PREFACE. Vii 



me. It may perhaps be well to mention here that, owing 

 to the complete severance of communication between Great 

 Britain and the enemy countries since the beginning of 

 hostilities, I have no knowledge of any literature which may 

 have appeared in the latter countries during the past two 

 years and a half. 



In conclusion, I must acknowledge my obligation to 

 Dr. G. A. K. Marshall for the punctilious care which he 

 has devoted to the preparation of my manuscript for the 

 press and the detection of errors and discrepancies, to 

 Mr. Horace Knight for the great pains he has bestowed 

 upon securing the accuracy, as well as the artistic finish, 

 of the illustrations, and, lastly, to my wife for relieving me 

 of much of the more mechanical part of my work. 



G. J. ARROW. 



March, 1917. 



