In this comparison, by Lieut. Maury, the South Seas themselves, in all
their vast intertropical extent, are the boiler for the engine, and the
northern hemisphere is its condenser. The mechanical power exerted by
the air and the sun in lifting water from the earth, in transporting
it from one place to another, and in letting it down again, is
inconceivably great. The utilitarian who compares the water-power that
the Falls of Niagara would afford if applied to machinery is astonished
at the number of figures which are required to express its equivalent
in horse-power. Yet what is the horse-power of the Niagara, falling
a few steps, in comparison with the horse-power that is required to
lift up as high as the clouds and let down again all the water that is
discharged into the sea, not only by this river, but by all the other
rivers in the world? The calculation has been made by engineers; and
according to it, the force of making and lifting vapour from each area
of one acre that is included on the surface of the earth, is equal to
the power of thirty horses; and for the whole of the earth, it is 800
times greater than all the water-power in Europe.
HOW DOES THE RAIN-MAKING VAPOUR GET FROM THE SOUTHERN INTO THE NORTHERN
HEMISPHERE?
This comes with such regularity, that our rivers never go dry, and
our springs fail not, because of the exact _compensation_ of the
grand machine of _the atmosphere_. It is exquisitely and wonderfully
counterpoised. Late in the autumn of the north, throughout its
winter, and in early spring, the sun is pouring his rays with the
greatest intensity down upon the seas of the southern hemisphere; and
this powerful engine, which we are contemplating, is pumping up the
water there with the greatest activity; at the same time, the mean
temperature of the entire southern hemisphere is about 10° higher than
the northern. The heat which this heavy evaporation absorbs becomes
latent, and with the moisture is carried through the upper regions
of the atmosphere until it reaches our climates. Here the vapour is
formed into clouds, condensed and precipitated; the heat which held
their water in the state of vapour is set free, and becomes sensible
heat; and it is that which contributes so much to temper our winter
climate. It clouds up in winter, turns warm, and we say we are going
to have falling weather: that is because the process of condensation
has already commenced, though no rain or snow may have fallen. Thus we
feel this southern heat, that has been collected by the rays of the sun
by the sea, been bottled away by the winds in the clouds of a southern
summer, and set free in the process of condensation in our northern
winter.
Thus the South Seas should supply mainly the water for the engine just
described, while the northern hemisphere condenses it; we should,
therefore, have more rain in the northern hemisphere. The rivers tell
us that we have, at least on the land; for the great water-courses of
the globe, and half the fresh water in the world, are found on the
north side of the equator. This fact is strongly corroborative of this
hypothesis. To evaporate water enough annually from the ocean to cover
the earth, on the average, five feet deep with rain; to transport it
from one zone to another; and to precipitate it in the right places at
suitable times and in the proportions due,--is one of the offices of
the grand atmospherical machine. This water is evaporated principally
from the torrid zone. Supposing it all to come thence, we shall have
encircling the earth a belt of ocean 3000 miles in breadth, from which
this atmosphere evaporates a layer of water annually sixteen feet in
depth. And to hoist up as high as the clouds, and lower down again,
all the water, in a lake sixteen feet deep and 3000 miles broad and
24,000 long, is the yearly business of this invisible machinery. What a
powerful engine is the atmosphere! and how nicely adjusted must be all
the cogs and wheels and springs and _compensations_ of this exquisite
piece of machinery, that it never wears out nor breaks down, nor fails
to do its work at the right time and in the right way!--_Maury._