472 PROCEEDINGS OF THE ACADEMY OF [August. 



A POSSIBLE PARTIAL EXPLANATION OF THE VISIBILITY AND BRILLIANCY 



OF COMETS. 



BY DANIEL M. BARRINGER. 

 WITH AN ADDENDUM BY ELIHU THOMSON. 



From a careful study of the so-called "shale-ball" meteorites 

 described in my previous papers on the Meteor Crater of Arizona, I 

 am impelled to make a suggestion that their peculiar and more or 

 less uniform shape may give us a hint of at least a partial cause of 

 the brilliancy of the head of comets and the gradually fading visi- 

 bility of their tails. 



It should not be forgotten that these so-called "shale-ball" 

 meteorites have never before been described, and we would know 

 little of their original shape were it not that the several hundred 

 which have been found by us have been dug out of exceeding finely 

 pulverized silicious dust, so abundantly found with larger rock 

 fragments on the rim of the crater. This finely divided silicious 

 dust is due to the pulverization of a portion of the 1,000-foot white 

 or gray sandstone stratum during the passage through it of the 

 meteoric mass which, by its impact with the earth, as is now 

 proved, made the crater described in my previous papers. The 

 great mass of this exceedingly finely pulverized "silica," so-called, 

 most of which is so fine that it will easily pass a 100-mesh screen, is 

 practically impervious to water, hence these shale-ball meteorites 

 in many cases have retained their original shapes. As previously 

 stated by me, they have no sharp corners, but are, generally speaking, 

 either round, oval or pear shaped, and in fact closely resemble in 

 shape ordinary river gravel. In nearly all cases oxidation has 

 penetrated from the outer surface inward for varying distances, but 

 in most cases not sufficiently far to make it impossible to know what 

 the original shape of the iron mass was when it fell to the earth and 

 was imbedded in the outpouring from the crater, like flour from a 

 barrel, of the finely divided silica dust commingled with rock frag- 

 ments. In all of the many thousands of iron meteorites found around 

 the crater only one aerolite, or perhaps more accurately siderolite, 

 has been found. This was found by me on June 24, 1905. Inasmuch 

 as by far the greater part of this aerolite was stony in nature, it had 



