the sides, covering the
vineyards and olive orchards, and driving the people out of their homes
in terror, it seems as if the earth's crust must be but a thin and frail
affair, covering a fiery interior, which might at any time break out.
The people who live near volcanoes might easily get this idea.

But they do not. They go back as soon as the lava streams are cooled,
and rebuild their homes, and plant more orchards and vineyards. "It is
_so_ many years," say they to one another, "since the last bad eruption.
Vesuvius will probably sleep now till we are dead and gone."

This is good reasoning. There are few active volcanoes left on the
earth, compared with the number that were once active, and long ago
became extinct. And the time between eruptions of the active ones grows
longer; the eruptions less violent. Terrible as were the recent
earthquakes of San Francisco and Messina, this form of disturbance of
the earth's crust is growing constantly less frequent. The earth is
growing cooler as it grows older; the crust thickens and grows stronger
as centuries pass. We have been studying the earth only a few hundred
years. The crust has been cooling for millions of years, and
mountain-making was the result of the shrinking of the crust. That
formed folds and clefts, and let masses of the heated substance pour out
on the surface.

My first geography lesson I shall never forget. The new teacher had very
bright eyes and _such_ pretty hands! She held up a red apple, and told
us that the earth's substance was melted and burning, inside its crust,
which was about as thick, in proportion to the size of the globe, as the
skin of the apple. I was filled with wonder and fear. What if we
children jumped the rope so hard as to break through the fragile shell,
and drop out of sight in a sea of fiery metal, like melted iron? Some of
the boys didn't believe it, but they were impressed, nevertheless.

The theory of the heated interior of the earth is still believed, but
the idea that flames and bubbling metals are enclosed in the outer layer
of solid matter has generally been abandoned. The power that draws all
of its particles toward the earth's centre is stated by the laws of
gravitation. The amount of "pull" is the measure of the weight of any
substance. Lift a stone, and then a feather pillow, much larger than
the stone. One is strongly drawn to the earth; the other not. One is
_heavy_, we say, the other _light_.

If a stone you