AUTHOR’S PREFACE. 
The present paper was read before the American Philosophical Society, 
June, 1870. Unfortunately, this Society publishes but a small edition of its 
Proceedings, the serial in which this article appeared, and only a part of 
this is sold. The number for the first half of 1870 is now on this account 
out of print, excepting a few copies retained for exchange. The demand 
for the present essay has been such as to induce me to reprint it with the 
folio in which it originally appeared. I use the opportunity to add a few 
notes at its close. 
Philadelphia, March 15, 1877. 
