1910] on BalUffs Comet. 755 



the law of s^ravitation, and realised, from the form of Kepler's third 

 law, that there might exist a force, varying inversely as the square 

 of the distance, which would approximately explain the movements 

 of the planets round the sun and of the moon round the earth ; 

 possibly also the rate at which bodies fell towards the earth. But 

 nothing practical came of his speculations, for several reasons : he 

 was shy and reserved as to his discoveries, and he realised a grave 

 difficulty which no one else seems to have suspected — viz. that 

 though the huge sun and planets might be considered to attract one 

 another as mere particles when separated by the planetary distances, 

 the same ideas were not applicable to bodies attracted close to the 

 earth's surface. His reserve was overcome by Halley, who visited 

 Cambridge in August, 1684, with the express purpose of finding out 

 how far Newton had got in applying the idea of gravity. Halley 

 was overjoyed to find that Newton had already proved the proposi- 

 tion that bodies attracted gravitationally would describe ellipses ; 

 he insisted on this " and much more " being published, and paid 

 for the publication himself, and his generous insistence was the 

 means of Newton discovermg the wonderful proposition that spheres 

 attract as particles at all (external) distances however small : — the 

 theory of gravity was complete ! The Principia appeared in 1687. 



For nearly twenty years Halley had no opportunity of following 

 up this great success. We may here glance at a few leading events in 

 his life, and it may help us in rememl)ering dates to remark that he 

 was a contemporary of the famous Yicar of Bray. Born under 

 Cromwell on November 8, 1656, educated at St. Paul's School and at 

 Queen's College, Oxford, " in good King Charles's golden days," he 

 undertook a voyage to St. Helena to catalogue the stars of the Southern 

 Hemisphere, and showed his gratitude to the King, who had aided 

 his expedition, by re-naming one of the Southern Constellations the 

 " Royal Oak "—a name which has not survived. From his study of 

 star positions, Halley was led to remark that some of them must have 

 moved, and he thus laid the foundation of our modern knowledge of 

 " proper motions." 



It was in the reign of " Royal James " that Halley published 

 Newton's Principia. Wilham of Orange commissioned Halley a 

 captain in the Royal Navy, and gave him command of H.M. 

 Pink, the Paramour, for " an expedition to improve the knowledge 

 of the longitude and variations of the compasse," an expedition which 

 he carried out with great scientific success and without losing a man 

 from sickness, though he had to return from his first start in conse- 

 quence of mutiny. 



" AVhen gracious Anne Ijecame our Queen," Halley was appointed 

 Savilian Professor of Geometry at Oxford, and it was then that he 

 made his famous discovery a])out the return of comets. George the 

 Fii-st made Halley Astronomer Royal, and he died on January 14, 

 1742, in the reign of George the Second, at the ripe old age of 85. 



