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POPULAR SCIENCE MONTHLY. 



question, the New York Geological Survey, under Professor Hall's 

 direction, had a careful trigonometric survey of the Horseshoe Fall 

 made in 1842, erecting monuments at the points at which their angles 



FIG. 1. Looking north from below the Whirlpool, showing the electric road at the bottom 

 of the east side of the gorge, and the steam road descending the face about halfway to 

 the top. 



were taken, so that, after a sufficient lapse of time, the actual rate 

 of recession could be more accurately determined. In 1886 Mr. 

 Woodward, of the United States Geological Survey, made a new 

 survey, and found that the actual amount of recession in the center 

 of the Horseshoe Fall had proceeded at an average rate of about five 

 feet per annum. The subject was thoroughly discussed by Drs. 

 Pohlman and Gilbert, at the Buffalo meeting of the American Asso- 

 ciation in 1886, when it was proved, to the satisfaction of every one, 

 that, if the supply of water had been constant throughout its history, 

 the whole work of eroding the gorge from Lewiston to the Falls 

 would have been accomplished, at the present rate of recession, in 

 about seven thousand years. 



But the question was immediately raised, Has the supply of water 

 in Niagara River been constant? Tt was my privilege, in the autumn 

 of 1892 (see Bulletin of the Geological Society of America, vol. iv, 

 pp. 421-427), to bring forth the first positive evidence that the water 

 pouring over Niagara had for a time been diverted, having been 



