where this law does not operate. On the other hand, of course, he
never grasped the conception of the operation of this law beyond
the close proximity of the earth. To extend the reach of
gravitation out to the moon and to the stars, including within
its compass every particle of matter in the universe, was the
work of Newton, as we shall see in due course. Meantime we shall
better understand that work if we recall that the mere local fact
of terrestrial gravitation has been the familiar knowledge of all
generations of men. It may further help to connect us in sympathy
with our primeval ancestor if we recall that in the attempt to
explain this fact of terrestrial gravitation Newton made no
advance, and we of to-day are scarcely more enlightened than the
man of the Stone Age. Like the man of the Stone Age, we know that
an arrow shot into the sky falls back to the earth. We can
calculate, as he could not do, the arc it will describe and the
exact speed of its fall; but as to why it returns to earth at
all, the greatest philosopher of to-day is almost as much in the
dark as was the first primitive bowman that ever made the
experiment.

Other physical facts going to make up an elementary science of
mechanics, that were demonstratively known to prehistoric man,
were such as these: the rigidity of solids and the mobility of
liquids; the fact that changes of temperature transform solids to
liquids and vice versa--that heat, for example, melts copper and
even iron, and that cold congeals water; and the fact that
friction, as illustrated in the rubbing together of two sticks,
may produce heat enough to cause a fire. The rationale of this
last experiment did not receive an explanation until about the
beginning of the nineteenth century of our own era. But the