PLANETARY SYSTEMS. 93 



Such general considerations regarding certain characteristic 

 properties appertaining to whole groups, can not, however, be 

 applied with equal justice to the individual planets of every 

 group, nor to the relations between the distances of the revolv- 

 nig planets from the central body, and their absolute size, 

 density, period of rotation, eccentricity, and the inclination of 

 their orbits and the axes. We know as yet of no inherent ne- 

 cessity, no mechanical natural law, similar to the one which 

 teaches us that the squares of the periodic times are propor- 

 tional to the cubes of the major axes, by which the above- 

 named six elements of the planetary bodies and the form of 

 their orbit are made dependent either on one another, or on 

 their mean distance from the Sun. Mars is smaller than the 

 Earth and Venus, although further removed from the Sun 

 than these last-named planets, approaching most nearly in size 

 to Mercury, the nearest planet to the Sun. Saturn is smaller 

 than Jupiter, and yet much larger than Uranus. The zone 

 of the telescopic planets, which have so inconsiderable a vol 

 ume, immediately precede Jupiter (the greatest in size of an) 

 of the planetary bodies), if we consider them with regard to 

 distance from the Sun ; and yet the disks of these small aster- 

 oids, which scarcely admit of measurement, have an areal sur- 

 face not much more than half that of France, Madagascar, or 

 Borneo. However striking may be the extremely small dens- 

 ity of all the colossal planets, which are furthest removed from 

 the Sun, we are yet unable in this respect to recognize any 

 regular succession.* Uranus appears to be denser than Sat- 

 urn, even if we adopt the smaller mass, -34 o o 5> assumed by 

 Lament ; and, notwithstanding the inconsiderable difference 

 of density observed in the mnermost planetary group,t we find 

 both Venus and Mars less dense than the Earth, which lies 

 between them. The time of rotation certainly diminishes 

 with increasing solar distance, but yet it is greater in Mars 

 than in the Earth, and in Saturn than in Jupiter. The el- 



* See Kepler, on the increasing density and volume of the planets in 

 proportion with their increase of distance from the Sun, which is de- 

 scribed as the densest of all the heavenly bodies ; in the Epitome As- 

 iron. Copern. in vii. libros digesta, 1618-1622, p. 420. Leibnitz also in- 

 clined to the opinions of Kepler and Otto von Guericke, that the plan- 

 ets increase in volume in proportion to their increase of distance from 

 the Sun. See his letter to the Magdeburg Burgomaster (Mayence, 

 1671), in Leibnitz, Deutschen Schriften, herausg. von Guhrauer, th. i., 

 I 264. 



t On the arrangement of masses, see Encke, in Schum., Astr. Nachr. 

 1843 Nr. 488, $ 114. 



