4 POPULAR SCIENCE MONTHLY. 



ago Prof. James Hall laid the foundation of all future calculations 

 when he made the first instrumental survey of the crest of the falls. 

 The changes in the crest have been measured three times since, 

 and from these surveys the mean recession for nearly half a cen- 

 tury is now known and has been found to be much more rapid 

 than was formerly supposed. If the whole history of the falls of 

 Niagara were thus told, then it would appear that their age is 



Fui. 3. — Falls of Muntmokency. (AIjoliI i'lit lict liif^li.) 



about nine thousand or ten thousand years. Indeed, some writers, 

 among others Mr. Gilbert, took this reduced estimate and minim- 

 ized it to seven thousand years, not knowing or overlooking the 

 history of the river which tells of the changing conditions ; but 

 these views he now abandons. The chronometer needed correc- 

 tion. During a term of several years of actual work, but ex- 

 tended through a decade and a half, for the investigations were 

 often blocked with difficulties, the writer has been able to decipher 



