76 



COSMOS. 



beyond that of the Earth,* which appears to us in a pyramidal 

 form, and is known as the Zodiacal Light ; and a host of very 

 small asteroids, whose orbits either intersect, or very nearly 

 approach that of our earth, and which present us with the 

 phenomena of aerolites and falling or shooting stars. When 

 we consider the complication of variously formed bodies which 

 revolve round the Sun in orbits of such dissimilar eccentricity 

 although we may not be disposed, with the immortal author 

 of the Mecanique Celeste, to regard the larger number of comets 

 as nebulous stars, passing from one central system to another,! 

 we yet cannot fail to acknowledge that the planetary system, 

 especially so called, (that is, the group of heavenly bodies 

 which, together with their satellites, revolve with but slightly 

 eccentric orbits round the Sun,) constitutes but a small por- 

 tion of the whole system with respect to individual numbers, 

 if not to mass. 



It has been proposed to consider the telescopic planets, 

 Vesta, Juno, Ceres, and Pallas, with their more closely inter- 

 secting, inclined, and eccentric orbits, as a zone of separation, 

 or as a middle group in space ; and if this view be adopted, we 

 shall discover that the interior planetary group (consisting of 

 Mercury, Venus, the Earth, and Mars,) presents several very 

 striking contrastsj when compared with the exterior group, 

 comprising Jupiter, Saturn, and Uranus. The planets nearest 



the most recent observations, that the mass of Neptune, instead of 

 being, as at first stated, g J ,-, is only about .^ that of the Sun, whilst 

 its periodic time is now given with a greater probability at 166 years, 

 and its mean distance from the Sun nearly 30. The planet appears to 

 have a ring, but as yet no accurate observations have been made regard- 

 ing its system of satellites. See Trans. Astron. Soc., and The Planet 

 Neptune, 1848, by J. P. Nicholl.] Tr. 



* " If there should be molecules in the zones diffused by the atmo- 

 sphere of the Sun of too volatile a nature either to combine with one 

 another or with the planets, we must suppose that they would in circling 

 round that luminary present all the appearances of zodiacal light, with- 

 out opposing any appreciable resistance to the different bodies com- 

 posing the planetary system, either owing to their extreme rarity, or 

 to the similarity existing between their motion and that of the planets 

 with which they come in contact." Laplace, Expos, du Syst. du Monde t 

 (ed. 5.) p. 415. 



f* Laplace, Exp. du Syst. du Monde, pp. 396, 414. 



Littrow, Astronomic, 1825, bd. xi. 107. MUdler, 

 1841, 212. Laplace, Exp. du Syst. du Monde, p. 210. 



