vi PREFACE 



The author is indebted to various manufacturing companies 

 for their cooperation in supplying material and illustrations for 

 the text; to Professor R. R. Lawrence of the Massachusetts 

 Institute of Technology for his careful review of the manuscript 

 and his many helpful suggestions given during its preparation; 

 and particularly to Professor H. E. Clifford of The Harvard 

 Engineering School, for his helpful advice during the preparation 

 of the manuscript and for the thorough manner in which he 

 has edited the material contained in this volume. 



C. L. D. 



HARVARD UNIVERSITY, CAMBRIDGE, MASS. 

 Jan., 1922. 



