us imagine the particle moving toward the
equator with the speed of an ordinary wind. At every step of its
journey toward lower latitudes it would come into regions having a
greater movement than those which it had just left. Owing to its
inertia, it would thus tend continually to lag behind the particles of
matter about it. It would thus fall off to the westward, and, in place
of moving due south, would in the northern hemisphere drift to the
southwest, and in the southern hemisphere toward the northwest. A good
illustration of this action may be obtained from an ordinary