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Young Folks' Library: Wonders of Earth, Sea and Sky

Rain And Snow
(From The Forms of Water.)
By
JOHN TYNDALL.
At the equator, and within certain limits north and south of it, the sun at certain periods of
the year is directly overhead at noon. These limits are called the Tropics of Cancer and of
Capricorn. Upon the belt comprised between these two circles the sun's rays fall with
their mightiest power; for here they shoot directly downwards, and heat both earth and
sea more than when they strike slantingly.
When the vertical sunbeams strike the land they heat it, and the air in contact with the hot
soil becomes heated in turn. But when heated the air expands, 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 [pg 343] 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.
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.
344]
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
It was supposed that if the sun's heat were diminished, greater glaciers than those now
existing would [pg
 
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