COMETS. 557 



32 years have elapsed. Yvon Villarceau has also given 

 elliptic elements for the last-named in Schumacher's Astron. 

 Naclir. no. 773, and has at the same time with Valz put 

 forward some conjectures as to its identity with the Comet of 

 1678, observed by La Hire, and calculated by Douwes. Two 

 other comets, apparently of from 5 to 6 year periods of revo- 

 lution, are the 3rd of 1819, discovered by Pons, and calculated 

 by Encke ; and the 4th of 1819, discovered by Blanpain, and 

 according to Clausen identical with the 1st of 1743. But 

 neither of these can be classed with those which, from longer 

 and more accurate observations, present a greater certainty 

 and completeness of their elements. 



The inclination of the orbits of the interior comets to the 

 plane of the ecliptic is, upon the whole, small, between 3 and 

 13; that of Brorsen's Comet alone is very considerable, and 

 reaches 31. All the hitherto discovered interior comets have, 

 like the principal and secondary planets of the entire solar 

 system, a direct motion (from west to east, proceeding in their 

 orbits). Sir John Herschel has directed attention to the great 

 rarity of retrograde motion of comets having a slight inclina- 

 tion to the plane of the ecliptic This opposite direction of 

 motion which occurs only with a certain class of planetary 

 bodies, is of great importance in reference to the very univer- 

 sally prevailing opinion as to the formation of the planetary 

 bodies belonging to one system, and as to the primitive, impul- 

 sive, and projectile force. It shows us the cometary world, 

 although subject even at the remotest distances to the attraction 

 of the central body, in greater individuality and independence. 

 Such a mode of viewing the subject has led to the idea of 

 considering the comets as older 33 than the planets as it were 



32 Outlines, 601. 



33 Laplace, Expos, du Systeme du Monde, pp. 396 and 414. 

 The special view of Laplace as to the comets being " wandering 

 nebulae," (petites nebuleuses errantes de systemes en systeme, 

 solaires,) " stands in opposition to the progress which has been 



