viii PREFACE 



developed; hence, a textbook .such as this one should be regarded as an 

 adjunct to this process and never as a substitute for it. 



It is a pleasant duty to acknowledge indebtedness to authors and 

 commercial firms who have so generousl}^ contributed illustrations. 

 The author is especially grateful to his associates at Cornell University 

 for^ their willing help: to Professors Lewis Knudson, Arthur J. Eames, 

 Lowell F. Randolph, Robert T. Clausen, and Dr. Victor M. Cutter for 

 criticisms of certain chapters, to Miss Louise Raynor for executing the 

 original drawings and preparing the index, and to Miss Lillian Piepei- 

 for her assistance with the manuscript. 



Lester W. Shakp. 

 Ithaca, New York, 

 June, 1943. 



