946 



EVENING DISCO UKSES. 



superstitious dread of comets, which are now (a) much more familiar and (b) much 

 belter understood. 



(a) They are more familiar through the developmeut of the telescope and 

 increase of assiduity in searching for them. Half a dozen are found every year, 

 and It would be difficult to identify their malign influences. Some are found 

 quite by accident, as when Mr. Holmes in 1892 turned his telescope to look at the 

 nebula in Andromeda and found a comet ; or when IVofessor Barnard found the 

 I'aint trail of a comet on one of his photographic plates. Others spring into view 

 almost suddenly, so brigbt as to be seen by several observers simultaneously. 

 Others, again, are found by most patient searching. Tlie great mHJority are never 

 seen by the naked eye, but can readily be photographed. Such photographs show 

 beautiful structure in the tail, which always points away from tbe sun, as if blown 

 by a current of wind outwards from tbe sun. It seerns probable that the light 

 emitted by the sun acts much as a current of air would, and by its pressure drives 

 away the lighter particles to form the tail. 



(6) They became better understood from the moment when Newton 

 announced the great law of gravitation. It was at once suggested tliat comets 

 might move in orbits round the sun under his attraction. The orbits were 

 clearly not circular like those of the planets, but Newton's work pointed to long 

 ellipses or parabolas as alternative forms. Halley, the devoted disciple of 

 Newton, on being appointed Savilian Professor of Geometry at Oxford in 1704 

 set about the task of computing the orbits of as many comets as had been well 

 enough observed, and in his famous paper, ' A-'-tronomiaj CometicjE Synopsis,' ^ 

 gives computed elements of twenty-four comets, from 1337 to ICUH. This 

 involved almost incredible labour, and is in itself a title to fame. But, as 

 often happens in scientitic work, the most imporlant outcome was unexpected. 

 Three of the orbits were so nearly the same as to suggest a recurrence of the 

 tame comet. The elements were approximately as follows : 



It will be seen that there are .some slight difljjrences ; but the point which 

 chiefly arrests attention is the difl'erence in interval between the returns. Halley 

 ilivined the explanation that this was due to the disturbing action of the planets. 

 It was known that the intervals between successive returns of the planets 

 Jupiter and Saturn were not exactly equal, and he ascribed this to their mutual 

 attractions. How much greater might not such errors be in the case of a comet 

 which moved much more slowly ? He saw no dilhculty that could not be 

 explained, and confidently predicted the return of the comet in 1758, or there- 

 abouts. In a later edition (not published until 1749) of his paper he reflects 

 with just national pride that, though he could n >t hope to live to see the return 

 ! he was born in 165(i and died at the age of eighly-six in 17-12), posteiitv would 

 remember to credit an Knglishman with the prediction. ' Quocirca ai secundum 

 predicta nostra redierit iterum circa annum 1758, hoc primum ab homine Anglo 

 inventum fuisse non inficiabitur teqna posteritas.' 



Tt was in everyway appropriate tliat this grand discovery should fall to 

 Halley. He was, as above mentioned, Newton's devoted disciple, and, more than 

 that. It was largely due to Halley that Newton's 'Principia' was ever published. 

 It was Halley who sought from Newton, after failing elsewhere, the answer to the 



* riul. Trans, vol. xxv., March 1705. 



