536 COSMOS. 



certainly presents the phenomenon of an extreme inclination ; 

 but with all the want of parallelism between their own 

 orbits, still they do not intersect in a cometary manner any 

 one of the orbits of the large older, i. e. earlier discovered, 

 planets. This circumstance, so extremely essential in every 

 assumption of a primitive projectile direction and projectile 

 velocity, appears, besides the difference in the physical con- 

 stitution of the interior comets, and the entirely vapourless 

 small planets, to render the similarity of origin of both kinds 

 of cosmical bodies very improbable. Laplace also, in his 

 theory of planetary genesis from rings of vapour revolving 

 round the Sun, in which matter aggregates into spheres 

 around a nucleus, considered it necessary to separate 

 the comets from the planets : " Dans Vliypothese des zones 

 de vapeurs et d'un noyau s 'accroissant par la condensation 

 *? V atmosphere qui Venvironne, les cometes sont etrangeres 

 M systeme planetaire."* "According to the hypothesis of 

 zones of vapour, and of a nucleus increasing by the con- 

 densation of the atmosphere which surrounds them, the comets 

 are strangers to the planetary system." 



We have already directed attention, in the Delineations of 

 Nature]* to the fact that the comets at the same time possess 

 the smallest mass, and occupy the largest space, cf any bodies 

 in the solar regions ; in their number also they exceed all 

 other planetary bodies, the theory of probabilities, applied 

 to the data of the equable distribution of the orbits, the 

 boundaries, the periJielions, and the possibility that some 

 remain invisible, indicates the existence of many thousands. 

 We except the aerolites or meteoric asteroids, as their 

 nature is still enveloped in great obscurity. Among the 



* Laplace, Expos, du Syst. du Monde (ed. 1824), p. 414. 

 4 On Comets: in the Delineation of Nature, see 



vol. i. pp. 8497. 



