76 



GEOLOGY. 



Fig. 30.— A diagram intended to illustrate the proportion of normal cases of collision 

 that will tend to give forward rotation as against retrograde rotation, a uniform 

 distribution of planetesimals being assumed. To avoid unnecessary complication, 

 a belt of planetesimals in the minimum normal orbits that permit collision, and 

 another belt of those in the maximum normal orbits that permit collision, each 

 having the breadth of the planetary nucleus, are chosen, as these are the cases of 

 greatest differential velocities, and hence the most effective in producing rotation. 

 The intermediate cases are not only less effective because of less differential veloc- 

 ity, but because the collisions of the two classes are more nearly equal in number 

 and more nearly neutralize one another. In inspecting, let it be noted that the 

 nucleus, E, at the left, is moving faster than the planetesimals in the area of pos- 

 sible collision, and hence that the planetesimals which it overtakes on its inner 

 side tend to cause forward rotation, while those which it overtakes on its outer 

 side tend to produce retrograde rotation. The collisions in the larger area, F, 

 favoring forward rotation, are much more numerous than those in the small area, 

 R, favoring retrograde rotation. At the right, the planetesimals are moving 

 faster than the nucleus E', and those that overtake it on the outside in the area 

 F / tend to forward rotation, while those that overtake it on the inner side in 

 the area R' tend to retrograde rotation. In both cases, forward rotation is 

 favored. If belts between these limiting belts be drawn, the difference between 

 the two classes of areas will be less, but of the same phases. 





