278 WAVES OF THE SEA 



often cusped and foaming, and they shift some- 

 what rapidly for a short distance up and down 

 about a mean position. 



Numerous standing waves originate from 

 successive positions on the banks, the position 

 being determined doubtless by changes of pres- 

 sure and, consequently, of velocity. Such change 

 of pressure and velocity is obviously produced in 

 some places by rocky obstructions, but in some 

 instances, particularly opposite the railway station, 

 the change of direction of the current probably 

 contributes to the effect. No long and regular 

 trains of waves can be observed, except just at 

 the commencement, but the positions of the prin- 

 cipal mounds of water are perfectly definite, so 

 that (as long as the river remains at a certain 

 level) these waves are recognisable individuals. 

 Nevertheless, I noticed from my elevated stations 

 on the bridge and the cliffs that in this terrible 

 rapid (where the combined depth, speed, and 

 volume are so unusually great) the throbbing, 

 which is so slight a feature in most wavy streams, 

 had developed to a remarkable extent. The sta- 

 tionary waves of the river differed from those 

 of quieter streams in the same way that the bow- 

 wave of a steamer in a heavy sea differs from the 

 bow -wave in smooth water. In the latter the bow- 



