ADVERTISEMENT. 



The following memoir was referred for examination to Dr. John Torroy and Dr. 

 F. A. P. Barnard, of Columbia College, New York. They recommended its pub- 

 lication provided certain changes were made in the manuscript. These having 

 been made by the author, the work is published as a part of the series of " Smith- 

 sonian Contributions to Knowledge." 



JOSEPH HENRY, 



Secretary, S. I. 

 Washington, October, 18'i2. 



(iii) 



