83 



of about twenty-four hours, are less compressed at the 

 poles, and, with one exception, that of the Earth, are without 

 satellites. The external planets, more distant from the sun, 

 are of much greater magnitude, five times less dense, more 

 than twice as rapid in their rotation round their axes, more 

 compressed at their poles, and richer in moons in the pro- 

 portion of 17 to 1 ; if Uranus has really six satellites as 

 supposed. 



In viewing these general characteristics of the two groups, 

 we must admit, however, that they cannot be strictly applied 

 to each of the planets in particular ; nor are there any con- 

 stant relations between the distances of the planets from the 

 central body round which they revolve, and their absolute mag- 

 nitudes, their densities, times of rotation, eccentricities, and 

 inclinations of orbit and of axis. We know as yet of no inhe- 

 rent necessity, no natural mechanical law, (such as the great 

 law of the proportionality of the squares of the periodic 

 times to the cubes of the mean distances from the sun), con- 

 necting the above-named six elements of the planets, and 

 the forms of their orbits, either inter se, or with their mean 

 solar distances. We find Mars, though more distant from 

 the Sun than either the Earth or Yenus, inferior to them 

 in magnitude; being, indeed, that one of the long known 

 greater planets which most nearly resembles in size Mercury, 

 the nearest planet to the solar orb. Saturn is less than 

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

 telescopic planets, which are so inconsiderable in point of 

 volume, viewed in the series of distances commencing from 

 the Sun, comes next before Jupiter, the greatest in size of 

 all the planetary bodies ; and yet the disks of these small 

 planets (whose apparent diameters scarcely admit of measure- 



