TEE LAW OF GRAVITATION. 485 



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 

 impiilse 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 demonstra- 

 tion 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 ex- 

 periment 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 167-i 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 par- 

 ticulars 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 substan- 

 tially 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 veri- 

 fied, 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 cer- 

 tain 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 en- 

 deavoring 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 dis- 

 covered 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 pre- 

 cise 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 



