COMETS. 183 



of a comiioii uiiffixi of tli'e asteroids between Mars ami Ju 

 piter, with some or even all of the comets. The grounds of 

 analogy which have been deduced from the nebulous envel- 

 opes of the asteroids must, according to all more recent and 

 accurate observations, be renounced. The orbits of the small 

 planets are not parallel to each other ; that of Pallas certain- 

 ly 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, ap- 

 pears, besides the difference in the physical constitution of the 

 interior comets, and the entirely vaporless small 2^lanets, to 

 render the similarity of origin of both kinds of cosraical bodies 

 very improbable. Laplace, also, in his theory of planetary 

 genesis from rings of vapor revolving round the Sun, in which 

 matter aggregates into spheres around a nucleus, considered 

 it necessary to separate the comets from the planets : " Dans 

 V hyiDothcse des zones de vapeurs et d\pi noyau s'accroissant 

 'par la condensation de V atmosphere qui Venvironne, les co- 

 mctes sont etrangeres au systhne planetaireT^ " According 

 to the hypothesis of zones of vapor, and of a nucleus increas- 

 ing by the condensation of the atmosphere which surrounds 

 them, the comets are strangers to the planetary system. ' 



"VYe have already diiected attention, in the Delineations of 

 Kature,^ to the fact that the comets at the same time pos- 

 sess the smallest mass, and occupy the largest space, of any 

 bodies in the solar regions ; in their number, also, they ex- 

 ceed all other planetary bodies ; the theory of probabilities, 

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

 the boundaries, the p)erihelions, and the possibility that some 



20, p. 181. The author distinguishes, with Hind (Schum., Astr. Nachr., 

 No. 724), "the comets of short period, whose semi-axes are ail nearly 

 the same with those of the small planets between Mars and Jupiter; 

 and the other class, including the comets whose mean distance or semi- 

 axis is somewhat less than that of Uranus." He concludes the first es- 

 say with this remark: '' DitFerent facts and coincidences agree in indi- 

 cating a near appulse, if not an actual collision, of Mars with a large 

 comet in 1315 or 1316, that the comet was thereby broken into three 

 parts, whose orbits (it may be presumed) received even then their pres- 

 ent form, viz., that still presented by the Comets of 1812, 1815, and 

 1846, which are fragments of the dissevered comet.'' 



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



t On Comets : in the Delineation of Nature, s^-e Cosmos, vol. i , p 



ino-110 



