398 BRORSEN'S COMETS. [SECT, 



Every twenty- third year, or after seven revolutions of Encke's 

 comet, its greatest proximity to Jupiter takes place, and at 

 that time his attraction increases the period of its revolution 

 by nine days a circumstance which took place in the end 

 of the years 1820 and 1843. But from the position of the 

 bodies there is a diminution of three days in the six following 

 revolutions which reduces the increase to six days in seven 

 revolutions. Thus, before the year 1819, the periodic time 

 of Encke's comet was 1204 days, and it was 1219 days in 

 accomplishing the revolution that ended in 1845. By this 

 progressive increase the orbit of the comet will reach that 

 of Jupiter in seven or eight centuries, and then by the 

 very near approach of the two bodies it will be completely 

 changed. 



At present the earth and Mercury have the most powerful 

 influence on the motions of Encke's and Biela's comets ; and 

 have had for so long a time that, according to the computation 

 of Mr. Airy, the present orbit of the latter was formed by the 

 attraction of the earth, and that of Encke's by the action of 

 Mercury. With regard to the latter comet, that event must 

 have taken place in February 1776. In 1786 Encke's comet 

 had both a tail and a nucleus, now it has neither ; a singular 

 instance of the possibility of their disappearance. 



In 1846 Biela's comet was divided into two distinct bo- 

 dies, by what strange accident is altogether a mystery. 

 The nuclei of the two comets were separated by about 

 150,000 miles, and they travelled together with their tails 

 parallel, and an arch of light over their heads. Till that 

 time Biela's comet never had been seen with a tail. The 

 new head was dull at first, but increased in size and bright- 

 ness till it surpassed its companion in both ; besides, it had 

 a bright flashing diamond-like point in its centre gradually 

 it resumed its dull appearance, and, as its period was com- 

 puted to be eight days longer than that of the original 

 head, they will probably separate before their next return. 



A comet discovered by M. Brorsen of Kiel, on the 26th of 



