3C8 COSMIC PIITLOSOrilY [pt. U. 



who have treated of the nebular hypothesis. Since the 

 mass of Mercury is four-fifths that of Venus, while the 

 circumference of his orbit is about one-half that of the orbit 

 of Venus, it follows that his ancestral ring must have been 

 much thicker than that of Venus. Again, the earth is but 

 little larger than Venus, while the circumference of its orbit 

 exceeds that of the latter nearly in the ratio of five to three, 

 so that it must have originated from a thinner ring. Mars, 

 with an orbit exceeding the earth's in the ratio of eight to 

 five, and containing but one-eighth as much planetary matter 

 as the earth, must have been formed from a still thinner ring. 

 And since the asteroids, if all piled together, would not make 

 a planet as large as 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 marvellous 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 

 r.ote that the velocities of the planetary rotations supply 



1 It may be objected tbat we have probably not yet discovered 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. 



3 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 Saturnian satellites, the four outer 

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

 largest of Jupiter's moous is not the outermost, but the third ; and of 

 Saturn's moons the largest is not the eighth, but the sixth. To these inte- 

 resting facts which Mr. Spencer has pointed, out, I will add one which he has 

 not observed. 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 in- 

 crease in the thickness of the rings, from the outermost to the innermost. 

 Similar evidence 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. And concerning the Uranism 

 system our knowledge is still more inadequate. It will be observed, how 

 ever, thac even the facts here fragmentarily collated point clearly to some 



