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 were sent to London about the end of 1683, and were soon afterward
communicated to the Royal Society.

About this period other philosophers had been occupied with the same
subject. Sir Christopher Wren had many years before endeavored to explain
the planetary motions "by the composition of a descent toward the sun, and
an impressed motion; but he at length gave it over, not finding the means
of doing it." In January, 1683-1684, Dr. Halley had concluded from Kepler's
law of the periods and distances, that the centripetal force decreased in
the reciprocal proportion of the squares of the distances, and having one
day met Sir Christopher Wren and Dr. Hooke, the latter affirmed that he had
demonstrated upon that principle all the laws of the celestial motions. Dr.
Halley confessed that his attempts were unsuccessful, and Sir Christopher,
in order to encourage the inquiry, offered to present a book of forty
shillings value to either of the two philosophers who should, in the space
of two months, bring him a convincing demonstration of it. Hooke persisted
in the declaration that he possessed the method, but avowed it to be his
intention to conceal it for time. He promised, however, to show it to Sir
Christopher; but there is every reason to believe that this promise was
never fulfilled.

In August, 1684, Dr. Halley went to Cambridge for the express purpose of
consulting Newton on this interesting subject. Newton assured him that he
had brought this demonstration to perfection, and promised him a copy of