194 LIGHT SCIENCE FOR LEISURE HOURS. 



then summarily dismissed. So long as the comet re- 

 mained within that system, the aphelion of its orbit 

 lay just beyond the orbit of Jupiter, and this would 

 be the case with any comet introduced in a similar 

 manner. But for the coincidence which led to its 

 expulsion, Lexell's comet would have continued to 

 revolve as a short-period comet. It seems also clear, 

 that in the course of many ages, its period and orbit 

 would have grown gradually smaller, through the 

 operation of the same cause (whatever that may be) 

 which is now reducing the period and orbit of Encke's 

 comet. At length it must have attained a path safe 

 within the orbit of the great disturbing planet. In 

 the list of short-period comets, then, we seem to see 

 illustrations of the successive stages through which 

 Lexell's comet would have passed in attaining the sort 

 of orbit in which Encke's comet is now moving. And 

 it seems permissible to assume that all the short-period 

 comets have been introduced to their present position 

 within the solar system by the same cause which led 

 to the temporary appearance of Lexell's comet as a 

 comet of short period that is, by the attractive 

 energy of the planet Jupiter. 



Chambers' 's Journal, July 1868. 



