ANOMALY IN THE MOVEMENT OF SATELLITES. 267 



will differ considerably from the average plane of the mass ; 

 while the planes of those detached latest will differ from it 

 less. Here, again, inference to a considerable extent agrees 

 with observation. Though the j^rogression is irregular, yet 

 on the average the inclinations decrease on approaching the 

 sun. 



Consider next the movements of the planets on their 

 axes. Laplace alleged as one among other evidences of 

 a common genetic cause, that the planets rotate in a direc- 

 tion the same as that in which they go round the sun, and 

 on axes approximately perpendicular to their orbits. Since 

 he wrote, an exception to this general rule has been discov- 

 ered in the case of Uranus, and another still more recently 

 in the case of Neptune — judging, at least, from the mo- 

 tions of their respective satellites. This anomaly has been 

 thought to throw considerable doubt on his speculation ; 

 and at first sight it does so. But a little reflection will, 

 we believe, show that the anomaly is by no means an insol- 

 uble one ; and that Laplace simply went too far in putting 

 down as a certain result of nebular genesis, what is, in some 

 instances, only a probable result. The cause he pointed 

 out as determining the direction of rotation, is the greater 

 absolute velocity of the outer part of the detached ring. 

 But there are conditions under which this difference of ve- 

 locity may be relatively insignificant, even if it exists : and 

 others in which, though existing to a considerable extent, it 

 will not suffice to determine the direction of rotation. 



Note, in the first place, that in virtue of their origin, 

 the different strata of a concentrating nebulous spheroid, 

 will be very unlikely to move with equal angular veloci- 

 ties : only by friction continued for an indefinite time will 

 their angular velocities be made uniform ; and especially 

 will the outermost strata, for reasons just now assigned, 

 maintain for the longest time their differences of move- 

 ment. Hence, it is possible that in the rings first detached 



