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intellectual nature was left unsatisfied by the mere act of
observation. He sought after the principle which ruled
the fact. Whether this anecdote be true or not, it illus-
trates how the ordinary operations of Nature, which most
people take for granted as perfectly plain and simple, are '
often those which most puzzle the scientific man. To the
conception of the matter of the apple, Newton added that
'of the force that moved it. The falling of the apple was
due to an attraction exerted mutually between it and the
earth. He applied the idea of this force to suns, and plan-
ets, and moons, and showed that all their motions were
necessary consequences of this attraction.

Newton, you know, was preceded by a grand fellow
named John Kepler—a true working-man—who, by analyz-
ing the astronomical observations of his master, Tycho
Brahe, had actually found that the planets moved as they are
now known to move. As a matter of fact, Kepler knew as
much about the motion of the planets as Newton did; in
fact, Kepler taught Newton and the world generally the
facts of planetary motion. But this was not enough. The
question arose—Why should the facts be so ? This was
the great question for Newton, and it was the solution of
this question which renders his name and fame immortal.
He proved that the planetary motions were what observa-
tion made them to be, because every particle of matter in
the solar system attracts every other particle by a force
which varies as the inverse square of the distanCe between
the particles. He showed that the moon fell toward the
earth, and thatthe planets fell toward the sun, through the
Operation of the same force that pulls an apple from its
tree. This all-pervading force, which forms the ‘solder of
the material universe, and the conception of which was
necessary to Newton’s intellectual peace, is called the force
of gravitation.

All force may be ultimately reduced to a push or a pull in

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