combustion of the ton of coal, by an amount
exactly equivalent to the work performed.
Suppose that work to consist in lifting a weight of
seven thousand seven hundred and twenty
pounds a foot high; the heat produced by the
coal would fall short of its maximum by a
quantity just sufficient to warm a pound of
water ten degrees Fahrenheit. In an elaborate
series of experiments, executed with
extraordinary assiduity and on a grand scale by M.
Hirn, a civil engineer at Colmar, this theoretic
deduction has been reduced to fact.

In earthly, and we may add in planetary,
affairs, the sun is the great worker who keeps
the whole business of life and action going. It
has been asserted that there is no life in certain
planets. A few years ago, Dr. Whewell wrote
a book to prove that the more distant planets
of our system are uninhabitable. Applying the
law of inverse squares to their distances from
the sun, the diminution of temperature was
found to be so great as to preclude the
possibility of human life in the more remote members
of the solar system. But—not to mention the
hazardous task of attempting to prove a
negative—the influence of an atmospheric envelope
was overlooked in those calculations. The
omission vitiates the whole argument. It is
perfectly possible to find an atmosphere which
would act the part of a barb to the solar rays,
permitting their entrance towards the planet,
but preventing their withdrawal.  For example,
Professor Tyndall tells us, a layer of air only
two inches in thickness, and saturated with the
vapour of sulphuric ether, would offer very
little resistance to the passage of the solar rays,
but would cut off fully thirty-five per cent of
the planetary radiation. It would require no
inordinate thickening of the layer of vapour to
double this absorption; and it is evident that,
with a protecting envelope which permits heat
to enter but prevents its escape, a comfortable
temperature might be obtained on the surface
of our most distant planet.

It is the presence of a protective atmosphere
that renders the earth itself habitable; and
in regions where it is so modified by the
absence of aqueous vapour as to lose its
protective power, man cannot live. One cause
of the coldness of high mountain-tops, is their
being lifted beyond the protection of the layer
of moist air which lies close to the earth. The
withdrawal of sunshine from any region over
which the atmosphere is dry, must be followed
by quick refrigeration. The moon would be
rendered entirely uninhabitable by beings like
ourselves, through the operation of this single
cause. With a radiation uninterrupted by
aqueous vapour, the difference between her
monthly maxima and minima of temperature
must be enormous. The winters of Thibet are
almost unendurable, from the same cause.
Humboldt dwelt upon the "frigorific power" of
the central portions of the Asiatic continent,
and controverted the idea that it was to be
explained by reference to their elevation; there
being vast expanses of country, not much above
the sea level, with an exceedingly low temperature.
He did not seem to be aware of this one
most important cause which contributes to the
observed result. The absence of the sun at
night causes powerful refrigeration when the
air is dry. The removal, for a single summer
night, of the aqueous vapour from the
atmosphere which covers England, would be attended
by the destruction of every plant which a
freezing temperature could kill. In Sahara,
where "the soil is fire and the wind is flame," the
refrigeration at night is often painful to bear.
Ice has been formed in this region at night.
In Australia also, the diurnal range of temperature
is very great, amounting, commonly, to
between forty and fifty degrees. In short, it
may be safely predicted that, wherever the air
is dry, the daily thermometric range will be great.
This, however, is quite different from saying that
where the air is clear, the thermometric range will
be great. Great clearness as to light is perfectly
compatible with great opacity as to heat. The
atmosphere may be charged with aqueous
vapour, while a deep blue sky is overhead; and
on such occasions the terrestrial radiation would,
notwithstanding the "clearness," be intercepted.
It is consequently impossible for any one on
earth to be sure that the distant planets are
uninhabitable, and that the sun cannot be to
them, as to us, a vivifyer as well as a worker.

Years ago, Sir John Herschel wrote: "The
sun's rays are the ultimate source of almost
every motion which takes place on the surface
of the earth. By its heat are produced all
winds, and those disturbances in the electrical
equilibrium of the atmosphere which give rise
to the phenomena of lightning, and probably
also to terrestrial magnetism and the Aurora.
By their vivifying action vegetables are enabled
to draw support from inorganic matter, and
become in their turn the support of animals and
man, and the source of those deposits of
dynamical efficiency which are laid up for human
use in our coal strata. By them the waters of
the sea are made to circulate in vapour through
the air, and irrigate the land, producing springs
and rivers. By them are produced all
disturbances of the chemical equilibrium of the
elements of nature; which, by a series of
compositions and decompositions, originate new
products and a transfer of materials."

Professor Tyndall applies the new philosophy
to illustrate and expand Herschel's proposition.
He reminds us that late discoveries have taught
that winds and rivers have their definite thermal
values; and that, in order to produce their
motion, an equivalent amount of solar heat has
been consumed. While they exist as winds and
rivers, the heat expended in producing them has
ceased to exist as heat, being converted into
mechanical motion; but when that motion is
arrested, the heat which produced it is restored.
A river, in descending from an elevation of
seven thousand seven hundred and twenty feet,
generates an amount of heat competent to
augment its own temperature ten degrees
Fahrenheit. This amount of heat has been