THE ORIGIN OF THE EARTH. 



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that, in the shifting of the inner orbits, it will be possible for the planet- 

 esimals to collide with the inner side of a nucleus earlier than with 

 the outer side, and hence forward rotation is favored. So also, by 

 an examination of the orbits of the outer planetesimals, a similar fact 



Fig. 31. — Diagram showing that in the shifting of orbits, the first contingencies of 

 collision favor forward rotation. B represents a smaller elliptical orbit within a 

 larger one, A. If B be shifted progressively in the direction B' ', B", it will first 

 come into possible collisional relations with A on its inner side, and at this point 

 a body in the orbit A is moving faster than a body in the orbit B, as shown by 

 the large orbit the former describes, and, the collision being on the inner side, 

 forward rotation is favored. 



is made obvious. Thus the order in which the possibilities of collision 

 are brought into effect favors direct rotation. 



The collecting zones. — From the previous discussion it will be 

 seen that a planetary nucleus gathers planetesimals that have orbits 

 both smaller and larger than itself, and hence in effect it sweeps a 

 space both outside and inside its own zone. The breadth of this space 



