6 PREFACE 



kindly placed cuts at my disposal, and have thus materially 

 aided in the preparation of the illustrations : American Radi- 

 ator Company; Commercial Museum, Philadelphia; General 

 Electric Company ; Hershey Chocolate Company ; Scientific 

 American; The Goulds Manufacturing Company; Victor 

 Talking Machine Company. 



Mr. W. D. Lewis, Principal of the William Penn High 

 School, has read the manuscript and has given me the benefit 

 of his experience and interest. Miss Helen Hill, librarian 

 of the same school, has been of invaluable service as regards 

 suggestions and proof reading. Miss Droege, of the Baldwin 

 School, Bryn Mawr, has also been of very great service. 

 Practically all of my assistants have given of their time and 

 skill to the preparation of the work, but the list is too long 



for individual mention. 



BERTHA M. CLARK. 

 WILLIAM PENN HIGH SCHOOL. 



