vi PREFACE. 



We desire to express our great indebtedness to the various 

 authors who have given us permission to reproduce their 

 diagrams and illustrations. Our requests have in every case 

 been met with the greatest courtesy and consideration. The 

 source of these illustrations is, we believe, acknowledged in each 

 instance. Our thanks are also due to Mr. Lapidge and Mr. C. 

 Douglas for the care they have bestowed upon the preparation of 

 the drawings from which many of the other illustrations have 

 been made. 



C. K. B. 



W. Me A. E. 

 LONDON, W. 

 January, 1906. 



