centre of the earth, viz., at its surface, with the space through which the
moon, as it were, falls to the earth in a second of time while revolving
in a circular orbit. Being at a distance from books when he made this
computation, he adopted the common estimate of the earth's diameter then
in use among geographers and navigators, and supposed that each degree of
latitude contained sixty English miles.

In this way he found that the force which retains the moon in her orbit,
as deduced from the force which occasions the fall of heavy bodies to the
earth's surface, was one-sixth greater than that which is actually
observed in her circular orbit. This difference threw a doubt upon all his
speculations; but, unwilling to abandon what seemed to be otherwise so
plausible, he endeavored to account for the difference of the two forces
by supposing that some other cause must have been united with the force of
gravity in producing so great velocity of the moon in her circular orbit.
As this new cause, however, was beyond the reach of observation, he
discontinued all further inquiries into the subject, and concealed from his
friends the speculations in which he had been employed.

After his return to Cambridge in 1666 his attention was occupied with
optical discoveries; but he had no sooner brought them to a close than his
mind reverted to the great subject of the planetary motions. Upon the death
of Oldenburg in August, 1678, Dr. Hooke was appointed secretary to the
Royal Society; and as this learned body had requested the opinion of Newton
about a system of physical astronomy, he addressed a letter to Dr. Hooke
on November 28, 1679. In this letter he proposed a direct experiment for
verifying the motion of the earth, viz., by observing whether or not bodies
that fall from a considerable height descend in a vertical direction; for
if the earth were at rest the body would describe exactly a vertical line;
whereas if it revolved round its axis, the falling body must deviate from