POETION OF THE COSMOS. COMETS. 407 



tion, and the state of the older comet a short time previously, 

 escaped observation. Did the separated comet become in- 

 visible solely from increasing distance and great faintness of 

 light, or did it dissolve ? Will it again appear and be re- 

 cognised as a companion ? and will Biela's comet on future 

 reappearances present to us similar anomalous phsenomena ? 



The production of a new planetary cosmical body by 

 division, naturally suggests the question, whether, in the 

 multitude of comets revolving round the Sun, several may 

 not have originated, or may originate from time to time, 

 by a similar process ? and whether, by retardation, i. e. un- 

 equal velocity of revolution, and by unequal influence of 

 perturbations, different orbits may not be produced ? In a 

 memoir by Stephen Alexander, to which I have already 

 alluded, it is attempted to explain the origin of all the 

 interior comets by the adoption of such an hypothesis, which 

 cannot, indeed, be said to rest on any adequate foundation. 

 It would seem as if similar cosmical events had been ob- 

 served, but not sufficiently well described, by the ancients. 

 Seneca relates, but, indeed, as he himself states, not on 

 trustworthy testimony, that the comet, to which the 

 downfall of the cities of Helice and Bura was attributed, 

 divided asunder into two parts. He adds, mockingly 

 " Why is it that no one has yet seen two comets unite into 

 one ?" ( 672 ) The Chinese astronomers speak of three 

 " double," or " couples of," comets, which appeared in 896, 

 and went through their course together ( 673 ). 



Among the great number of comets whose course has 

 been computed, there are eight whose periods of revolution 

 are of shorter duration than the period of revolution of Nep- 

 tune. Of these eight, six are interior comets, i. e. comets 



VOL. in. z 



