o compute whether
the moon falls from the tangent she would describe if the earth ceased
to act upon her by a quantity proportional to that observed in the case
of bodies falling near the surface. In the first calculations made by
Newton, he found that the moon is deflected from the tangent thirteen
feet every minute; but, if the hypothesis of gravitation were true, her
deflection should be fifteen feet. It is no trifling evidence of the
scrupulous science of this great philosopher that hereupon he put aside
the subject for several years, without, however, abandoning it. At
length, in 1682, learning the result of the measures of a degree which
Picard had executed in France, and which affected the estimate of the
magnitude of the earth he had used, and therefore the distance of the
moon, he repeated the calculations with these improved data. It is
related that "he went home, took out his old papers, and resumed his
calculations. As they drew to a close, he became so much agitated that
he was obliged to desire a friend to finish them." The expected
coincidence was verified. And thus it appeared that the moon is retained
in her orbit and made to revolve round the earth by the force of
terrestrial gravity.

[Sidenote: The cause of Kepler's laws.] These calculations were founded
upon the hypothesis that the moon moves in a circular orbit with a
uniform velocity. But in the "Principia" it was demonstrated that when a
body moves under the influence of an attractive force, varying as the
inverse square of the distances, it must describe a conic section, with
a focus at the centre of force, and under the circumstances designated
by Kepler's laws. Newton, therefore, did far more than furnish the
expected solution of the problem of elliptical motion, and it was now
apparent that the existence of those laws might have been foreseen,
since they arise in the very necessities of the case.

[Sidenote: Resistless spread of the heliocentric theory.] This point
gained, it is obvious that the evidence was becoming unquestionable,
that as the moon is made to revolve round the earth through the
influence of an attractive force exercised by the earth, so likewise
each of the planets is compelled to move in an elliptical orbit round
the sun by his attractive force. The heliocentric theory, at this stage,
was presenting physical evidence of its truth. It was also becoming
plain that the force we call gravitation must be imputed to the sun, and