and this rushing in
of the cold air is what we call wind.

It may surprise many people to be told that there are only two great and
never-ceasing courses of the winds of this world--namely, north and
south. They flow perpetually from the equator to the poles, and from
the poles to the equator. All the irregularities and interruptions that
we observe are mere temporary and partial deflections from this grand
course. The heated air at the equator rises continually and flows in an
upper current towards the pole, getting gradually cooled on its way
north. That from the pole flows in an under current towards the
equator, getting gradually heated on its way south. We speak only of
the Northern Hemisphere, for the sake of simplifying explanation,--the
action of the great wind-current in the Southern Hemisphere is precisely
similar.

But our broad simple statement about the upper current from the equator,
and the under current from the pole, requires a slight modification,
which we thought it best not to mingle with the statement itself. The
heated air from the equator does indeed _commence_ to flow in an upper
current, and the cooled air from the pole in an under current; but, as
the upper currents of air are speedily cooled by exposure to space, and
the under currents are heated by contact with the earth's surface, they
constantly change places--the lower current becoming the upper, and
_vice versa_. But they do not change _direction_. The Equatorial
Current ascends, rushes north to a point about latitude 30 degrees,
where, being sufficiently cooled, it swoops down, and continues its
Northward rush along the earth. At another point the Polar Current
quits the earth, and soaring up, in consequence of its recently acquired
heat, becomes the upper current. This change in the two currents takes
place twice in their course.

Of course, the effect of these changes is to produce north winds in one
latitude and south winds in another, according to the particular wind
(equatorial or polar) that happens to be in contact with the earth. At
the points where these two currents cross, in changing places, we
necessarily have calms, or conflicting and variable winds.

Here, then, we have the first of the constant disturbing causes, and of
apparent irregularities, in the winds. The Earth, as every one knows,
whirls rapidly on its axis from west to east. At the equator the whirl
is so rapid that the atmosphere does not at onc