vi PREFACE 



The writer wishes to express his thanks to Prof. William 

 T. Home, Professor Guy West Wilson, Professor G. W. Martin, 

 Dr. J. J. Taubenhaus, Dr. W. IT. Martin and many others for 

 valuable suggestions and for assistance in reading the manuscript 

 and proof. Also to Miss Gertrude E. Macpherson, Mr. R. F. 

 Poole and to Mr. E. West for the many illustrations prepared 

 by them. 



THE AUTHOR. 

 August, 1920. 



