THE ORIGIN OF THE EARTH. 



69 



is to be noted that the heat generated after a solid nucleus was developed 

 must have been superficial, and hence readily radiated away. While 

 the nuclei remained assemblages of small bodies, perhaps gaseous in 

 part in the larger ones, plane tesimals from without may have pene- 



Fig. 26. — -Diagram to illustrate the evolution of circularity where the orbits of the 

 uniting bodies are as far as possible from having concentric positions. The bodies 

 in the orbits A and B, meeting and uniting at D, necessarily take an intermediate 

 orbit C, which is obviously more circular than either. In less symmetrical cases, 

 the tendency would be of the same order. 



trated to the interior, and there developed heat not so readily lost. 

 But this -state is only assignable to the early stages of growth. 



The collisions all overtakes. — A further consideration bearing upon 

 the critical subject of temperature is the manner of collision. Since 

 all the planetesimals and planetary nuclei were revolving in the same 

 direction about the solar mass, the collisions were all overtakes, and 

 could have been violent only to the extent of their differences of orbital 

 velocities, modified by their mutual attractions. Since their orbits 

 were elliptical, their velocities varied considerably in different portions 

 of their paths, and hence overtake collisions of considerable velocity 

 were possible. These velocities were, however, of a much lower order 

 than the average velocities of meteoritic collisions. Many of the 

 overtakes must obviously have been due to differences of velocity barely 

 sufficient to bring about an overtake. When the average mildness of 

 impact is considered, in connection with the intervals between impacts 

 at a given spot, the conviction can scarcely be avoided that the sur- 

 face temperature would not necessarily have been high. It seems prob- 



