﻿Jan., 1857.] 



ELLIOTT SOCIETY. 



99 



from the point D, gradually lowered 10 feet by reason of the dip 

 (which is about 25 feet per mile) it is also true, if I correctly 

 understand the statements in his report, page 402, that the surface 

 of the water at the main Fall is lower than the surface at the 

 American Fall by about 8 feet, so that there has been but little 

 gain in depth during the recess. There must have been, however, 

 a gain in velocity from the increase of declivity from the top of 

 the rapids to the top of the Fall, and there may have been a conse- 

 quent diminution in volume of water at the American Fall. But, 

 even if this gain in depth of water be granted, since our compari- 

 sons made above, with extreme numbers, taken from the Fall at 

 their period of greatest inequality in volume barely serve on this view 

 to explain the difference of rate of recess by difference in volume 

 of water, the probability that this is the sole cause does not seem 

 to be increased greatly by this view. It will be understood that, 

 while the surface of the two streams stood at the same level at the 

 line A D, the floors of the two streams were at different average 

 levels, the larger volume of the main stream requiring a greater 

 depth. 



Throughout the preceding discussion we have compared the 

 recess of the Canada Fall with that of the American Fall ; but if 

 our view above presented be correct, that the partial Fail F H be 

 the true representative of the original Fall at A K, and have cut 

 its way from G to F, precisely as it had already done from A to D 

 and from D to F, then the comparison of the two Falls F H and F 

 G, which cannot greatly differ in depth and velocity of water, will 

 show still more strikingly that the different rate of recess in differ- 

 ent directions, cannot be referred to difference in volume of water 

 solely; nor is the gain in depth from dip of strata, adequate to 

 account for the difference. 



Good observations are wanting on the width, depth and velocity 

 of the streams at several points above the two Falls, in order to 

 estimate the volume of water passing over each, also of the ve- 

 locity at "or near the brink of each Fall; then from the known 

 volume, linear length of brink, known from surve}-, and the known 

 velocity at the brink, the depth of water there might be computed 

 for each, if required. The ratio of volume of water compared 

 with the ratio of recession would show how far inequality of resis- 

 tance entered as a factor in the final effect. 



Besides the usual modes of estimating the velocity of the 

 current, such as a space passed through in a given time by objects 



