he finds the clew to the secret, the key of the whole matter, in the
earth's vast central heat. This it is which produces the convulsions
that change the terrestrial configuration, and fill the minds of men
with fear and awe. Conceive of "a sea of fire, on which we are all
floating, land and sea,"--a boiling, seething, incandescent reservoir in
the centre of our planet; and the solution of the problem will seem to
you not difficult. Such a sea would necessarily roll its liquid matter
to and fro; and the removal of ever so small a portion from one point to
another on the earth's surface would tend to disturb the equilibrium of
the floating mass; just as, when a ship is launched into the river, the
water it displaces is carried to the opposite bank with greater or less
violence, according to the amount of displacement.

It is impossible, adds Herschel, but that this increase of pressure in
some places and relief in others must be very unequal in their bearings.
So that at some point or another our planet's floating crust must be
brought into a state of strain, and if there be a weak or a soft part a
crack will at last take place. This is exactly what happened in the
earthquake which originated the Allah Bund, or God's Wall, in Cutch.

Volcanic eruptions are easily explicable on this principle,--the volcano
being simply a vent for the passage of heated and molten matter, which
the elevating pressure of the liquid below tends to eject. It is a
well-known fact that volcanoes and earthquake-centres are nearly all
situated on the borders or in the immediate neighbourhood of seas and
oceans; and the reason would seem to be, that at such positions the
accumulation of transported matter would necessarily attain its maximum,
to whatever cause it might be due. Then again, as Herschel points out,
the eruption of scorite and lava from the mouths of volcanoes, the
result of the upward movement of the fiery liquid below, compensates in