PREFACE 



ro THE SECOND EDITION, 



Many notes have been added to the text in this 



Edition : the whole has been carefally revised ; and 



the Chapter of Directions, in particular, has been 



greatly augmented, and almost re-written. The 



work is thus brought down to the present state of 



our knowledge on the subject. 



P. H. G. 



London : August, 1856. 



