AND OTHER WATER WAVES 323 



stones made something resembling a dam at 

 the lower end of what was, in fact, a pool. 

 Here the wave seemed to originate by the sudden 

 emptying of the pool. The depth being very small 

 (generally not more than i inches or 2 inches 

 on the sill), and the pool being several yards 

 long, there was great sensitiveness to the slight 

 flicker always perceptible in this and similar 

 streams. Wherever in the course of these irregu- 

 lar flickerings a slight deficiency was followed 

 suddenly by a slight excess, the latter would have 

 to burst a way across the shallow sill as a small 

 wave with a steep front ; and this, clearing the 

 way before it, drained the pool considerably, thus 

 preparing for the formation of a second wave. 

 This reasoning, I repeat, is only valid for a certain 

 scale of things . If the dimensions be greatly 

 changed, the relative importance of the factors 

 would also be changed. This origin of the wave 

 by the filling and emptying of a sort of pool 

 accounts for the fact that the interval between 

 successive waves on the Guntenbach was never less 

 than several seconds, whereas on the Griinnbach 

 the period at first was much less than a second. 

 When the Guntenbach was flowing sluggishly, with 

 less water than usual, the period was always a long 

 one, for it took a long time to fill and empty the 



