This inward heat of the earth is not a mere theory,
but an ascertained fact ; for as we descend from the
surface in deep mines or borings, we find the tempera-
ture actually does increase at a rate which varies somewhat
in different localities, but which averages about 1° Fah-
renheit for every 60 feet of depth. At this rate of
increase water would boil at a depth of 10,000 feet, and
iron and all other metals be melted before we reached
100,000 feet. What actually occurs at great depths we
do not know with any certainty, for we are not sufficiently
acquainted with the laws under which matter may behave
when under enormous heat combined with enormous pres-
sure. But we do know from volcanoes and earthquakes
that masses of molten rocks and of imprisoned gases exist
in certain localities, at depths below the surface which,
although large compared with our deepest pits, are
almost infinitesimally small compared with the total depth
of 4,000 miles from that surface to the earth's centre.
This much is clear, that, in order to account for
observed facts, we must consider the extreme outer
crust, or surface of the earth as known to us, as



38 MODERN SCIENCE AND MODERN THOUGHT.
