J. W. Spencer — Duration of Niagara Falls. 461 



5. Discharge of the Niagara River. 



The corps of Engineers TJ. S. A., made the measurements 

 of the outflow of the Great Lakes between June 27th and 

 Sept. 17th, 1868 * That, of Lake Huron was 216,435 cubic 

 feet per second ; and of Lake Erie for the first part of the 

 season, 304,307 cubic feet, and 258,586 feet for the second part. 

 From these figures, I have taken the maximum proportional dis- 

 charge (as the volume is variable) of Lake Erie, which is found 

 to gather T y of the total drainage of the Niagara River but 

 the mean discharge is less than T ^. This is an important 

 factor in the following computations. 



6. Modem Recession of the Falls. 



The four surveys illustrated in figure 4, show the modern 

 recession of the horseshoe cataract. During 48 years, 275,400 

 square feet fell away. The mean width of the adjacent por- 

 tions of the gorge (as opposite Goat Island) is 1,350 feet. Thus 

 the mean recession would be 4*175 feet a year. The American 

 falls have undermined 32,900 square feet of rock which gives 

 a mean rate of 0*64 foot a year. But the rate is not uniform. 

 In 1819, the crest of the Canadian fall was very acute, it had 



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x 







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FrG. 4. — The four surveys of the Canadiau Falls showing the retreat of ^the 

 cataract (in which some inaccuracies are apparent). (Kibbe). 



become quite obtuse in 1842, acute in 1886, but it was broaden- 

 ing out again in 1890 ; thus there are cycles of slow and rapid 

 retreat. 



* Report of Chief of Engineers for 1869, p. 582. 



