PREFACE. 
The articles collected in this volume, like those 
formerly published under the title of “ Methods 
of Study,” were originally prepared from notes 
of extemporaneous lectures, and first appeared 
in the pages of the Atlantic Monthly. They stil 7 
retain something of the familiarity induced by 
the personal relation of a lecturer to his audi 
ence, so different from the more distant one of 
the author to his reading public. They musl 
indeed be considered as familiar talks on scien 
tific subjects rather than as scientific papers. 
The three concluding chapters on Glaciers are 
introduced with special reference to their geologi 
cal significance, and will be followed in a future 
volume by a number v p artic^s showing the 
former extension of glaciers on this continenl 
as well as in Europe, and giving at some length 
the history of their retreat. This will naturally 
