COMETS OF SHORT PERIOD. 193 



come to an object which has recently been rediscovered, 

 and will continue visible (in good telescopes) for 

 several weeks. On February 6, 1846, M. Brorsen 

 discovered a telescopic comet, whose motions soon 

 showed it to belong to the class of objects we are now 

 dealing with. It was found to have an orbit of mode 

 rate eccentricity, extending just beyond Jupiter s orbit, 

 and a period of about five and a half years. It was 

 not seen at its next return to perihelion ; but was re 

 discovered by M. Bruhns on March 18, 1857. In 

 1862, it again escaped undetected; but at its present 

 return, it has been rediscovered (by three observers 

 simultaneously), and it is now being carefully tracked 

 across the northern skies. 



In all, there have been recognised thirteen comets of 

 short period that is, having periods of less than seven 

 years. Amongst these are included several which have 

 only been seen once, and some which are known to have 

 been subjected to such disturbance as no longer to 

 travel in orbits of short period. Of these thirteen 

 comets, no less than ten have the aphelia of their 

 orbits just beyond the orbit of the planet Jupiter ; two 

 have their aphelia just within Jupiter s orbit; and 

 Encke s cornet alone has its aphelion at a safe distance 

 from that orbit. It appears to us that the peculiarity 

 thus exhibited is not without meaning. Eemembering 

 the history of Lexell s comet, we seem to find a satis 

 factory explanation of the peculiarity. We have seen 

 how Lexell s comet was first introduced into the system 

 of short-period comets by the giant planet Jupiter, and 



o 



