COMETS OF SHORT PERIOD. 183 



the only kind of path which they might expect a comet 

 to pursue was a long oval. They accordingly con- 

 fined their calculations, and limited the invention of 

 new mathematical processes, to the case of very eccen- 

 tric orbits. 



But in 1770 a comet appeared which led astrono- 

 mers to form wholly new views. No orbit which could 

 be devised (subject to the above-mentioned condition) 

 could be reconciled with the motions of the new arrival. 

 At length the astronomer Lexell discovered that the 

 path of the comet was not an oval of extreme eccen- 

 tricity, but an ellipse of such a figure that the comet's 

 period of revolution was less than six years. But here 

 a difficulty arose. The comet was sufficiently conspi- 

 cuous ; and it was asked, how could such an object 

 have gone on circulating so rapidly around the sun, 

 and yet have remained undiscovered ? A very singular 

 result rewarded the inquiry into this question. It was 

 found that the aphelion of the comet's path lay just 

 outside the orbit of Jupiter ; and, further, that when 

 the comet was last in aphelion, Jupiter was quite close 

 to it. Thus it became clear that the comet had been 

 travelling in another, and doubtless much wider orbit, 

 when its motions had brought it into the neighbour- 

 hood of the planet Jupiter the giant of the solar 

 system. The comet had actually approached the planet 

 nearer than his fourth satellite. ' It had intruded,' 

 says Sir J. Herschel, ' an uninvited member into his 

 family circle.' 



The result of this close appulse may be readily con- 



