supposed to have fallen on Newton's head; this version appears to have been invented by Isaac D'Israeli. Voltaire is the authority for the former version of the story. He had his information from Newton's favourite niece Catherine Barton, who married John Conduitt, a fellow of the Royal Society, and one of Newton's intimate friends. How much truth there is in what is a plausible and a favourite story can never be known, but it is certain that tradition marked a tree as that from which the apple fell, until 1866, when, owing to decay, the tree was cut down and its wood carefully preserved. Johannes Kepler had proved by an elaborate series of measurements that each planet revolves in an elliptical orbit around the Sun, whose centre occupies one of the foci of the ellipse, that the radius vector of each planet drawn from the Sun sweeps out equal areas in equal times, and that the squares of the periodic times of the planets are in the same proportion as the cubes of their mean distances from the Sun. The fact that heavy bodies have always a tendency to fall to the Earth, no matter at what height they are placed above the Earth's surface, seems to have led Newton to conjecture that the same tendency to fall to the Earth might have been the cause by which the Moon was retained in its orbit around the Earth. Newton, by calculating from Kepler's laws, and supposing the orbits of the planets to be circles with the sun at the centre, had already proved that the force of the Sun acting upon the different planets must vary as the inverse square of the distances of the planets from the Sun. He was therefore led to inquire whether if the Earth's attraction extended to the Moon, the force at that distance would be of the exact magnitude necessary to retain the Moon in its orbit. He found that the Moon by its motion in its orbit was deflected from the tangent in every minute through a space of 13 feet (3.96 m). But by observing the distance through which a body would fall in one second at the Earth's surface, and by calculating from that on the supposition of the force diminishing in the ratio of the inverse square of the distance, he found that the Earth's attraction at the distance of