COMETS. 199 



and reaches 31*^ All the hitherto discovered interior com- 

 ets have, jiko the principal and secondary planets of the en- 

 tire solar system, a direct motion (from west to east, pro- 

 ceeding in th«^ir orbits). Sir John Herschel has directed at- 

 tei *;ion to the great rarity of retrograde motion of comets 

 ^lainng a slight inclination to the plane of the ecliiitic.^ 

 Thii opposite direction of motion, which occurs only with a 

 certam class of planetary bodies, is of great importance in 

 reference to the very universally prevaihng opinion as to thf 

 formation of the planetary bodies belonging to one system, 

 and as to the primitive, impulsive, and projectile force. It 

 shows us the cmnetary 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 olderf than the planets — as it were primitive 

 forms of the loosely aggregating matter in space. Under 

 these presuppositions, it becomes a question whether, not- 

 withstanding the enormous distance of the nearest fixed 

 stars, whose parallax we know from the aphelion of the 

 Comet of 1680, some of the comets which appear in the 

 heavens may not be merely wanderers, through our solar 

 system, moving from one Sun to another ? 



Next in order to the group of comets, I shall speak of the 

 ring of the zodiacal light, as v/ith grea4; probability belong- 

 ing to our solar region, and after that of the swarms 'tf inc- 

 teoric asteroids which sometimes fall upon our eartfr, and 

 with regard to whoso existence, as bodies in spaco, by no 

 means unanimous opinions prevail. As, in acf;(3rd*anco with 

 the course adopted by Chladni, Olbers, Laplace, Arago, Sir 

 John Herschel, and Bessel, I consider the aerolites to bo of 

 decidedly extra-terrestrial cosmical origin, I may venture, at 

 the conclusion of the section upon the planets, confidently to 

 express the expectation that, by continued accuracy in the 

 observation of aerolites, fire-balls, and shooting- stars, the op- 

 posite opinion will disappear in the same way that the opin- 



* Oxitlims, $ 601. 



t Laplace, Expos, du Systtme du Monde, p. 396 and 414. The special 

 view of Laplace as to the comets being " wandeiniig nebulae" (petites 

 nebnlouses errantes de systemes en systeraes solaires), " stands in op- 

 position to the progress which has been made since the death ol iho 

 great man, in the resolvability of so many nebulous spots into crow ded 

 neaps of stars; the circumstance, also, that the comets have a portion 

 o{ reflected polarized light, which the self-luminous bodies are destitute 

 of. Compare Cosmos, vol. iii., p. 142 ; vol. iv., p. 22. 



