XIV PREFACE. 



this occasion. Mere verbal description can 

 never convey distinct ideas of the form and 

 structure of parts, unless aided by figures ; 

 and these I have accordingly introduced very 

 extensively in the course of the work.* 



Being compelled, from the nature of my 

 subject, and in order to avoid tedious and 

 fatiguing circumlocution, to employ many 

 terms of science, I have been careful to ex- 

 plain the meaning; of each when first intro- 

 duced : but as it might frequently happen that, 

 on a subsequent occurrence, their signification 

 may have been forgotten, the reader will gene- 

 rally find in the index, which I have, with this 

 view, made very copious, a reference to the 

 passage where the term is explained. 



I beg, in this place, to express my deep 

 sense of the obligation conferred on me by 

 Mr. Davies Gilbert, the late president of the 

 Royal Society, to whose kindness I owe my 

 being appointed to write this treatise. 



I also take this opportunity of conveying 

 my best thanks to my friend and colleague, 



* All the wood engravings have been executed by Mr. Byfield, 

 and the drawings for them were, for the most part, made by Miss 

 Catlow, whose assistance on this occasion has been most vahiable 

 to me. 



