2 



difficult to procure in consequence. Verification of the dates will be 

 found in Bulletin No. 13, a point to be observed in connection with Nos. 

 4 and 5, which were issued without date attached, through the inadver- 

 tence of the printer. In the original issues of Nos. 3, 4, 7, and 12, the 

 printers inserted at the head of each article "Read before the American 

 Philosophical Society" without corresponding date; a statement untrue at 

 .that time, though the papers were all so read at a subsequent date. This 

 insertion was not authorized by the author in any way, and is therefore 

 erased at present, excepting in the case of No. 12, where it was inserted 

 in spite of the especial request to have it omitted. 

 Philadelphia, July 16, 1873. 





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