92 COSMOS 



a pyramidal form, and is known as the Zodiacal Light ; an.1 

 a host of very small asteroids, whose orbits either intersect, or 

 very nearly approach, that of our earth, and which present ns 

 with the phenomena of aerolites and falling- or shooting stars 

 When we consider the complication of variously-formed bodicF^ 

 which revolve round the Sun in orbits of such dissimilar ec- 

 centricity — although we may not be disposed, with the im- 

 mortal author of the Mecanique Celeste, to regard the larger 

 number of comets as nebulous stars, passing from one central 

 system to another,* we yet can not lail to acknowledge that 

 the planetary systera, especially so called (that is, the group 

 of heavenly bodies which, together w^ith their satellites, re- 

 volve with but slightly eccentric orbits round the Sun), con- 

 stitutes but a small portion of the whole system with respect 

 to individual numbers, if not to mass. 



It has been proposed to consider the telescopic planets, ^'es- 

 ta, Juno, Ceres, and Pallas, with their more closely intersect- 

 ing, 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 contrastsf when compared with the exterior group, 

 comprising Jupiter, Saturn, and Uranus. The planets near- 

 est the Sun, and consequently included in the inner group, are 

 of more moderate size, denser, rotate more slowly and with 

 nearly equal velocity (their periods of revolution being almost 

 all about 24 hours), are less compressed at the poles, and, with 

 the exception of one, are without satellites. The exterior 

 planets, which are further removed from the Sun, are very 

 considerably larger, have a density five times less, more than 

 twice as great a velocity in the period of their rotation round 

 their axes, are more compressed at the poles, and if six satel- 

 lites may be ascribed to Uranus, have a quantitative prepon- 

 derance in the number of their attendant moons, which is as 

 seventeen to one. 



phere 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 cir- 

 cling round that luminary, present all the appearances of zodiacal light, 

 without 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 MoiuU 

 (ed. 5), p. 415. 



* Laplace, Exp. du Syst. du Monde, p. 396, 414. 



t Lit^row, Astronomic, 18-25, bd. xi., § 107. MJidler Aslron., 1841. 

 $ 212 Laplace, Exp. d» SysL du Monde, p. 210. 



