ed, with their
thousands of living inhabitants. Think of rivers of glowing lava
streaming up from regions below ground, and pouring along the surface
for a distance of forty, fifty, and even sixty miles, as in Iceland
and Hawaii. Think of red-hot cinders flung from a volcano-crater to a
height of ten thousand feet. Think of lakes of liquid fire in other
craters, five hundred to a thousand feet across, huge cauldrons of
boiling rock. Think of showers of ashes from the furnace below of yet
another, borne so high aloft as to be carried seven hundred miles
before they sank to earth again. Think of millions of red-hot stones
flung out in one eruption of Vesuvius. Think of a mass of rock, one
hundred cubic yards in size, hurled to a distance of eight miles or
more out of the crater of Cotopaxi.

[Illustration: HOT WELLS.]

Think also of earthquake-shocks felt through twelve hundred miles of
country. Think of fierce tremblings and heavings lasting in constant
succession through days and weeks of terror. Think of hundreds of
miles of land raised several feet in one great upheaval. Think of the
earth opening in scores of wide-lipped cracks, to swallow men and
beasts. Think of hot mud, boiling water, scalding stream, liquid rock,
bursting from such cracks, or pouring from rents in a mountain-side.

Truly these are signs of a state of things in or below the solid crust
on which we live, that may make us doubt the absolute security of
"Mother Earth."

Different explanations have been put forward to explain this seemingly
fiery state of things underground.

Until lately the belief was widely held that our earth was one huge
globe of liquid fire, with only a slender cooled crust covering her, a
few miles in thickness.

This view was supported by the fact that heat is found to increase as
men descend into the earth. Measurements of such heat-increase have
been taken, both in mines and in borings for wells. The usual rate is
about one degree more of heat, of our common thermometer, for every
fifty or sixty feet of descent. If this were steadily continued, water
would boil at a depth of eight thousand feet below the surface; iron
would melt at a depth of twenty-eight miles; while at a depth of forty
or fifty miles no known substance upon earth could remain solid.

The force of this proof is, however, weakened by the fact that the
rate at which the heat increases differs very much in different
places. Also it is now generally su