that the whole interior of the
earth is at the temperature of incandescence, and that the eruptions
of volcanoes and the shocks of earthquakes are merely consequences of
the gradual shrinkage of the external crust, as it continually strives
to accommodate itself to the lessening bulk of the fluid interior.
But whichever view we may adopt, it is at least obvious that the earth
is in part, at all events, a heated body, and that the heat is not in
the nature of a combustion, generated and sustained by the progress of
chemical action. No doubt there may be local phenomena of this
description, but by far the larger proportion of the earth's internal
heat seems merely the fervour of incandescence. It is to be likened to
the heat of the molten iron which has been run into the sand, rather
than to the glowing coals in the furnace in which that iron has been
smelted.

There is one volcanic outbreak of such exceptional interest in these
modern times that I cannot refrain from alluding to it. Doubtless
every one has heard of that marvellous eruption of Krakatoa, which
occurred on August 26th and 27th, 1883, and gives a unique chapter in
the history of volcanic phenomena. Not alone was the eruption of
Krakatoa alarming in its more ordinary manifestations, but it was
unparalleled both in the vehemence of the shock and in the distance to
which the effects of the great eruption were propagated. I speak not
now of the great waves of ocean that inundated the coasts of Sumatra
and Java, and swept away thirty-six thousand people, nor do I allude
to the intense darkness which spread for one hundred and eighty miles
or more all round. I shall just mention the three most important
phenomena, which demonstrate the energy which still resides in the
interior of our earth. Place a terrestrial globe before you, and fix
your attention on the Straits of Sunda; think also of the great
atmospheric ocean some two or three hundred miles deep which envelopes
our earth. When a pebble is tossed into a pond a beautiful series of
concentric ripples diverge from it; so when Krakatoa burst up in that
mighty catastrophe, a series of gigantic waves were propagated through
the air; they embraced the whole globe, converged to the antipodes of
Krakatoa, thence again diverged, and returned to the seat of the
volcano; a second time the mighty series of atmospheric ripples spread
to the antipodes, and a second time returned. Seven times did that
series of waves cour