PREFACE. vii 



Institute of Technology, not only for favoring me with the 

 two articles (Qu'est-ce que 1'filectricite ? and L'filectricite' 

 Industrielle), but also for giving me many valuable sugges- 

 tions. I must also express my thanks to Professors Cyrus 

 F. Brackett and Henry B. Cornwall, and Mr. Frederic C. 

 Torrey, of Princeton, for their kind assistance in the difficult 

 matter of technical terms. It will give me great pleasure to 

 receive suggestions or corrections from my esteemed col- 

 leagues ; and for any such communications I thank them 

 in advance. 



A. W. H. 



PRINCETON, October, 1894. 



