PLANETARY SYSTEMS. 77 



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 arc 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 satellites may be ascribed to Uranus, have a quantitative 

 preponderance in the number of their attendant rnoons, which 

 is as seventeen to one. 



Such general considerations regarding certain characteristic 

 properties appertaining to whole groups, cannot, however, be 

 applied with equal justice to the individual planets of every 

 group ; nor to the relations between the distances of the re- 

 volving 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 

 necessity, no mechanical natural law, similar to the one 

 which teaches us that the squares of the periodic times are 

 proportional 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 

 volume, immediately precede Jupiter, (the greatest in size of 

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

 to distance from the Sun ; and yet the discs of these small 

 asteroids, which scarcely admit of measurement, have an areal 

 surface not much more than half that of France, Madagascar, 

 or Borneo. However striking may be the extremely small 

 density of all the colossal planets, which are furthest removed 

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

 regular succession.* Uranus appears to be denser than Saturn, 



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

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



