being the same as if they were all
concentrated in its centre. To this central force, thus determining the
fall of bodies, the designation of gravity was given. Up to this time,
no one, except Kepler, had considered how far its influence reached. It
seemed to Newton possible that it might extend as far as the moon, and
be the force that deflects her from a rectilinear path, and makes her
revolve in her orbit round the earth. It was easy to compute, on the
principle of the law of inverse squares, whether the earth's attraction
was sufficient to produce the observed effect. Employing the measures
of the size of the earth accessible at the time, Newton found that the
moon's deflection was only thirteen feet in a minute; whereas, if his
hypothesis of gravitation were true, it should be fifteen feet. But in
1669 Picard, as we have seen, executed the measurement of a degree more
carefully than had previously been done; this changed the estimate of
the magnitude of the earth, and, therefore, of the distance of the moon;
and, Newton's attention having been directed to it by some discussions
that took place at the Royal Society in 1679, he obtained Picard's
results, 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 established. It was proved that the moon is retained
in her orbit and made to revolve round the earth by the force of
terrestrial gravity. The genii of Kepler had given place to the vortices
of Descartes, and these in their turn to the central force of Newton.

In like manner the earth, and each of the planets, are made to move
in an elliptic orbit round the sun by his attractive force, and
perturbations arise by reason of the disturbing action of the planetary
masses on one another. Knowing the masses and the distances, these
disturbances may be computed. Later astronomers have even succeeded with
the inverse problem, that is, knowing the perturbations or disturbances,
to find the place and the mass of the disturbing body. Thus, from the
deviations of Uranus from his theoretical position, the discovery of
Neptune was accomplished.

Newton's merit consisted in this, that he applied the laws of dynamics
to the movements of the celestial bodies, and insisted that scientific
theories must be substantiated by the agreement of observations with
calculations.

When Kepler