Sir Isaac Newton (1643-1727) expounded on the geometrical plan of movement in
the solar system significance by showing that the same uniformly acting force
regulates celestial revolutions, and compels heavy bodies to fall towards the
earth's surface. The law of gravity, published in 1687 in "Philosophiae
Naturalis Principia Mathamatica" is to the following effect: every particle
of matter attracts every other with a force directly proportional to their
masses, and inversely proportional to the squares of their distances apart. Its
validity was tested by comparing the amount of the moon's orbital deflection in
a second with the orbital deflection in a second with the rate at which an apple
(say) drops in an orchard. Allowance being made for the distance of the moon,
the two velocities proved to tally perfectly, and the identity of terrestrial
gravity with the force controlling the revolutions of the heavenly established.
But this was only a beginning.
The colossal work remained to be accomplished of calculating the consequences
of the law, in the minute details of its working, and of comparing them with the
heavens. It was carried foreword first by Newton himself, and in the ensuing
century, by Euler, Clairaut, d' Alembert, Lagrange, and Laplace. Urbain
Leverrier (1811- 77) inherited from these men of genius a task never likely to
be completed; and the intricacies of lunar theory have been shown, by the
researches of John Cough Adams (1819-92), of Hansen and Delaunay, of Professors
Hill and Newcomb, and many more, to be fraught with issues of unexpected and
varied interest.
