404 PERIODIC COMETS. 



parabolas or hyperbolas, it seems probable, nevertheless, that such has been 

 the case ; and we may therefore consider with propriety comets to consist of 

 two classes : first, those which revolve round the sun in regular periods, 

 reappearing in the system after equal intervals of time ; and secondly, those 

 which enter it once, and depart from it, never to return. 





