PEEFACE. ix 



reading of the greater part of the volume — a work which he has performed 

 with care and skill. 



For the last ten years I have habitually laid before Professor Henry 

 all of my scientific work, and have during that time received the benefit 

 of his judgment on these matters, and to a great extent I am indebted to 

 him for advice, encouragement, and influence. In expressing my gratitude 

 to the Professor, I beg also to express the hope that the results of my work 

 will not wholly disappoint him. 



J. W. POWELL. 



