.

Because of this incongruity he let the matter drop for a time. But, in
1679, his mind again reverted to the subject; and in 1682, having
obtained a correct measurement of the diameter of the earth, he repeated
his calculations of 1666. In the progress of his calculations he saw
that the result which he had formerly expected was likely to be
produced, and he was thrown into such a state of nervous irritability
that he was unable to carry on the calculation. In this state of mind he
entrusted it to one of his friends, and he had the high satisfaction of
finding his former views amply realised. The force of gravity which
regulated the fall of bodies at the earth's surface, when diminished as
the square of the moon's distance from the earth, was found to be
exactly equal to the centrifugal force of the moon as deduced from her
observed distance and velocity.

The influence of such a result upon such a mind may be more easily
conceived than described. The whole material universe was opened out
before him; the sun with all his attending planets; the planets with all
their satellites; the comets wheeling in every direction in their
eccentric orbits; and the system of the fixed stars stretching to the
remotest limits of space. All the varied and complicated movements of
the heavens, in short, must have been at once presented to his mind as
the necessary result of that law which he had established in reference
to the earth and the moon.

After extending this law to the other bodies of the system, he composed
a series of propositions on the motion of the primary planets about the
sun, which was sent to London about the end of 1683, and was soon
afterwards communicated to the Royal Society.

Newton's discovery was claimed by Hooke, who certainly aided Newton to
reach the truth, and was certainly also on the track of the same law.

Between 1686 and 1687 appeared the three books of Newton's immortal
work, known as the "Principia." The first and second book are entitled
"On the Motion of Bodies," and the third "On the System of the World."

In this great work Newton propounds the principle that "every particle
of matter in the universe is attracted by, or gravitates to, every other
particle of matter with a force inversely proportional to the squares of
their distances." From the second law of Kepler, namely, the
proportionality of the areas to the times of their description, Newton
inferred that the force which keeps a plan