INTRODUCTORY NOTE. 



This paper was originally an official report presented to the Navy Department by Professor 

 Harkness; but, as that department made no use of it, the National Academy of Sciences, in 

 August, 1867, passed a resolution asking for the manuscript. This request was complied with; 

 and, an abstract of the paper having been read to the Academy in April, 1869, it was referred to a 

 commission consisting of the 'President of the Academy, Professors J. H. C. Coffin, and F. Rogers, 

 in accordance with whose recommendation it is now published by the Smithsonian Institution. 



Joseph Henry, 



Secretary S. I. 



COLLINS, PRINTER, 

 PHILADELPHIA. 



