eated than any other parts of
the earth, with the exception of certain circumscribed portions of the
land. The result is that this heated equatorial zone is constantly
sending up warm air caused by the inrush of colder air, which is heavier
than the air at the equator, expanded by the heat. The warm air at the
equator is forced up into the higher regions of the atmosphere, and here
it overflows each way, north and south, causing a current of air in the
upper regions counter to that of the lower. As it travels north and
south it gradually drops as it becomes cooler, and finally at some point
north and south its course is changed and it flows in again toward the
equator. As a matter of fact, the trade winds do not flow apparently
from the north and south directly toward the equator, but in an oblique
direction. On the north side of the equator we have a northeasterly
wind, and a southeasterly wind on the south side. This is caused by the
rotation of the earth from west to east. The direction of the trade
wind, however, is more apparent than real.

The earth in its diurnal revolutions travels at the rate of a little
more than 1000 miles an hour at the equator. But if we should travel
northward to within four miles, say, of the north pole, the surface
point would be moving at the rate of only about a mile an hour. At some
point equidistant between the north pole and the equator the surface of
the earth will be moving at a rate, say, of 500 miles an hour. If we
could fire a projectile from this point that would have a carrying power
to take it to the equator some time after the projectile was fired,
although it would fly in a perfectly direct line, it would appear to
anyone at the equator who observed its approach to be moving from a
northeasterly direction. The reason is that the earth is traveling twice
as fast at the equator as it is at the point whence the projectile is
fired. Therefore it will overshoot, so to speak, at the equator, and not
be dragged around by the increased motion we find there.

To make this still plainer, suppose the earth to be standing still and a
projectile be fired directly across from the north pole in the direction
of the lines of longitude and required one hour to reach the equator,
the projectile would appear to anyone standing at the equator to come
directly from the north. If, however, the earth is revolving at the rate
of 1000 miles an hour at the equator to the eastward, and the
projecti