viii 



PREFACE TO SECOND EDITION 



The sources of those illustrations on the following 

 pages which are not original with the author, are 

 indicated in the legends below the picttires. 



C. M. W. 



A^cz^' HaiupsJurc College of Agrieultnrc and flic Mechanic Arts. 

 Durham, MarcJi, 1895. 



