PREFACE. 



This work, to the preparation of which the last sixteen years of Dr. 

 Pickering's life were devoted, forms the last of the contributions to 

 knowledge due to the zeal and industry in the accumulation of facts 

 which early distinguished him. Unhappily, he did not live to see its 

 publication, but died March 17, 1878, while it was passing through the 

 press. 



The manuscript, however, was complete, even including the alphabeti- 

 cal index, the whole of which he had prepared, with the exception of the 

 paging, which it was his custom to render conformable to the printed 

 sheets of the work as fast as he received proofs of them. The manu- 

 script was written with great care, and he had made very few changes 

 in the pages, about six hundred and fifteen in number, which he had 

 seen in type. The remainder of the work has accordingly been printed 

 in exact conformity with the manuscript. 



Three biographical notices of the author have been added ; one from 

 the " Unitarian Review" for April, 1878; another from the "Proceedings" 

 of the American Academy of Arts and Sciences, Boston, Massachusetts; 

 and a third from the " Proceedings " of the Academy of Natural Sciences, 

 Philadelphia, Pennsylvania. 



A steel engraving of a photograph of Dr. Pickering, taken a few 

 years ago, has also been prefixed to the volume. 



S. S. P. 



Boston, U. S. A., May 1, 1879. 



