COMETS. 195 



may not acquire different orbits by retardation, i. c, unequal 

 velocity of revolution, and the unequal influence of perturba- 

 tions ? In a treatise already alluded to, Stephen Alexander 

 has attempted to explain the genesis of all the interior com- 

 ets by the assumption of such an hypothesis, certainly but in- 

 adequately founded. In antiquity, also, similar occurrences 

 appear to have been observed, but not sufficiently described. 

 Seneca states, upon the authority, as he himself says, of an 

 unreliable witness, that the comet which was considered to 

 have caused the destruction of the two towns of Helice and 

 Bura separated into two parts. He adds ironically, why has 

 no one seen two comets unite to form one ?* The Chinese 

 astronomers speak of " three dome-formed comets," which ap- 

 peared in the year 896, and pursued their course together.! 

 Among the great number of calculated comets, there are, 

 up to the present time, eight known, whose period of revolu- 

 tion is shorter than that of Neptune. Of these eight, six are 

 interior comets, i.e., such whose aphelia are within the orbit 

 of Neptune, viz., the comets of Encke (aphelion, 4 - 09), of 

 De Vico (5-02), Brorsen (564), Faye (5-93), Biela (6-19), 

 and D'Arrest (6*44). If the distance of the Earth from the 

 Sun is taken as = 1 , the orbits of all these six interior com- 

 ets have aphelia which are situated between Hygeia (3*15), 

 and a limit which is nearly \\ the Earth's distance from the 

 Sun beyond Jupiter. The two other comets, likewise of a 

 shorter period of revolution than Neptune, are the 74-year 

 Comet of Olbers, and the 76-year Comet of Halley. Up to 

 the year 1819, when Encke first discovered the existence of 

 an interior comet, these two latter ones were those of the 

 shortest period among the then calculated comets. Olbers's 

 Comet of 1815, and Halley's Comet are, since the discovery 

 of Neptune, situated in their aphelia only 4 and 5| times the 

 Earth's distance from the Sun — beyond the limits which 

 would allow of their being considered interior comets. Al- 

 though the term interior comet may suffer alteration from the 



* " Ephorus non religiosissima? fidei, 6aepe decipitur, stepe decipit. Si- 

 cut hie Cometem, qui omnium mortalium oculis custoditus est, quia in- 

 gentis rei traxit eventus, cum Helicen et Burin ortu suo merserit, ait 

 ilium discessisse in duas Stellas : quod praeter ilium nemo tradidit. Quia 

 enim posset observare illud momentum, quo Cometes solutus et in duas 

 partes redactus est? Quomodem autem, si est qui viderit Cometem in 

 duas dirimi, nemo vidit fieri ex duabus?" — Seneca, Nat. Qucest., lib. 

 vii., cap. 16. 



t Edward Biot, Reckerches sur les Cometes de la Collection de Ma' 

 tuan-lin, in the Comptes Reyidus, torn, xx., 1845, p. 334. 



