in the glories of Krakatoa. The marvellous sunsets
in the autumn of 1883 are attributable to this cause; and thus once
again was brought before us the fact that the earth still contains
large stores of thermal energy.

Attempts are sometimes made to explain volcanic phenomena on the
supposition that they are entirely of a local character, and that we
are not entitled to infer the incandescent nature of the earth's
interior from the fact that volcanic outbreaks occasionally happen.
For our present purpose this point is immaterial, though I must say it
appears to me unreasonable to deny that the interior of the earth is
in a most highly heated state. Every test we can apply shows us the
existence of internal heat. Setting aside the more colossal phenomena
of volcanic eruptions, we have innumerable minor manifestations of its
presence. Are there not geysers and hot springs in many parts of the
earth? and have we not all over our globe invariable testimony
confirming the statement, that the deeper we go down beneath its
surface the hotter does the temperature become? Every miner is
familiar with these facts; he knows that the deeper are his shafts the
warmer it is down below, and the greater the necessity for providing
increased ventilation to keep the temperature within a limit that
shall be suitable for the workmen. All these varied classes of
phenomena admit solely of one explanation, and that is, that the
interior of the earth contains vast stores of incandescent heat.

We now apply to our earth the same reasoning which we should employ on
a poker taken from the fire, or on a casting drawn from the foundry.
Such bodies will lose their heat by radiation and conduction. The
earth is therefore losing its heat. No doubt the process is an
extremely slow one. The mighty reservoirs of internal heat are covered
by vast layers of rock, which are such excellent non-conductors that
they offer every possible impediment to the leakage of heat from the
interior to the surface. We coat our steam-pipes over with
non-conducting material, and this can now be done so successfully,
that it is beginning to be found economical to transmit steam for a
very long distance through properly protected pipes. But no
non-conducting material that we can manufacture can be half so
effective as the shell of rock twenty miles or more in thickness,
which secures the heated interior of the earth from rapid loss by
radiation into space. Even were the ea