ften
continues to be red hot at a few feet depth for years after the surface
is consolidated, and is hardly any warmer than that of the surrounding
land. A still more remarkable case is that of a glacier on the
south-east side of the highest cone of Etna underneath a lava stream
with an intervening bed of volcanic sand only ten feet thick. This was
visited by Sir Charles Lyell in 1828, and a second time thirty years
later, when he made a very careful examination of the strata, and was
quite satisfied that the sand and the lava stream together had actually
preserved this mass of ice, which neither the heat of the lava above it
at its first outflow, nor the continued heat rising from the great
volcano below it, had been able to melt or perceptibly to diminish in
thirty years. Another fact that points in the same direction is the
existence over the whole floor of the deepest oceans of ice-cold water,
which, originating in the polar seas, owing to its greater density sinks
and creeps slowly along the ocean bottom to the depths of the Atlantic
and Pacific, and is not perceptibly warmed by the internal heat of the
earth.

Now the solid crust of the earth is estimated as at least about twenty
miles in average thickness; and, keeping in mind the other facts just
referred to, we can understand that the heat from the interior passes
through it with such extreme slowness as not to be detected at the
surface, the varying temperatures of which are due entirely to solar
heat. A large portion of this heat is stored up in the surface soil, and
especially in the surface layer of the oceans and seas, thus partly
equalising the temperatures of day and night, of winter and summer, so
as greatly to ameliorate the rapid changes of temperature that would
otherwise occur. Our dense atmosphere is also of immense advantage to us
as an equaliser of temperature, charged as it almost always is with a
large quantity of water-vapour. This latter gas, when not condensed into
cloud, allows the solar heat to pass freely to the earth; but it has the
singular and highly beneficial property of absorbing and retaining the
dark or lower-grade heat given off by the earth which would otherwise
radiate into space much more rapidly. We must therefore always remember
that, very nearly if not quite, the _whole_ of _the warmth we experience
on the earth is derived from the sun._[8]

[Footnote 8: Professor J.H. Poynting, in his lecture to the British
Association a