68 



GEOLOGY, 



The bearings of the rate of infall on temperature. — The rate of 

 accretion is a matter of radical geologic importance; indeed, it is, 

 in some measure, the most critical feature of the whole nebular prob- 

 lem; for the rate of accretion determines whether the average tem- 

 perature on the surface of the growing body will be high or low. The 

 surface temperature is not determined by the total heat produced by 



Fig. 25. — Diagram to illustrate the tendency toward circularity when the orbits of 

 the uniting bodies have concentric positions. The two bodies revolving in the 

 orbits A and B, and uniting at D, necessarily take an intermediate orbit, C, with 

 an obvious advance toward circularity. In less simple and symmetrical cases, the 

 result is less obvious, but it would be of the same order in all cases involved in the 

 problem in hand. 



the collisions, but by the heat produced in a given time which, in turn, 

 is determined by the frequency and force of the collisions on a given area. 

 If the succession of collisions on a given square mile is not rapid enough 

 to generate heat beyond the concurrent radiation from the square mile, 

 a high average temperature for the whole cannot be reached, how- 

 ever great the sum total of heat generated in the course of time. It 



