424 COSMOS. 



Egeria (0-086), and Vesta, (0*089) have orbits less eccentric 

 than Mars (0'093), without, however, attaining to the approxi- 

 mative circular orbits of the other planets (Jupiter, Saturn, 

 and Uranus). The diameter of the telescopic planets is immea- 

 surably small; and according 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 580 geographical miles in diameter ; that is, -|- of that of 

 Mercury, y 1 ^ that of the earth. 



If the 4 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 4 exterior planets, those which are more remote from the 

 sun, situated between the ring of asteroids, and the, to us, 

 unknown 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 number 

 of moons greater in the proportion of 20 to 1. The interior 

 planets are all smaller than the earth (Mercury and Mars 

 ~ and \ 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 y 1 ^; 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 ; Saturn, indeed, is only i, almost 

 only half the density of the other exterior planets and the sun. 

 The exterior planets present the solitary phenomenon of the 

 whole solar system, the wonderful circumstance of one of its 

 principal planets being surrounded by an unattached ring; 



