COMETS. 19'J 



Rtid readies 31*^ All tl.e hitherto discovered i?ilcrior com- 

 ets have, ]iko the principal and secondary planets of the en- 

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

 ceeJing in th'^ir orbits). Sir John Herschel has directed at- 

 tei ^ion to the great rarity of retrograde motion of conietg 

 ^lamng a sligh': indi nation to the plane of the ecliptic.'^ 

 Thii opposite direction of motion, which occurs only with a 

 certain class of planetary bodies, is of great importance in 

 reference to the very universally prevailing 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 comet ary icorld, 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 iha 

 Comet of 1660, some of the comets which appear in the 

 heavens may not be merely iva7iderers 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 with grea-t probability belong- 

 ing to our solar region, and after that of the sivarms "f Die- 

 teoric asteroids which sometimes fall upon oui^ eart.'', and 

 with regard to whose existence, as bodies in space, f)y no 

 means unanimous opinions prevail. As, in acf ordnance with 

 the course adopted by Chladni, Olbers, Laplace, Arag'.^, Sir 

 John Herschel, and Bessel, I consider the aerolites to be 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- 



* Oiit!in\s, § GOl. 



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

 view of Laplace as to the comets bein^ " wandering nebul;e" (pelites 

 uebnlcuses errantes ile systemes en systeines solaires), "stands in op- 

 jKJsition to the progress which has been made since the death ol ihe 

 groat man, in the regolvability of so many nebulous spots into crovN ded 

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

 o( rejliicted polarized light, whicli the self-luminous bodies are deftituls 

 of. Compare Cosmos, vol. iii., p. 142 ; vol. iv., j,*. r>2. 



