PmST EVENING DISCOURSE. 045 



EVENING DISCOURSES. 



FRIDAY, SEPTEMBER 4. 



Halleijs Comet. Bij Professor H. H. Turner, D.Sc, F.R.S. 



The British Association is to meet next year at AVinnipeg. Before it reassembles 

 on this side of the Atlantic in 1910 we may expect to haye seen Halley's Comet, 

 which^last appeared in 1835, and is calculated to return to perihelion in April 

 1910. The comet will not be so striking an object as Donati's in 1858 ; but it is 

 the most famous of all comets for two reasons : — 



(a) Its long sequence of appearances at intervals of about seventy-five years, 

 which have now been verified (by Messrs. Cowell and Crommelin, of the Royal 

 Observatory, Greenwich) back to 240 B.C. 



(6) The" circumstances under which it became associated with the name of 

 Halley, who in|1705 first realised its periodic returns and predicted that of 1759. 



Halley's discovery of the periodicity followed naturally from the great work of 

 Newton on gravitation, which first suggested the character of the movements of 

 comets.fjNewton's ' Priucipia ' was presented to the Royal Society in 1686. Only 

 twenty years earlier, in the very first number of the ' Philosophical Transactions,' 

 we find anV'attempt by a certain Monsieur Auzout to predict the motion of the 

 comet of 1664. And he claims this as a ' design, which never yet was undertaken 

 by any Astronomer ; all the World having been hitherto perswaded, that the 

 motions of Comets were so irregular, that they could not be reduced to any Laws.' 

 M, Auzout, however, got no further than the suggestion that the orbit lay iu 

 a plane ; and in'a later paper he extends his method to the comet of 1665. 



What is not understood is apt to be disquieting and even terrifying, and 

 comets had been in olden times, and were still, at that date, regarded as causing 

 pestilence and war. Milton writes in ' Paradise Lost,' Book II. 708 :— 



like a comet burn'd 

 That fires the length of Ophiuchus huge 

 In th' Arctic sky, and from his horrid hair 

 Shakes pestilence and war. 



' Paradise Lost ' was finished in 1665, and these words have been supposed to 

 refer to the very comets of 1664 and 1665 considered by M. Auzout, which were 

 held responsible for the Butch War and the Great Plague of London. But 

 Milton was by that time blind and did not see these comets. He probably had in 

 his mind, at any rate iu addition, the much larger comet of 1618, which he 

 had seen as a bov of ten, and which had a tail 104° long, and was actually in 

 Ophiuchus, as suggested in the passage quoted. The great Thirty Years' War was 

 attributed to this comet (see Evelyn's ' Diiry '). , , . 



In the few centuries elapsed since Milton the world has forgotten ita 



1908. 3 p 



