n supposed to rest.

By application of Fourier's theorem (or definition) to the observed rate
of increment of heat in descending from the geothermal _couche_ of
invariable temperature, and the co-efficients of conductivity of the
rocks of our earth's crust, as given by the long-continued observations
made beneath the Observatories of Paris and of Edinburgh, it results
that the annual loss of heat into space of our globe at present is equal
to that which would liquefy into water, at 32 deg. Fahr., about 777 cubic
miles of ice; and this is the measuring unit for the amount of
contraction of our globe now going on. The figures are not probably
exact, for the data are not on a basis sufficiently full or exactly
established as yet; but they are not very widely wrong, and their
precise exactness is not material here. Now, how is this annual loss of
heat (great or small, as we may please to view it) from the interior of
our globe disposed of?

What does it _do_ in the interior? We have already seen that it is
primarily disposed of by conversion into work; into the work of
diminishing the earth's volume as a whole, and in so doing crushing
portions of the solid surrounding shell.

But does the transformation of lost heat into the work of vertical
descent, and of the crush as it follows down after the shrinking
nucleus, end the cycle? No. A very large portion of the mechanical work
thus produced, and resolved, as we have seen, into tangential crushing
pressure, is retransformed into heat again in the very act of crushing
the solid material of the shell. If we see a cartload of granite
paving-stones shot out in the dark, we see fire and light produced by
their collision; if we rub two pieces of quartz together, and crush thus
their surfaces against each other, we find we heat the pieces and evolve
light.

The machinery used for crushing by steam-power, hard rocks into road
metal, gets so hot that the surfaces cannot be touched.

These are familiar instances of one result of what is now taking place
by the crushing of the rocky masses of our cooling and descending
earth's crust, every hour beneath our feet, only upon a vastly greater
scale. It is in this local transformation of work into heat that I find
the true origin of volcanic heat within our globe. But if we are to test
this, so as in the only way possible to decide is it a true solution of
this great problem, we must again ask the question, _How much?_ and to
answer t