PREFACE ix 



I must also express my gratitude to the numerous authors 

 whose work I have made use of, and whose names are mentioned 

 in the appropriate places. 



I am indebted to the Council of the Royal Society of Arts for 

 permission to make use of the Aldred Lecture which I delivered 

 before the Society in 1909, and which is to a large extent 

 reprinted from their Journal in Chapter XIV. 



ARTHUR DENDY. 



KING'S COLLEGE, LONDON, 

 December, 1911. 



