ORIGIN OF THE SOLAR SYSTEM 61 



the diameter of the planets : if we can grant then that these 

 planets were formerly much more distended than at present, 

 the viscosity remaining the same, the available time may be 

 adequate. At the same time, solar tidal friction may be 

 adequate to explain the facts that one of the satellites of Mars 

 and the particles at the inner edge of Saturn's ring revolve 

 more rapidly than their primaries rotate, which would not be 

 the case on the unmodified Nebular Hypothesis. 



Direct rotation and revolution of satellites on the Plane- 

 tesimal Theory are shown by Moulton to be probable as a result 

 of a very ingenious argument involving the mode of accretion. 

 Whether it is quantitatively adequate is not proved, and the 

 present writer would prefer to regard these motions as having 

 been direct since the initial disruption. Let us suppose, for 

 instance, that disruption would occur when the disruptive force 

 had reached a definite fraction of surface gravity. It can 

 easily be seen that both are proportional to the diameter of 

 the disturbed body, and hence their ratio is independent of it. 

 Other things being equal, then, a nucleus of any size would be 

 equally likely to be broken up and give a set of dependent 

 nuclei, which would then revolve round it in the direct sense. 

 Secondary nuclei expelled at the same time and close together 

 would remain together, and their relative motion might be in 

 either sense. Thus we should expect both direct and retrograde 

 revolutions, but the former would predominate. The fact 

 that the retrograde satellites are on the outsides of their systems 

 is to be attributed partly to the greater stability of retrograde 

 orbits of large size and partly to the fact that they would 

 experience less resistance from the medium. Capture may be 

 possible ; in the present state of our knowledge we can neither 

 affirm nor deny it. Direct rotation is presumably to be attri- 

 buted to the attraction of the disturbing body on the tidal 

 protuberance before and during expulsion, and to secondary 

 nuclei with direct motions falling back into the parent body. 

 Subsequent evolution would take place in a similar way to that 

 indicated by Darwin. 



The Hypothesis of a Heterogeneous Nebula. — A system of 

 nuclei revolving in a tenuous gaseous nebula would experience 

 a viscous resistance from it, and hence would probably evolve 

 in much the same way as See has indicated in the Capture 

 Theory ; accretion must probably be almost negligible, so that 



