We saw, the other day, how winds were
caused, like currents of the sea, by inequalities
of temperature. The heated air expands near
the equator, rises and runs over towards
either pole in two grand upper currents,
under which there flow from north and south
two deluges of colder air, to occupy the space
vacated. These currents do not flow from
due north and due south, because, as the
earth rolls every day once round itself from
west to east, air that has acquired slow
movement at the poles, finds the globe travelling
too fast for it at the equator, and is obliged,
therefore, to drop more and more behind.

The current from the north becomes a
north-east wind; that from, the south is not
due south, but south-east. These winds are
constant, where there is no local interference,
within about twenty-eight degrees on each
side of the equator, being, in fact, the north-
east and south-east trade-winds. Why do
they not blow all the way from pole to tropic?

There, you have the upper current to consider;
the hot air that ascended at the equator
has been gradually cooling, and becoming
therefore denser— heavier— as it ran over the
cold current below. The cold air from the
pole, too, had been getting warmer, therefore
lighter, on its travel; so that in temperate
climates, to which we belong, it becomes a
disputable point between the two currents
which shall have the upper, which the lower
seat. In these regions, therefore, there is no
uniform wind; but as the currents from the
equator generally succeed in maintaining that
it is now their turn to go below, winds from
the south prevail outside the trade-winds
north of the equator, which are, of course,
represented by north winds on the other
hemisphere. South-west and north-west these
winds are, because they are fast currents,
which started from the earth where it was
rapidly revolving, and vote polar regions slow.
Winds from the south-west then prevail in
Europe; and the south-wester now blowing
whistles with immoderate exultation at a
victory over some polar current with which
it has for the last few days been exchanging
blows.

Well, you say, there must be a pretty
clashing of cymbals when the great trade-
winds from the north and south run against
one another  and they must do that
somewhere near the equator. Yes, the scene of
their collision occupies a broad band about
six degrees north of the equator, more or less.
The trade-winds of the southern hemisphere
encroach over the line at all seasons, owing to
peculiarities of land and water; but the limits
of the trade-winds are not marked out by a
fixed straight line. They vary, in extent, with
the season, and their outline varies with the
nature of the earth or water over which they
blow. But, the scene of collision, as we said,
forms a broad zone always north of the
equator, which is called the zone of the
variable winds and calms. Here it is that
a great ascending current strikes off the
antagonists on either hand; and then if we are
in the current, we perceive no wind; and if
we hold a lighted candle in the air, its flame
ascends unwavering; but a few feet from the
ground we can feel nothing of the upward
rush which we denominate a calm. With
this current rises a vast mass of vapour, and
the sun's decline, or a touch from the trade-
wind, or the coldness of the upper air,
condenses this; and down come sheets of rain,
attended with electrical explosions. How the
trade-winds, when they come together, twist
and twirl, and generally how two winds cause
an eddy, and a veering of the weathercock
when they come down upon each other, any
man may understand who ever sat by a brookside.
Currents of water coming upon each
other, round the stones, from different directions,
act upon each other just as the air-
currents act: carving miniature gales and
model whirlwinds from a kindred element.

Within this zone of variable winds and
calms, vapour ascends perpetually, and rain
falls almost every day; the rainy season being
distinguished only by a more determined
drench, just as a doctor, paid by items, pours
forth more bottles in the season of an
epidemic, though he at all times is unmercifully
liberal. That vapour rises from water and
from every moist body under the influence of
heat, anybody knows. The greater the heat,
the more the vapour; but even in winter,
from the surface of an ice-field, vapour rises.
The greater the heat, the greater the
expansion of the vapour. It is the nature of
material things to expand under heat, and to
contract under cold; so water does, except in
the act of freezing, when for a beneficent purpose
it is constituted an exception to the rule.
Vapour rises freely from lakes, rivers, and
moist land; but most abundantly, of course, it
rises from the sea, and nowhere more abundantly
than where the sun is hottest. So it
rises in the zone of variable winds and calms,
abundant, very much expanded, therefore
imperceptible. There comes a breath of colder
air on the ascending current; its temperature
falls; it had contained as much vapour as it
would hold in its warm state; when cooled it
will not hold so much; the excess, therefore,
must part company, and be condensed again;
clouds rapidly form, and as the condensation
goes on in this region with immense rapidity,
down comes the discarded vapour in the
original state of water, out of which it had
been raised; down it comes, a hogshead in
each drop. Sudden precipitation, and the
violent rubbing against each other, of two air-
currents unequally warmed, developes
electricity; and then in this zone you can hear
such thunder, and behold such lightning, as
we quiet folks at home are never plagued
with. We may stop here to remark that in
all climates this is the whole theory of rain.
Our present weather is sufficient illustration.
There was a noisy wind from the south-west