PREFACE 



Dr. R. O. Greep kindly assembled for me the information 

 which appears in the Appendix. I am grateful to the authors 

 and publishers — American, British, French, and German — 

 who gave me permission to reproduce illustrations and tables. 

 These all receive acknowledgment where the material ap- 

 pears. 



H. B. VAN Dyke 



New Brunswick, New Jersey 

 February 1939 



