PLANETARY EVOLUTION 



Mars, 1 while they move through a very much 

 greater orbit, it follows that their parent-ring 

 must have been the thinnest of all. In marvel- 

 lous conformity to this general statement, it 

 also happens that the inner planets rotate in 

 planes which diverge more widely from their 

 orbit-planes than in the case of Jupiter and 

 Saturn, though less widely than in the case of 

 Uranus and Neptune. 2 And lastly let us note 



i It may be objected that we have probably not yet dis- 

 covered all the asteroids. Those not yet discovered, however, 

 must obviously be so small that the addition of them to the 

 aggregated mass of those already known would not materially 

 affect the truth of my statement. 



2 Curiously enough, if we examine the different systems of 

 satellites, we find a similar general contrast in size between the 

 members of outer and inner groups. The two outer satellites 

 of Jupiter are much larger than the two inner ones ; and the 

 same relation holds between the four acknowledged satellites 

 of Uranus ; while of the eight Sarurnian satellites, the four 

 outer ones seem to be decidedly larger than the four inner ones. 

 Moreover the largest of Jupiter's moons is not the outermost, 

 but the third ; and of Saturn's moons the largest is not the 

 eighth, but the sixth. To these interesting facts which Mr. 

 Spencer has pointed out, I will add one which he has not ob- 

 served. If instead of looking at the sizes of the moons, we 

 consider the thicknesses of their genetic rings, as determined 

 by comparing the size of a moon with the size of its orbit, we 

 find in the Jovian system a regular increase in the thickness of 

 the rings, from the outermost to the innermost. Similar evi- 

 dence from the Saturnian system is not yet forthcoming, since 

 the masses and even the volumes of Saturn's moons have not 

 yet been determined with sufficient accuracy for this purpose. 

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