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PROCEEDINGS OF THE 



[Jan., 1857. 



independently, or nearly so, of the action of the American Fall, it 

 will follow that, during the interval of time that the main Fall was 

 receding- from A K to F H, a distance of about 1,100 yards, the 

 American Fall must have receded from some line, A D, left by the 

 action of the main Fall to its present position. The amount of this 

 recession, where greatest, is estimated by Mr. Desor, at 40 yards, 

 and this would give a recession 25 times more rapid for the mam 

 Fall, than for the American Fall. This result must, however, be 

 reduced in some ratio on account of the gradual development of 

 the American Fall, while the main Fall was receding from A to D. 

 The result of this gradual development may be regarded as the 

 same as from the display of the whole force of the Fall for half 

 that time, and as*A D is about one-third of A F, if we suppose a 

 nearly uniform rate of recession of the main Fall, the reduction 

 will be one-sixth of the whole, leaving the ratio of recession as 1 

 to 20 or 21. If the angular or curved outline of the main Fall be 

 regarded as not arising from the junction of two portions F H and 

 F G, but as the normal form throughout the course from A G, it 

 will still be true, that the main Fall has cut its path of nearly 

 uniform width and direction, very nearly as though the American 

 Fall did not exist. In this case the vertex of the Fall will have 

 receded from some point B to the point F, and the ratio of recess 

 would then be as 40 yards to about 800 yards, or as 1 to 20; this 

 reduced one-fourth according to principles given above, would give 

 a final ratio of recess of 1 to 15. 



Having thus shown that the main Fall appears to have receded 

 at least fifteen times as fast as the American Fall, we may return to 

 the discussion of the remaining three causes above assigned to 

 explain this inequality of recess. The first of these is the most 

 probable cause by far, and to it the whole difference may possibly 

 be due, and that we will first examine. 



The volume of water passing over any linear yard of the brink 

 of either Fall depends on the velocity and depth, and I am unable 

 to form any close approximation to the value of either of these 

 factors, or of their relative magnitude at each Fall; nor can I find, 

 in the various publications relating to the Falls, data for making 

 such approximation. It is twenty years since I visited the Falls, 

 but from my own recollections, as well as from the representations 

 of visitors, and the impression given by views of the Falls, there 

 can be no doubt that the volume of the Canadian Fall is much 

 greater than that of the American Fall, not only from greater length 



