now quite familiar but at this time new and of exceeding interest. He suspended from the ceiling a long wire to the end of which a ball of wood was attached—a simple pendulum on a large scale. On removing the pendulum from the vertical position and then giving it a lateral impulse at right angles to the plane in which it tended to oscillate, the ball described an ellipse—the eccentricity of the ellipse varying with a variation of the intensity of the lateral impulse. An ocular demonstration was thus given of the important fact that elliptical motion could be produced by the combined action of two forces—one impulsive and the other central—and that the particular form of the ellipse depended upon the relative intensities of the two forces. Although in the experiment the attractive force was at the center of the ellipse, whilst in the case of planetary motion it was at one of the foci, still the fact exhibited must have been highly suggestive to any subsequent inquirer as to the cause of planetary motion.
In 1674 Hooke published a dissertation entitled 'An Attempt to prove the motion of the earth by observations,' in which he says: "I shall hereafter explain a system to the world, differing in many particulars from any yet known, depending upon three suppositions." The first—which he gave at some length—is a distinct statement of the universality of the attraction of gravitation. The second is substantially Kepler's law of inertia. The third is "that the attractive powers of the heavenly bodies are so much the more powerful, by how much nearer the body wrought upon is to their own centers." And, he adds, "Now what these several degrees are, I have not yet experimentally verified, but it is a notion which, if fully prosecuted, as it ought to be, will mightily assist the astronomers to reduce all celestial motions to a certain rule, which, I doubt not, will never be done without it." From this declaration it is evident, first, that at this time he was still in doubt as to the true law of gravitation; and second, that he was endeavoring to discover it by experiment—a method by which he could never have arrived at the truth. A few years later, as appears from his correspondence with Newton, Wren and Halley, he was fully convinced that the intensity of the attraction of gravitation was inversely as the square of the distance, and he even professed to be able to furnish a demonstration. In this he was either insincere at the time or discovered subsequently that his supposed demonstration was defective, as he never presented it, though repeatedly urged by Wren and Halley to do so.
We are now prepared to understand and appreciate aright the precise work which Newton performed in connection with the discovery of the law of gravitation. Born on Christmas day of the year 1642, the year in which Galileo died, in 1665 we find Newton a student of Trinity College, Cambridge, which he had entered in 1660. But
