en. v.] PLANETARY EVOLUTION. 365 



structure of the solar system, otherwise apparently inexpli- 

 cable, are beautifully explained by the theory of nebular 

 genesis. Let us first consider a case which would appear to 

 be an obstacle, not only to this, but to any other frameable 

 theory. We have already hinted that Uranus, while revolv- 

 ing in the same direction with the other planets, has a back- 

 ward rotation, so that to an observer placed upon Uranus the 

 sun would seem to rise in the west and set in the east. His 

 moons revolve about him in the same retrograde direction ; 

 and his axis, instead of standing at a great angle to his orbit- 

 plane, as is the case with all the nearer planets, lies down 

 almost upon the orbit-plane. It has been asserted that these 

 peculiarities are also manifested by Neptune; though our 

 opportunities for observing the latter planet are so few that 

 this point cannot yet be regarded as established. Why now 

 should such exceptional phenomena be manifested in the 

 case of either or both of these outermost planets? In his 

 essay on the Nebular Hypothesis, Mr. Spencer has shown 

 that these phenomena may be explained by a reference to the 

 shape of the rings from which the outermost planets were 

 formed. When the solar nebula was so large as to fill the 

 orbit of Neptune, its rotation must have been slower, and its 

 figure consequently less oblate, than at later stages of con- 

 traction. Now the ring detached from a very oblate spheroid, 

 which bulges greatly at the equator, must obviously be 

 shaped like a flat quoit, as is the case with Saturn's rings ; 

 wnile 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 occupied by these rings, compared 

 with the moderate sizes of the resulting planets, we see that 

 they must have been very thin hoops. Now in such a hoop 



