same thing, a loss of an unit of heat-making power
from the chemical or the mechanical energies. The substantial result is
the same; the heat, actual or potential, of the earth must be
decreasing. It should, of course, be observed that a great part of the
thermal losses experienced by the earth is of an obvious character, and
not dependent upon the slow processes of conduction. Each outburst of a
volcano discharges a stupendous quantity of heat, which disappears very
speedily from the earth; while in the hot springs found in so many
places there is a perennial discharge of the same kind, which in the
course of years attains enormous proportions.

The earth is thus losing heat, while it never acquires any fresh
supplies of the same kind to replace the losses. The consequence is
obvious; the interior of the earth must be growing colder. No doubt this
is an extremely slow process; the life of an individual, the life of a
nation, perhaps the life of the human race itself, has not been long
enough to witness any pronounced change in the store of terrestrial
heat. But the law is inevitable, and though the decline in heat may be
slow, yet it is continuous, and in the lapse of ages must necessarily
produce great and important results.

It is not our present purpose to offer any forecast as to the changes
which must necessarily arise from this process. We wish at present
rather to look back into past time and see what consequences we may
legitimately infer. Such intervals of time as we are familiar with in
ordinary life, or even in ordinary history, are for our present purpose
quite inappreciable. As our earth is daily losing internal heat, or the
equivalent of heat, it must have contained more heat yesterday than it
does to-day, more last year than this year, more twenty years ago than
ten years ago. The effect has not been appreciable in historic time; but
when we rise from hundreds of years to thousands of years, from
thousands of years to hundreds of thousands of years, and from hundreds
of thousands of years to millions of years, the effect is not only
appreciable, but even of startling magnitude.

There must have been a time when the earth contained much more heat than
at present. There must have been a time when the surface of the earth
was sensibly hot from this source. We cannot pretend to say how many
thousands or millions of years ago this epoch must have been; but we may
be sure that earlier still the earth was ev