408 SPECIAL RESULTS IN THE URANOLOGTCAL 



whose aphelia are less distant from the Sun than is a point 

 in the orbit of Neptune : they are Ericke's comet (aphelion 

 4-09); de Vico's (5'02); Brorsen's (5-64); Faye's (5-98); 

 Biela'a (6-19) ; and d' Arrest's (6-44). The Earth's mean 

 distance from the Sun being unity, the paths of all these 

 six interior comets have aphelia situated between Hygeia 

 (3*15) and a limit which is placed almost 1^- distances of 

 the Earth beyond Jupiter (5 -20). The two other comets 

 which have also shorter periods of revolution than Neptune, 

 are Olbers's comet, having a period of 74, and Halley's, 

 having a period of 76 years. Up to the year 1819, when 

 the existence of an interior comet was first recognised by 

 Encke, the above-named two comets were those which had 

 the shortest known periods of revolution. The aphelia of 

 Gibers' s comet of 1815, and of Halley's comet, are situated 

 only 4 and 5-f distances of the Earth from the Sun beyond 

 the limit at or within which, according to the discovery of 

 Neptune, they would be considered interior comets. Al- 

 though the application of the term " interior comet" may 

 undergo alteration by the future discovery of trans-Neptu- 

 nian planets, since the limit which renders comets " inte- 

 rior" is a variable one, yet the term is preferable to that of 

 " comets of short period/' inasmuch as it depends at every 

 epoch of our knowledge on something definite at that epoch. 

 The six interior comets which have now been securely com- 

 puted, only vary indeed in their periods of revolution from 

 3*3 to 7 '4 years; but if the expectation of the return at 

 the end of 16 years of the comet discovered by Peters at 

 Naples, on the 26th of June, 1846 (the 6th comet of the 

 year 1846, with a semi-major axis of 6'32), should be con- 

 firmed ( 6 ? 4 ), it may be anticipated that, as respects the 



