PLANETARY EVOLUTION 



oblate spheroid, which bulges greatly at the 

 equator, must obviously be shaped like a flat 

 quoit, as is the case with Saturn's rings ; while 

 conversely the ring detached from a spheroid 

 which bulges comparatively little at the equator, 

 will approximate to the shape of a hoop. Hence 

 the rings which gave rise to Neptune and 

 Uranus, having been detached before the solar 

 nebula had attained the maximum of oblateness, 

 are likely to have been hoop-shaped ; and when 

 we consider the enormous circumferences occu- 

 pied by these rings, compared with the moder- 

 ate sizes of the resulting planets, we see that 

 they must have been very thin hoops. Now 

 in such a hoop the angular velocities of the 

 inner and outer surfaces respectively will be 

 nearly equal, and the planetary mass into which 

 such a hoop concentrates will have its greatest 

 diameter at right angles (or nearly so) to the 

 plane of its orbit ; so that its tendency to ro- 

 tate in the line of its revolution will be so slight 

 as to be easily overcome by any one of a hun- 

 dred possible disturbing circumstances. With- 

 out feeling required to point out the precise 

 nature of such circumstances, we may readily see 

 that, in the case of the outermost planets, the 

 causes which ordinarily make the rotation co- 

 incide with the line of revolution were at their 

 minimum of efficiency. So that this retro- 

 grade rotation of Uranus, though not perhaps 

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