PREFACE TO SECOND EDITION. 



A NOTHEE edition of these Essays being called for, 

 -LA- I have carefully revised the whole volume, and 

 particularly the scientific articles ; making such ad- 

 ditions as seemed desirable for the more complete 

 elucidation of the subjects under review. It is a 

 striking proof of the rapid advancement of physical 

 science, that even in the short interval of two months 

 since the first edition was published, discoveries have 

 been made known which no scientific record could 

 rightly omit to notice. The addition of a few pages 

 has enabled me, either in the text or notes, to advert 

 to the more remarkable of these discoveries. I should 

 gladly have dwelt much longer on them (as upon many 

 other instances of the progress of physical knowledge), 

 had not the original plan of these articles restricted me 

 to those more general views, of which I have spoken 

 in the preface to the former edition. To this preface 

 I may perhaps be allowed to refer the reader, in ex- 

 planation of the circumstances under which they were 

 written, and the objects they were intended to fulfill. 



July 15, 1862, 



