e the circumstances of our system which give
to heat its potency. Look first at our earth, which at present seems--on
its surface, at all events--to be a body devoid of internal heat; a
closer examination will dispel this idea. Have we not the phenomena of
volcanoes, of geysers, and of hot springs, which show that in the
interior of the earth heat must exist in far greater intensity than we
find on the surface? These phenomena are found in widely different
regions of the earth. Their origin is, no doubt, involved in a good deal
of obscurity, but yet no one can deny that they indicate vast reservoirs
of heat. It would indeed seem that heat is to be found everywhere in the
deep inner regions of the earth. If we take a thermometer down a deep
mine, we find it records a temperature higher than at the surface. The
deeper we descend the higher is the temperature; and if the same rate of
progress should be maintained through those depths of the earth which we
are not able to penetrate, it can be demonstrated that at twenty or
thirty miles below the surface the temperature must be as great as that
of red-hot iron.

We find in the other celestial bodies abundant evidence of the present
or the past existence of heat. Our moon, as we have already mentioned,
affords a very striking instance of a body which must once have been
very highly heated. The extraordinary volcanoes on its surface place
this beyond any doubt. It is equally true that those volcanoes have
been silent for ages, so that, whatever may be the interior condition of
the moon, the surface has now cooled down. Extending our view further,
we see in the great planets Jupiter and Saturn evidence that they are
still endowed with a temperature far in excess of that which the earth
has retained; while, when we look at our sun, we see a body in a state
of brilliant incandescence, and glowing with a fervour to which we
cannot approximate in our mightiest furnaces. The various fixed stars
are bodies which glow with heat, like our sun; while we have in the
nebulae objects the existence of which is hardly intelligible to us,
unless we admit that they are possessed of heat.

From this rapid survey of the different bodies in our universe one
conclusion is obvious. We may have great doubts as to the actual
temperature of any individual body of the system; but it cannot be
doubted that there is a wide range of temperature among the different
bodies. Some are hotter than others. The