X PREFACE 



Professor Laird, the head of the Department, and 

 Dr. Shields read this book in manuscript during 

 the summer of 1918. Miss Shields has followed the 

 general method and incorporated some of the ma- 

 terial in a course for second year students. For this 

 proximate use of the manuscript and for the sugges- 

 tions which have arisen from it the writer expresses 

 his thanks to Miss Shields. Thanks are also expressed 

 to Dr. K. K. Darrow of the Western Electric Company, 

 who has read the proof. 



J. M. 



RESEARCH LABORATORIES 

 THE WESTERN ELECTRIC COMPANY, INC. 

 March 1, 1919 



