the vertical line toward the east.

The Royal Society attached great value to the idea thus casually suggested,
and Dr. Hooke was appointed to put it to the test of experiment. Being
thus led to consider the subject more attentively, he wrote to Newton that
wherever the direction of gravity was oblique to the axis on which the
earth revolved, that is, in every part of the earth except the equator,
falling bodies should approach to the equator, and the deviation from the
vertical, in place of being exactly to the east, as Newton maintained,
should be to the southeast of the point from which the body began to move.

Newton acknowledged that this conclusion was correct in theory, and Dr.
Hooke is said to have given an experimental demonstration of it before the
Royal Society in December, 1679. Newton had erroneously concluded that the
path of the falling body would be a spiral; but Dr. Hooke, on the same
occasion on which he made the preceding experiment, read a paper to the
society in which he proved that the path of the body would be an eccentric
ellipse _in vacuo_, and an ellipti-spiral if the body moved in a resisting
medium.

This correction of Newton's error, and the discovery that a projectile
would move in an elliptical orbit when under the influence of a force
varying in the inverse ratio of the square of the distance, led Newton, as
he himself informs us in his letter to Halley, to discover "the theorem
by which he afterward examined the ellipsis," and to demonstrate the
celebrated proposition that a planet acted upon by an attractive force
varying inversely as the squares of the distances, will describe an
elliptical orbit in one of whose _foci_ the attractive force resides.

But though Newton had thus discovered the true cause of all the celestial