THE PLANETS. 103 



however, attaining to the approximative circular orbits of the 

 other planets (Jupiter, Saturn, and Uranus). The diameter 

 of the telescopic planets is immeasurably small ; and accord- 

 ing to observations made by Lament in Munich, and Madler 

 with the Dorpat refractor, it is probable that the largest of 

 the small planets is at the utmost only 145 geographical 

 miles in diameter ; that is, one fifth of that of Mercury, one 

 twelfth of that of the Earth. 



If the four planets nearest to the Sun, situated between the 

 ring of the asteroids (the small planets) and the central body, 

 are called interior planets, they will all agree in presenting 

 a moderate size, a greater density, less flattened at the poles, 

 and, at the same time, rotating slowly round their axes (in 

 periods of rotation of nearly 24 hours), and, with the excep- 

 tion of one (the Earth), without moons. On the contrary, the 

 four exterior planets, those which are more remote from the 

 Sun, situated between the ring of asteroids, and the, to us, un- 

 known limits of the solar system (Jupiter, Saturn, Uranus, 

 and Neptune), are considerably larger, five times less dense, 

 their axial rotation more than twice as rapid, and their num- 

 ber of moons greater in the proportion of 20 to 1. The in- 

 terior planets are all smaller than the Earth (Mercury and 

 Mars f and 4- smaller in diameter) ; the exterior planets, on 

 the contrary, are from 4'2 to 11-2 larger than the Earth. 

 The density of the Earth being taken as =1, the densities 

 of Venus and Mars are the same to within less than r ! F ; the 

 density of Mercury is also but very little more, according to 

 Encke's determination of his mass. On the contrary, none 

 of the exterior planets exceed in density 1 ; Saturn, indeed, 

 is only ^, almost only half the density of the other exterior 

 planets and the Sun. The exterior planets present the soli- 

 tary phenomenon of the whole solar system, the wonderful 

 circumstance of one of its principal planets being surrounded 

 by an unattached ring ; also atmospheres which, in conse- 

 quence of the peculiarity of their condensation, appear to us 

 variable ; in Saturn, indeed, sometimes as interrupted bands. 



Although in the important classification of the planets into 

 two groups of interior and exterior planets, the general char- 

 acters of absolute magnitude, density, flattening at the poles, 

 velocity of rotation, absence of moons, present themselves as 

 dependent upon the distances, i. e.,.from their semi-orbital 

 axes, this dependence can not be affirmed of each one of these 

 groups. Up to the present time we are ignorant, as I have 

 already remarked, of any internal necessity, any mechanical 



