756 



:)//•. Herbert Hall Tamer 



[Feb. 18, 



It was probably lack of leisure, due to his many absorbing interests, 

 which prevented his earlier following up the possibilities of work 

 opened out l\y the great discovery of the law of gravitation. But on 

 being appointed Savilian Professor of Geometry at Oxford in 1704, 

 he set to work to calculate the orbits of as many comets as he could 

 find records of, according to the principles and methods furnished by 

 Newton, and after " incredible labour " published the elements of 24 

 comets. In three cases, the elements bore a close resemblance to one 

 another, as shown in this table : — 



It will be seen that the last four columns have nearly the same 

 figures, and the quadruple coincidence suggested to Halley that the 

 same comet had appeared three times, travelling in an elliptic orldt. 

 One circumstance, however, was puzzling : the interval between the 

 first two appearances was 76 years 2 months, between the second and 

 thii'd 74 years 11 months. But Halley was ready with an explanation 

 which we now know to be correct. He had noticed that the planets 

 Jupiter and Saturn disturbed each other (as they should according to 

 the great law of gravitation). He argued that they might also dis- 

 turb a comet and the effect Ayould l)e much greater : for a not very 

 large disturbance was capable of sending a comet away for ever — 

 outside the sun's sphere of influence : hence a smaller disturbance 

 could lengthen its journey appreciably. 



In fact he saw no difficulty which could not be explained away in 

 concluding that the three sets of elements in Table I. referred really 

 to the same comet ; and he predicted that it would again return in 

 another seventy-five or seventy-six years, say in 1758 or thereabouts. 

 This return he could himself hope to witness (he died in 1742 at the 

 ripe age of eighty-five), l)ut he trusted posterity, when the comet did 

 reappear, to credit an Englishman with the prediction. ' Quocirca 

 si secundum predicta nostra redierit iterum circa annum 1758, lioc 

 primum ab homine Anglo inventum fuisse non inficiabitur aequa 

 posteritas.' * 



Before the reappearance of the comet was due there was ample 



* These words are not in the original paper, but were added in a later 

 edition, which, however, was not published until after HaUey's death, in 1749. 



