vi PREFACE 



The writer wishes to expiress his thanks to Prof. William 



T. Hornei, Professor Guy West Wilsqp, Professor G. W. ilartin, 



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



valtiable suggestions and for assistance in reading the manuscript 



and proof. Also to Miss Gerti-ude E. Macpherson, Mr. K. F. 



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



by them. 



The AtTTHOB. 

 August, 1920. 



