pands, and when it expands it becomes lighter. This lighter
air rises, like wood plunged into water, through the heavier air
overhead.

When the sunbeams fall upon the sea the water is warmed, though not so
much as the land. The warmed water expands, becomes thereby lighter,
and therefore continues to float upon the top. This upper layer of
water warms to some extent the air in contact with it, but it also
sends up a quantity of aqueous vapor, which being far lighter than
air, helps the latter to rise. Thus both from the land and from the
sea we have ascending currents established by the action of the sun.

When they reach a certain elevation in the atmosphere, these currents
divide and flow, part towards the north and part towards the south;
while from the north and the south a flow of heavier and colder air
sets in to supply the place of the ascending warm air.

Incessant circulation is thus established in the atmosphere. The
equatorial air and vapor flow above towards the north and south poles,
while the polar air flows below towards the equator. The two currents
of air thus established are called the upper and the lower trade
winds.

But before the air returns from the poles great changes have occurred.
For the air as it quitted the equatorial regions was laden with
aqueous vapor, which could not subsist in the cold polar regions. It
is there precipitated, falling sometimes as rain, or more commonly as
snow. The land near the pole is covered with this snow, which gives
birth to vast glaciers.

It is necessary that you should have a perfectly clear view of this
process, for great mistakes have been made regarding the manner in
which glaciers are related to the heat of the sun.

It was supposed that if the sun's heat were diminished, greater
glaciers than those now existing would be produced. But the lessening
of the sun's heat would infallibly diminish the quantity of aqueous
vapor, and thus cut off the glaciers at their source. A brief
illustration will complete your knowledge here.

In the process of ordinary distillation, the liquid to be distilled is
heated and converted into vapor in one vessel, and chilled and
reconverted into liquid in another. What has just been stated renders
it plain that the earth and its atmosphere constitute a vast
distilling apparatus in which the equatorial ocean plays the part of
the boiler, and the chill regions of the poles the part of the
condenser. In this process of dist