in. v.J PLANETARY EVOLUTION. 367 



only ought Mercury to be the smallest, but Neptune ought 

 to be the largest. The facts, however, do not accord with 

 this view. The four outer planets are indeed much larger 

 than the four inner ones. But of the inner group the largest 

 is not Mars, but the earth ; while in the outer group we find 

 Jupiter three-and-a-half times as large as Saturn, which in 

 turn is seven times larger than Uranus. Now the key to 

 these apparent anomalies must, I think, be sought in the 

 shapes of the rings from which the planets were respectively 

 formed. Neptune and Uranus, formed from very thin hoop- 

 like rings, at a period when the solar equator protruded but 

 slightly, are indeed large planets, but not so large as would 

 be inferred from the size of their orbits alone. But as the 

 solar nebula continued to contract, its increasing equatorial 

 \ elocity rendered it more and more oblate in figure, so that 

 the rings next detached were quoit-shaped. Hence the 

 resulting planets not only had their major diameters but 

 little inclined to their orbit-planes, but they were also larger 

 in size. The very broad quoits which gave rise to Jupiter 

 and Saturn may well have contained more than fourteen times 

 as much planetary matter as the extensive but slender hoops 

 which formed the two oldest planets. If instead of looking 

 at the sizes of the resulting planets, we consider the thick- 

 nesses of the genetic rings, as determined by comparing the 

 size of a planet with the size of its orbit, we shall see that, 

 from Neptune to Jupiter, there was a regular increase in the 

 thickness of the rings, such as the theory might lead us to 

 anticipate. 



But now after the separation of Jupiter from the parent- 

 mass, we encounter a break in this series of phenomena. The 

 thickness of the detached rings sinks to a minimum in the 

 case of the asteroids, and then steadily increases again until 

 in Mercury there is once more an approach to the quoit- 

 shape. Observe the curious sequence of facts, which hitherto, 

 so far as I know, has never been noticed by any of the writers 



