_The Constituents of Climate._
We have spoken of the steady average of the climate at each place,
of the difference of this average at different places, and of the
adaptation of organized beings to this character in the laws of
the elements by which they are affected. But this steadiness in
the general effect of the elements, is the result of an extremely
complex and extensive machinery. Climate, in its wider sense,
is not one single agent, but is the aggregate result of a great
number of different agents, governed by different laws, producing
effects of various kinds. The steadiness of this compound agency is
not the steadiness of a permanent condition, like that of a body
at rest; but it is the steadiness of a state of constant change
and movement, succession and alternation, seeming accident and
irregularity. It is a perpetual repose, combined with a perpetual
motion; an invariable average of most variable quantities. Now, the
manner in which such a state of things is produced, deserves, we
conceive, a closer consideration. It may be useful to show how the
particular laws of the action of each of the elements of climate
are so adjusted that they do not disturb this general constancy.
The principal constituents of climate are the following:--the
temperature of the earth, of the water, of the air:--the
distribution of the aqueous vapour contained in the
atmosphere:--the winds and rains by which the equilibrium of the
atmosphere is restored when it is in any degree disturbed. The
effects of light, of electricity, probably of other causes also,
are no doubt important in the economy of the vegetable world,
but these agencies have not been reduced by scientific inquirers
to such laws as to admit of their being treated with the same
exactness and certainty which we can obtain in the case of those
first mentioned.
We shall proceed to trace some of the peculiarities in the laws of
the different physical agents which are in action at the earth’s
surface, and the manner in which these peculiarities bear upon the
general result.
_The Laws of Heat with respect to the Earth._
One of the main causes which determine the temperature of each
climate is the effect of the sun’s rays on the solid mass of the
earth. The laws of this operation have been recently made out with
considerable exactness, experimentally by Leslie, theoretically by
Fourier, and by other inquirers. The theoretical inquiries have
required the application of very complex and abstruse mathematical
investigations; but the general character of the operation may,
perhaps, be made easily intelligible.
The earth, like all solid bodies, transmits into its interior the
impressions of heat which it receives at the surface; and throws
off the superfluous heat from its surface into the surrounding
space. These processes are called _conduction_ and _radiation_, and
have each their ascertained mathematical laws.
By the laws of conduction, the daily impressions of heat which the
earth receives, follow each other into the interior of the mass,
like the waves which start from the edge of a canal;[6] and like
them, become more and more faint as they proceed, till they melt
into the general level of the internal temperature. The heat thus
transmitted is accumulated in the interior of the earth, as in a
reservoir, and flows from one part to another of this reservoir.
The parts of the earth near the equator are more heated by the
sun than other parts, and on this account there is a perpetual
internal conduction of heat from the equatorial to other parts
of the sphere. And as all parts of the surface throw off heat by
radiation, in the polar regions, where the surface receives little
in return from the sun, a constant waste is produced. There is
thus from the polar parts a perpetual dispersion of heat in the
surrounding space, which is supplied by a perpetual internal flow
from the equator towards each pole.
Here, then, is a kind of circulation of heat; and the quantity and
rapidity of this circulation, determine the quantity of heat in the
solid part of the earth, and in each portion of it; and through
this, the _mean_ temperature belonging to each point on its surface.
If the earth _conducted_ heat more rapidly than it does, the
inequalities of temperature would be more quickly balanced,
and the temperature of the ground (below the reach of annual
and diurnal variations) would differ less than it does. If the
surface _radiated_ more rapidly than it does, the flow of heat
from the polar regions would increase, and the temperature of the
interior of the globe would find a lower level; the differences of
temperature in different latitudes would increase, but the mean
temperature of the globe would diminish.
There is nothing which, so far as we can perceive, determines
necessarily, either the conducting or the radiating power of
the earth to its present value. The measures of such powers, in
different substances, differ very widely. If the earth were a globe
of pure iron, it would conduct heat, probably, twenty times as
well as it does; if its surface were polished iron, it would only
radiate one-sixth as much as it does. Changes in the amount of the
conduction and radiation far less than these, would, probably,
subvert the whole _thermal_ constitution of the earth, and make it
uninhabitable by any of its present vegetable, or animal tenants.
One of the results of the laws of heat, as they exist in the globe,
is, that, by their action, the thermal state tends to a limiting
condition, which, once reached, remains constant and steady, as it
now is. The oscillations or excursions from the mean condition,
produced by any temporary cause, are rapidly suppressed; the
deviations of seasons from their usual standard produce only a
small and transient effect. The impression of an extremely hot
day upon the ground melts almost immediately into the average
internal heat. The effect of a hot summer, in like manner, is soon
lost in its progress through the globe. If this were otherwise,
if the inequalities and oscillations of heat went on, through the
interior of the earth, retaining the same value, or becoming larger
and larger, we might have the extreme heats or colds of one place
making their appearance at another place after a long interval;
like a conflagration which creeps along a street and bursts out at
a point remote from its origin.
It appears, therefore, that both the present differences of
climate, and the steadiness of the average at each place, depend
upon the form of the present laws of heat, and on the arbitrary
magnitudes which determine the rate of conduction and radiation.
The laws are such as to secure us from increasing and destructive
inequalities of heat; the arbitrary magnitudes are elements to
which the organic world is adjusted.