nd returns
to the point of beginning, forming a double circle, or rather two
ellipses, the longer sides of which are parallel with the line of the
air current. Watching more closely, aiding the sight by the particles
which float at various distances below the surface, we note the fact
that the motion which was at first imparted to the surface gradually
extends downward until it affects the water to the depth of some
inches.

In the trade-wind belt the ocean waters to the depth of some hundreds
of feet acquire a continuous movement in the direction in which they
are impelled by those winds. This motion is most rapid at the surface
and near the tropics. It diminishes downwardly in the water, and also
toward the polar sides of the trade-wind districts. Thus the trades
produce in the sea two broad, slow-moving, deep currents, flowing in
the northern hemisphere toward the southwest, and in the southern
hemisphere toward the northwest. Coming down upon each other
obliquely, these broad streams meet about the middle of the tropical
belt. Here, as before noted, the air of the trade winds leaves the
surface and rises upward. The waters being retained on their level,
form a current which moves toward the west. If the earth within the
tropics were covered by a universal sea, the result of this movement
would be the institution of a current which, flowing under the
equator, would girdle the sphere.

With a girdling equatorial current, because of the intense heat of the
tropics and the extreme cold of the parallels beyond the fortieth
degree of latitude, the earth would be essentially uninhabitable to
man, and hardly so to any forms of life. Its surface would be visited
by fierce winds induced by the very great differences of temperature
which would then prevail. Owing, however, to the barriers which the
continents interpose to the motions of these windward-setting tropical
currents, all the water which they bear, when it strikes the opposing
shores, is diverted to the right and left, as was the stream in the
experiment with the basin and the breath, the divided currents seeking
ways toward high latitudes, conveying their store of heat to the
circumpolar lands. So effective is this transfer of temperature that a
very large part of the heat which enters the waters in the tropical
region is taken out of that division of the earth's surface and
distributed over the realms of sea and land which lie beyond the
limits of the vertical s