1916.] NATURAL SCIENCES OF PHILADELPHIA. 475 



origin of the Arizona crater and that there is much stronger reason 

 now than ever before to believe in the general correctness of Chamber- 



lin's and Moulton's theory of the building up of planetary systems. 

 It is calculated that some 2,000,000 meteorites reach our atmosphere 

 every twenty-four hours, and it is highly probable that in the early 

 history of the earth it was abundantly bombarded by cometary bodies, 

 that are probably, after all, merely masses of meteoric material which 

 have gotten together and in some way not known to us have assumed 

 orbits of their own. It would seem, however, that most of them, 

 as well as most of the meteoric material originally forming the 

 nebula out of which our solar system has been built up, have long 

 since been gathered into the sun, its planetary bodies or the moons 

 revolving about them. Our moon, having been without an atmos- 

 phere for perhaps a great many millions, if not billions, of years, show- 

 evidences of some of the more recent accretions to its mass, outside of 

 the more or less steady rain of cosmic dust. Its numerous craters 

 probably merely represent the gathering in of cometary bodies or 

 clusters of meteorites, for they are apparently exactly similar to our 

 Arizona crater, except that most of them are vastly larger. 



If this theory of the building up of solar systems be correct, is it 

 not wonderful to reflect that when one holds in the hand one of these 

 pieces of meteoric material he is probably holding something older 

 than our sun, our own earth, or any of the planetary bodies which 

 with their moons revolve about the sun? That is to say, he is 

 literally holding, practically unchanged through countless eons of 

 time, a part of the nebula out of which our solar system was con- 

 structed and which nebula in turn probably represented the wreck- 

 age of a previously existing system. 



Addendum by Elihu Thomson. 



The following comments on the above paper were received by 

 Mr. Barringer from Prof. Elihu Thomson: 



I have your letter of June 14, enclosing a communication which 

 you are thinking of sending to The Academy of Natural Sciences of 

 Philadelphia. I think there is little question that the explanation put 

 forward in your paper as to the rounded shape of the shale-ball 

 meteorites is the true one. I have, in fact, often spoken of the 

 inevitable readjustments that may take place in a small cluster 

 revolving around the sun and the attrition between the parts as 

 accounting for the steady production of very finely divided material 

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