AND OTHER WATER WAVES 281 



wave is constant in size and steady in position, but 

 in a heavy sea the vessel throws off from her bows 

 a succession of waves. Similarly, I saw the stand- 

 ing waves near each shore periodically wax and 

 wane (though never waning to less than, say, 

 three -fourths of their maximum), and as each 

 passed its maximum it disengaged a conspicuous 

 and considerable travelling wave, which surged for- 

 ward (facing diagonally up-stream) towards the 

 centre of the stream. The waves travelling out- 

 wards from each bank met in the middle of the 

 stream with much commotion and throwing up of 

 spray, after which they could be clearly seen to 

 have passed right through each other, and to con- 

 tinue each its course towards the opposite shore. 

 The wave, however, is drifting down -stream all 

 the time ; the resulting motion of the mound of 

 foaming water being more down -stream than across 

 stream, for the current velocity is greater than the 

 wave velocity relatively to the bank. 



Viewed from the edge of the torrent at Whirl- 

 pool Rapids Station, the great standing mounds of 

 water in the centre of the stream towered up above 

 the line of sight. They are formed (as I knew 

 from my bird's-eye reconnaissance) where the 

 standing waves, diverging from the banks, cross 

 one another. They fluctuated continually in height 



