DIASTROPHISM AND THE FORMATIVE PROCESSES 667 



in its own path under control of its own inertia and the gravitative 

 stresses of its environment. The planetesimals shot out from the 

 nuclei had the simpler history, and are easiest followed to gain 

 typical pictures of planetesimal behavior as a basis for estimates 

 of their modes and rates of infall. 



Let us picture the earth nucleus as pursuing a nearly circular 

 orbit about the sun while certain of its outer molecules were 

 escaping from it in various directions by reason of exceptional 

 velocities given them by cumulative successions of rebounds from 

 favorable collisions. It is easy to fall into the error of supposing 

 that these molecules, thus escaping in different directions, would 

 take orbits quite discordant with that of the nucleus and thus 

 pass into the meteoritic rather than the planetesimal class. As 

 constituents of the nucleus, they already had motions relative to 

 the sun, and of course carried these with them they went into 

 orbits of their own, except in so far as these motions were reduced 

 or increased by their ejection from the nucleus. The velocity of 

 the nucleus in its orbit should have been of the order of eighteen 

 miles per second, and that of all the molecules of the nucleus about 

 the same, some a little more, some a little less, by reason of their 

 participation in rotation, et cet. It was the new and additional 

 velocity which the escaping molecule had been given, measured at 

 the border of the sphere of control of the nucleus, which determined 

 its orbit after it had escaped. Only in extremely rare cases would 

 molecular interaction give to an escaping molecule a speed greater 

 than the parabolic velocity respecting the nucleus, that is a velocity 

 sufficient to carry the molecule to infinity so far as the restraining 

 attraction of the nucleus was concerned. The parabolic velocity 

 of even the full-grown earth at the border of its sphere of control 

 is 1 .75 miles per second, so that we leave a large margin of safety 

 if we assume that molecules almost never were shot away from 

 the border of the sphere of control of the nucleus at more than 

 two miles per second. Now, as the nucleus was moving at eighteen 

 miles per second relative to the sun, a molecule shot directly back- 

 ward would still have a velocity of sixteen miles per second rela- 

 tive to the sun, and a molecule shot directly forward would have 

 a velocity of twenty miles per second relative to the sun, while 



