Il8 ON COMETS. 



Neptune. But here we seemed to have quite a sort of 

 tame comet keeping within bounds, and within call. Of 

 course its return was watched for with eagerness, but 

 alas ! it never made its appearance again. At its next 

 return in 1776 this was well accounted for, as owing to 

 the relative situations of the earth, sun, and comet, it 

 could not have been visible; but at the next, in 1781, 

 the earth was favourably situated, since 5I years would 

 place the sun in the opposite part of its orbit; but 11 

 years in the same, and the calculators for a time were 

 puzzled. The solution of the enigma was a very strange 

 one. The poor comet had got bewildered. It had 

 plunged headlong into the immediate sphere of Jupiter's 

 attraction had intruded, an uninvited guest, into his 

 family circle actually nearer to him than his fourth 

 satellite, and into a situation where Jupiter's attraction 

 for it was two hundred times that of the sun. Of course 

 its course was for a time commanded entirely by this 

 new centre of motion, and the comet was completely 

 diverted from its former orbit. 



(31.) So far all was clear enough. But people began 

 to ask how, with so short a period, and being a tolerably 

 large comet, it had never been seen before 1 Here again 

 Lexell called Jupiter to the rescue. As he had taken 

 away, so it turned out he had given. Jupiter, it will be 

 borne in mind, comes round to the same point of his 

 orbit in 11 years and 10 months; two of the comet's re- 

 volutions w^ould occupy 11 years and 3 months, so that 

 tracing back the comet two revolutions in its ellipse, and 

 Jupiter rather less than one in his circle from the \)\dce 



