PREFATORY NOTE. 



inent is here made. I should not fail to mention also the indispensable 

 facilities and helps of various sorts provided through the unfailing 

 interest and kindness of Dean W. C. Sabine, of the Graduate Schools 

 of Applied Science of Harvard University. Thanks are also due to my 

 assistants, Messrs. S. Wright and H. D. Fish, who have helped with 

 statistical computations as indicated in the text. 



W. E. CASTLE. 



