ger active the earth cools down rapidly and soon passes the point
where there is an equilibrium between the land and water. The water
possesses the opposite quality. It is slow to become heated, because of
a much larger mass that is affected, and is equally slow to give up the
heat. And the consequence is that after the sun has set, the land cools
so much faster than the water that we soon have the opposite condition,
and the sea is warmer than the land, which makes the air at that point
lighter, and which in turn causes the denser or colder air from the land
to flow toward the ocean, and displace the lighter air and force it
upward; hence we have a land instead of a sea breeze. So that the
normal condition in summer is that of a breeze from the ocean toward the
land during part of the day and a corresponding breeze from the land to
the ocean during part of the night, with a period of no wind during the
morning and evening of each day.

The forces that work to produce all the varying phenomena of air
currents on different portions of the earth are difficult to explain, as
there are so many local conditions of heat and cold, and these are
modified by the advancing and receding seasons. The unequal distribution
of land and water upon the earth's surface; the readiness with which
some portions absorb and radiate heat as compared with others; the tall
ranges of mountains, many of them snow-capped; the lowlands adjacent to
them that become intensely heated under the sun's rays; the diversity of
coastline and the fact that there is a zone of continually heated earth
and water in the tropical regions--all these conditions, coupled with
the fact that the earth rotates on its axis once in twenty-four hours,
are certainly sufficient to account for all the complicated phenomena of
aerial changes on the various portions of the earth's surface.

The trade winds are so called because they blow in a certain definite
direction during certain seasons of the year, and can be reckoned upon
for the use of commerce. If you trace the line of the equator you will
notice that for more than three-quarters of the distance it passes
through the water. The water, as we have explained in the last chapter,
becomes gradually heated to a considerable depth, and when once
saturated with heat is slow to give it up. It can easily be seen that
there will be a zone extending each way from the equator for a certain
distance that will become more intensely h