our
argument that we should answer it. The fact being admitted that the heat
is there, all that we require is to apply one or two of the well-known
thermal laws to the interpretation of the facts. We have first to
consider the general principle by which heat tends to diffuse itself and
spread away from its original source. The heat, deep-seated in the
interior of the earth, is transmitted through the superincumbent rocks,
and slowly reaches the surface. It is true that the rocks and materials
with which our earth is covered are not good conductors of heat; most of
them are, indeed, extremely inefficient in this way; but, good or bad,
they are in some shape conductors, and through them the heat must creep
to the surface.

It cannot be urged against this conclusion that we do not feel this
heat. A few feet of brickwork will so confine the heat of a mighty blast
furnace that but little will escape through the bricks; but _some_ heat
does escape, and the bricks have never been made, and never could be
made, which would absolutely intercept all the heat. If a few feet of
brickwork can thus nearly mask the heat of a furnace, cannot some scores
of miles of rock nearly mask the heat in the depths of the earth, even
though that heat were seven times hotter than the mightiest furnace that
ever existed? The heat would escape slowly, and perhaps imperceptibly,
but, unless all our knowledge of nature is a delusion, no rocks, however
thick, can prevent, in the course of time, the leakage of the heat to
the surface. When this heat arrives at the surface of the earth it must,
in virtue of another thermal law, gradually radiate away and be lost to
the earth.

It would lead us too far to discuss fully the objections which may
perhaps be raised against what we have here stated. It is often said
that the heat in the interior of the earth is being produced by chemical
combination or by mechanical process, and thus that the heat may be
constantly renewed as fast or even faster than it escapes. This,
however, is more a difference in form than in substance. If heat be
produced in the way just supposed (and there can be no doubt that there
may be such an origin for some of the heat in the interior of the globe)
there must be a certain expenditure of chemical or mechanical energies
that produces a certain exhaustion. For every unit of heat which escapes
there will either be a loss of an unit of heat from the globe, or, what
comes nearly to the