ACKNOWLEDGMENT 
THE substance of several of the essays, or of parts of 
them, that follow, has been printed heretofore in Zhe Field 
(of London), Harper's Magazine, The Popular Science 
Monthly, The Evening Post (of New York), or Frank Leslie's 
Popular Monthly. The author desires to credit these publi- 
cations with this priority, and to thank them for permitting 
him to make this new and revised use of the material. 
