habits of our primitive
ancestors gave opportunities for constant observation of the
practicalities of this law. And, so soon as man had developed the mental
capacity to formulate ideas, one of the earliest ideas must have been
the conception, however vaguely phrased in words, that all unsupported
bodies fall towards the earth. The same phenomenon being observed to
operate on water-surfaces, and no alteration being observed in its
operation in different portions of man's habitat, the most primitive
wanderer must have come to have full faith in the universal action of
the observed law of gravitation. Indeed, it is inconceivable that he can
have imagined a place on the earth 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