iv PREFACE. 



lections. How far lie has succeeded he must leave others 

 to judge. He may, however, be permitted to add and 

 justice to those, to whose friendly aid he owes so much, 

 requires that he should do so that in gathering material 

 and in adapting it fur the purposes in question, he has been 

 so fortunate as to secure the assistance of some of the most 

 eminent men of science of this country, some of whom 

 have, in addition, furnished quite a number of original 

 communications containing accounts of personal discover- 

 ies and observations. 



In preparing the matter of the present volume, the de- 

 sign has been to furnish abstracts only, divesting the differ- 

 ent subjects, as far as practicable, of mere technicalities, 

 and omitting what was not properly relevant. In many 

 instances, indeed, a single article has been made to embrace 

 data from a number of different sources, while not unfre- 

 quently are included original ideas, unpublished elsewhere, 

 and derived from personal investigation or supplied by col- 

 la boration as already mentioned. In very few cases has a 

 1 iteral copy of any article been made, and this only where 

 farther condensation or other change appeared inexpedient. 



A large portion of the contents of this volume has al- 

 ready appeared in the " Editor's Scientific Record" of 

 Harper's Monthly, and the " Scientific Intelligence" of the 

 Weekly ; some of it also in the Philadelphia Public Ledger 

 and elsewhere. It is here, however, arranged somewhat in 

 systematic order for the benefit of special students, and 

 more or less revised, and in book form, with the original 

 references appended, as explained in the table on page 

 G14. The remainder of the book, embracing material 

 which could not be introduced into the journals referred to 

 for want of Bpace, is here presented for the first time. 



Smithsonias Institution, Washington, March 1, 1872. 



