80 COSMOS. 



by eighteen secondary planets (moons or satellites). Tht 

 principal planets are therefore themselves the central bodies 

 of subordinate systems. We seem to recognise in the fabric 

 of the universe the same process of arrangement so frequently 

 exhibited in the development of organic life, where we find in 

 the manifold combinations of groups of plants or animals, the 

 same typical form repeated in the subordinate classes. The 

 secondary planets or satellites are more frequent in the ex- 

 ternal region of the planetary system, lying beyond the inter- 

 secting orbits of the smaller planets or asteroids ; in the inner 

 region none of the planets are attended by satellites, with the 

 exception of the Earth, whose moon is relatively of great 

 magnitude, since its diameter is equal to a fourth of that of 

 the Earth; whilst the diameter of the largest of all known 

 secondary planets the sixth satellite of Saturn is probably 

 about one-seventeenth, and the largest of Jupiter's moons 

 the third, only about one twenty-sixth part that of the primary 

 planet or central body. The planets which &re attended by 

 the largest number of satellites are most remote from the 

 Sun, and are at the same time the largest, most compressed 

 at the poles, and the least dense. According to the most 

 recent measurements of Madler, Uranus has a greater plane- 

 tary compression than any other of the planets, viz. -g.^-%. In 

 our Earth and her moon, whose mean distance from one 

 another amounts to 207,200 miles, we find that the differences 

 of mass* and diameter between the two are much less con- 

 siderable than are usually observed to exist between the 

 principal planets and their attendant satellites, or between 

 bodies of different orders in the solar system. Whilst the 

 density of the Moon is five-ninths less than that of the Earth, 

 it would appear, if we may sufficiently depend upon the 

 determinations of their magnitudes and masses, that the 



* Tf, according to Burckhardt's determination, the Moon's radius be 

 0'2725 and its volume w , TlS , its density will be 0'5596, or nearly five- 

 ninths. Compare also Wilh. Beer und H. Madler, der Mond, 2, 10, 

 and Madler, Ast., 157. The material contents of the Moon are, 

 recording to Hauseii, nearly Jj (and according to Madler w '. 8 ) that of 

 the Earth; and its m;iss equal to 5T ' T3 that of the Earth. In the largest 

 of Jupiter's moons, the third, the relations of volume to the central bodj 

 are^ 7r ; and of mass TTI ^. On the polar flattening of Uranus, be 

 Bchum. 4fetm. N'tckr., 1844, Nr. 493. 



