Daily globe. [volume] (St. Paul, Minn.) 1878-1884, 1881-06-05
Air and ocean Currents.

The sun, as Is well known, Is never
exactly overhead in Ireland, nor in any
northern latitude. Neither is it in
southern ATTITUDES. But 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 the tropics of Cancer
and Of Capricorn. Upon the belt COM
prised between these two circles the
suns rays fall with their mightiest pow
er !; for here they shoot directly down.
ward, 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 arises, 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 warns
to some extent the air in contact with it,
but it also sends up a quantity of aque-
GUS vapor, which, being far lighter than
the 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 atmoshere these currents divide
and flow, part toward the north and part
toward the south; while from the north
and the south flow of heavier and cold
er air sets in to supply the place Of the
ascending warm air.
Incessant circulation is thus estab-
lished in the atmosphere.
The equatorial air and vapor flow
above toward the North and South poles,
while the polar air flows below toward
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 quilted the equatorial re-
gions 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 COW
cred with this snow, which gives birth
to vast glaciers.
It is necessary that we should have a
perfectly view Of this process, for
great mistakes have been made regard
ing the manner in which glaciers are re-
lated to the heat of the sun.
It was supposed that if the suns heat
were diminished, greater glaciers than
those now existing would be produced.
But the lessening Of the suns heat
would infallibly diminish the quantity
Of aqueous Sapor, and thus cut off the
glaciers at their source.
Let us Illustrate this
In the process of ordinary distilla-
ton, 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 con-
denser. In this process OF dlstlllatlon
heat plays quite as necessary a part as
cold, and before Bishop Heber could
speak of Greenland's icy mountains, :"
the equatorial ocean had to be warmed
by the sun.
The heating of the tropical air by the
sun is indirect. The solar beams have
scarcely any power to heat the air through
IoI, fIorr L IL
and ocean, and these communicate their
heat to the air in contact with them.
The air and vapor start upward charged
with the heat thus communicated.
But long before the air and vapor from
the equator reach the poles, precipita-
ton occurs. Wherever humid, warm
wind mixes with cold, dry one, rain
falls. Indeed, the heaviest rains occur
at those places where the sun is vertical
Iy overhead.