COMETS. 553 



nomers 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 aplielia are within the orbit 

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

 De Vico (5-02), Brorsen (5-64), 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 comets 

 have aphelia which are situated between Hygiea (3'15), and a 

 limit which is nearly 1 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 Others, 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. Gibers' Comet of 1815, 

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

 situated in their aphelia only 4, and 5f times the Earth's dis- 

 tance from the Sun beyond the limits which would allow of 

 their being considered interior comets. Although the term 

 interior comet may suffer alteration from the discovery of 

 Trans -neptunian planets since the boundary which determines 

 whether a comet is to be called an interior one is changeable, 



et Burin ortu suo merserit, ait ilium discessisse in duas 

 Stellas : quod pneter ilium nemo tradidit. Quis 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. yii. cap. 16. 



tt Edward Biot, Recherches sur les Cometes de la collection 

 de Ma-tuan-lin, in the Comptes rendus, torn. xx. 1845, 

 p. 334. 



