PREFACE. 



The present volume is the second of a series in which 

 it is proposed to present, year by year, the most important 

 discoveries in the various branches of science, theoretical 

 and applied the selection of subjects being made, prima- 

 rily, on the ground of their absolute importance, as marking 

 the stages of scientific advancement ; and, secondarily, as 

 being of interest to the general reader. A general sum- 

 mary of progress for the year in the different departments, 

 prefixed to the volume, is intended to give a connected 

 sketch of what has actually been accomplished. 



It will, of course, be readily understood that, in the com- 

 pass of a single duodecimo volume, it is impossible to do 

 more than touch very briefly upon what appear to be the 

 more noteworthy subjects. As far as the specialist is con- 

 cerned, he must necessarily have recourse, for full informa- 

 tion, to the Journals or Year-Books devoted to his partic- 

 ular department, of which scarcely any branch of science 

 is at present destitute. 



As no person, however learned in any one direction, is 

 competent to decide upon the relative importance of facts 

 and discoveries in departments other than his own, the ed- 

 itor would be far from arro^atinsr to himself even an av- 

 erage ability in this respect. He has, however, been so 

 fortunate as to secure the collaboration of some of the 

 most eminent men of science in this country ; and among 

 those to whom he has been indebted for the communica- 

 tion of original discoveries, abstracts of what has been done 

 by others, or summaries of progress in their respective 

 departments, he is permitted to mention the names of Pro- 

 fessors Henry, Gill, Harkness, Abbe, Newcomb, and Hay- 

 den, of Washington ; Professors Cope and Leidy, of Phila- 



