ly small and
so assumed as plane) of the shell, suppose a square mile, equal to the
effect of gravity upon that unit, r being the earth's radius, and if we
assume the unit square be also a unit in thickness, P is then the weight
of a cubic mile of its material; and if we take (roughly) the earth's
radius as 4,000 miles, the tangential pressure, T, is, on _each face_ of
the cubic mile, equal to

(4000/2) P,

or equal to the pressure of a column of the same material of 2,000 times
its weight.

If the cubic mile that we have thus supposed cut out of the earth's
crust at the surface were of the hardest known granite or porphyry, it
would be exposed to a crushing tangential pressure equal to between 400
and 500 times what it could withstand, and so must crush, even though
only left unsupported by the nucleus beneath, to the extent of 1/400 or
1/500 of its entire weight. And what is true here of a mile taken at the
surface, is true (neglecting some minute corrections for difference in
the co-efficient of gravity, etc.) if taken at any other depth within
the thick crust.[F]

The crust of our earth, then, as it now is, must crush, to follow down
after the shrinking nucleus--if so be that the globe be still cooling,
and constituted as it is; even to the limited extent to which we know
anything of its nature--it must crush unequally, both regarded
superficially and as to depth; generally the crushing lines being
confined to the planes or places of greatest weakness; and the crushing
will not be absolutely constant and uniform anywhere, or at any time, or
at any of those places of weakness to which it will be principally
confined, but will be more or less irregular, quasi-periodic, or
paroxysmal: as is, indeed, the way in which all known material
substances (more or less rigid) give way to a slow and constantly
increasing, steady pressure.

We have now to ask, _How much_ of this crushing is going on at present
year by year? And the answer to this depends upon what amount of heat
our world is losing into space year by year.

Geologists who have taken on trust the statement, that La Place has
proved that the world has lost no sensible amount of heat for the last
10,000 years seem generally to suppose that to be a fact; but in reality
La Place has _proved_ nothing of the sort, as those geological teachers
who have echoed the conclusion should have known, had they deciphered
the mathematical argument upon which it has bee