10 PREFACE 



Davison for Figs. 23, 27, 37, 38, 44-46, 71, 317; to G. W. 

 Hunter for Figs. 30, 306, 308, 311 ; to Messrs. Blackwelder and 

 Barrows for Figs. 5, 226, 228, 229, 234-237, 242, 244, 256, 257, 

 259, 260, 271, 276, 278, 284; to Messrs. Mayne and Hatch for 

 Figs. 302, 307, 321, 325, 333, 334; to J. G. Coulter for Figs. 43, 

 295; to Dr. C. R. Dryer for Figs. 216, 231, 258, 262, 267, 272, 

 275 ; to E. F. Andrews for Figs. 290-292, 299, 300, 303, 304. 

 Acknowledgment is also made to Miss Alma Waldie, Miss 

 Helen Shriver, Mr. A. H. Rosewig, and Mr. Philip Lee. 



Illustrations have been freely given by The American 

 Radiator Co., General Electric Co., Hershey Chocolate Co., 

 Scientific American, Aluminum Cooking Utensil Co., Goulds 

 Manufacturing Co., Carnegie Institution, Illuminating Engi- 

 neering Society, U. S. Bureau of Fisheries, U. S. Department of 

 Agriculture, Canadian Pacific R.R., Northern Pacific R.R., 

 Johns Hopkins Press, and many other firms and publishers. 



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

 has given the stimulus of his interest and approval. Miss 

 Helen Hill, Librarian of the school, has kept me supplied with 

 the most recent publications of practical scientific interest and 

 has been of invaluable service in numerous ways. Miss 

 Mathilde Droege of the Brearley School, New York, has read 

 the manuscript and made many helpful suggestions. 



Without the cooperation of the science teachers of the 

 William Penn High School, the writing of the book would have 

 been difficult. They have tested and retested projects, have 

 criticized again and again, and have cooperated in every way 

 to make successful the book and the course on which it is based. 

 Among those who have given most freely of their time are 

 Miss Jean Weber and Miss May Laramay. Thanks are also 

 due to Miss Helen Price, Miss Ruth Flanagen, Miss Anna Biddle. 

 and Miss Catherine Way. 



BERTHA MAY CLARK. 



