146 HISTOEY OF ASTRONOMY. 



comet should be at that time, from calculations based on 

 its path when seen in 1832. No doubt then Biela has 

 been separated into two parts, and the separation probably 

 took place in the latter part of the year 1844. With re- 

 gard to this last conclusion there is perhaps some room 

 for question. The extraordinary changes of brightness 

 which the two nuclei exhibited both in 1846 and 1852 

 clearly indicate that the brightness of these objects does 

 not depend merely upon their distance from the earth 

 and sun, but upon other unknown causes. These causes 

 might have developed sufficient brightness in the com- 

 panion, at its last two returns to the sun, to render it 

 visible to us ; while at its former returns, on account of 

 its unfavorable position, the companion was too faint to 

 be noticed. 



"What has caused this separation of the comet into two 

 portions ? Was it caused by collision with some foreign 

 body ? Such a collision would have materially changed 

 the figure of the orbit, and therefore we can not suppose 

 it to have taken place since the observation of the comet 

 in 1772, when it was found to be pursuing nearly the 

 same path as at present. It is probable that in case of an 

 encounter with some other body, both bodies would have 

 moved on in some new orbit. 



Was it caused by an explosion arising from some in- 

 ternal force ? Forces of this kind we see in operation in 

 our own globe, ejecting liquid mountains from the bowels 

 of the earth. The surface of our moon bears marks of 

 similar agency the sun appears agitated by powerful 



