PORTION OF THE COSMOS. COMETS. 409 



duration of the period of revolution, intermediate links will 

 gradually be discovered between the comets of Olbers and 

 of Faye, and that it will in future be difficult to determine 

 any fixed limit defining " shortness of period." I subjoin 

 the table in which Dr. Galle has collected the elements of 

 the six interior comets (see p. 410). 



It follows from this review, that from the recognition of 

 Encke's ( 675 ) as an interior comet, in 1819, to the discovery 

 of the last interior comet of d' Arrest, 32 years only have 

 elapsed. Elliptic elements for the last-named comet have 

 also been computed by Yvon Yillarceau, in Schumacher's 

 Astr. Nachr. No. 773, who, as well as Valz, has expressed 

 some conjectures respecting its possible identity with the 

 comet of 1678, observed by La Hire, and calculated by 

 Douwes. Two other comets, apparently also having periods 

 of revolution of five or six years, are the 3d of 1819, dis- 

 covered by Pons, and calculated by Encke ; and the 4th of 

 1819, found by Blanpain, and considered by Clausen to 

 be identical with the 1st of 1 743. Neither of these comets, 

 however, can yet be classed with those in regard to which 

 long-continued and exact observations permit greater cer- 

 tainty and completeness in the assigned elements. 



The inclination of the paths of the interior comets to the 

 ecliptic is, generally speaking, small, i. e. between 3 and 

 13: in Brorsen's comet only it is considerable, attaining 3 1. 

 All the interior comets which have yet been discovered, have, 

 like all the planets and satellites of the solar system, a direct 

 motion (advancing in their orbits from west to east). Sir 

 John Herschel has called attention to the greater rarity of 

 retrograde motion in those comets whose degree of inclina- 

 tion to the ecliptic is small ( 676 ). This opposite direction 



