ain facts in the ancient
history of the earth-moon system perhaps as astounding as those to
which the tides have conducted us. In one respect we may compare these
laws of heat with the laws of the tides; they are both alike
non-periodic, their effects are cumulative from age to age, and
imagination can hardly even impose a limit to the magnificence of the
works they can accomplish. Our argument from heat is founded on a
very simple matter. It is quite obvious that a heated body tends to
grow cold. I am not now speaking of fires or of actual combustion
whereby heat is produced; I am speaking merely of such heat as would
be possessed by a red-hot poker after being taken from the fire, or by
an iron casting after the metal has been run into the mould. In such
cases as this the general law holds good, that the heated body tends
to grow cold. The cooling may be retarded no doubt if the passage of
heat from the body is impeded. We can, for instance, retard the
cooling of a teapot by the well-known practice of putting a cosy upon
it; but the law remains that, slowly or quickly, the heated body will
tend to grow colder. It seems almost puerile to insist with any
emphasis on a point so obvious as this, but yet I frequently find that
people do not readily apprehend all the gigantic consequences that can
flow from a principle so simple. It is true that a poker cools when
taken from the fire; we also find that a gigantic casting weighing
many tons will grow gradually cold, though it may require days to do
so. The same principle will extend to any object, no matter how vast
it may happen to be. Were that great casting 2000 miles in diameter,
or were it 8000 miles in diameter, it will still steadily part with
its heat, though no doubt the process of cooling becomes greatly
prolonged with an increase in the dimensions of the heated body. The
earth and the moon cannot escape from the application of these simple
principles.

Let us first speak of the earth. There are multitudes of volcanoes in
action at the present moment in various countries upon this earth. Now
whatever explanation may be given of the approximate cause of the
volcanic phenomena, there can be no doubt that they indicate the
existence of heat in the interior of the earth. It may possibly be, as
some have urged, that the volcanoes are merely vents for comparatively
small masses of subterranean molten matter; it may be, as others more
reasonably, in my opinion, believe,