FIRST EVENING DISCOURSE. 017 



question, ' What curve will a particle describe if attracted to a centre by the 

 force of gravitation ? ' He journeyed specially to Cambridge to ask this question ; 

 and never has a more niomentous journey of an Oxford man to Cambridge 

 been taken. Newton was able to reply, to Halley's delight, ' An ellipse.' He 

 had solved the problem some time before, and then tossed it aside as solved — 

 so carelessly that he could uot find the solution. But he sent another solution to 

 Halley later, with much other new knowledge, which developed with extraordhiary 

 rapidity into the ' Principia.' When this was presented to the Royal Society they 

 had no funds to publish it ; but Halley, though then a poor man, had been formerly 

 a rich one, and retained the rich man's contempt for financial difficulties. He pub- 

 lished the ' Principia ' at his own expense, showing the same enthusiasm which 

 was capable of taking him all over the world as captain of his own ship to try 

 and solve the longitude problem, and later of gluing him to his chair to com- 

 pute cometary orbits with unprecedented labour. It was a tremendous epoch in 

 scientific history ; and the smallest details relating to it are of interest. Fresh 

 light was thrown upon it some years ago by the rediscovery of an autograph 

 letter from Newton to Hooke dated November 28, 1679. Hooke claimed that ue 

 had himself been the first to announce the solution of the question above men- 

 tioned, put by Halley to Newton, and asserted that Newton had in the first 

 instance supposed that an attracted body would move in a spiral to the centre, 

 and that he (Hooke) had first told Newton that the curve would be an ellipse. 

 The letter recently discovered and now in the possession of Trinity College, 

 Cambridge, is the actual letter wherein Newton draws a spiral curve ; but it 

 also makes it clear that he was at the moment, considering a totally different 

 problem, viz., what would be the path relative to the rotating earth (not the orbit 

 in space) of a falling body. This most interesting letter therefore bridges a gap in 

 a most important episode. 



Halley's Comet returned, as he had predicted, ' about 1758 '—really in 1759,_ 

 a little later than his rough date. The delay was due to the perturbing action of 

 the planets, and had been anticipated by calculation; so that Halley's prediction, 

 in making which such perturbations had been expressly recognised, was the more 

 completely verified. The comet went round once again and reappeared in 

 1835 ; and now we are eagerly awaiting the next return in 1910. Calculations 

 of the circumstances of return have meantime been chiefly made by foreign 

 astronomers, but in the last year or two Messrs. Crommelin and Cowell, of the 

 Royal Observatory at Greenwich, have done splendid work of this kind. Mr. 

 Cowell has suggested a method of work which greatly shortens the calculations, 

 and, with the able assistance of Mr. Crommelin, mistakes in other calculations 

 have been rectified, and a prediction made which should be close to the truth. 

 The comet will be brightest in May 1910, but a search will be made for it much 

 . earlier; indeed the search was commenced last autumn, though without success. 

 It will be renewed this autumn, when success is more probable, and the comet 

 when found will be followed along its orbit with the greatest interest by many 

 telescopes. , , • <, 



But although England until recently left to others the exact calculation ot 

 modern returns, a fine piece of work on the past history of the comet is due to an 

 Englishman, Mr. J. R. Hind. lu 1849 he examined old records, especially the 

 Chinese annals, and collected accounts of remarkable comets which could fairly be 

 identified with Halley's.^ 



The following is his list : — 



Probable Early Returns of Halley's Comet (Bind). 



A.D. 1456 1145 837 530 218 



1378 1066 760 451 141 



1301 y8<J 684 373 66 A.D 



1223 912 608 295 13 B.C. 



The comet of 1066 is represented on the Bayeux tapestry, and was held 



• Monthly Notices R.A.8., vol. x. p. 51. 



3p3 



