288 WAVES OF THE 



the river being here straight, there is less irregu- 

 larity than at the bend of the Whirlpool Rapids. 

 The distance from crest to crest in the direction of 

 the current was 100 feet, but this is not a true 

 " wave-length " in the physical sense. 



It is now incumbent to explain the co-existence 

 of the ordinary diagonal standing waves of rivers 

 with travelling waves facing diagonally up-stream 

 in the rapids of Niagara and the St. Lawrence. 



The absolute steadiness of standing waves can 

 only be secured by absolute steadiness of current. 

 In a river with a rough bed and a crooked course 

 it is impossible that the motion should be perfectly 

 steady if the current be even moderately swift, 

 for (to take the principal and most evident cause 

 of fluctuation) eddies are alternately formed and 

 released. By this means a standing wave is caused 

 to fluctuate as the " unwinding scrolls " and other 

 irregularities follow one another down the stream. 

 Generally speaking, the fluctuations so produced in 

 a wave near the right bank will not synchronise 

 with those of a wave near the left bank, for such 

 irregularities of flow of the river are generally 

 partial, not affecting simultaneously the whole 

 cross -section of the stream. 



Wave-motion is essentially differential, and if 

 the rate of change of motion be slow, the effect 



