330 WAVES OF THE SEA 



still less is it usually possible to know if they are 

 travelling through the current down -stream. If, 

 however, the streams have gently shelving shores, 

 and especially if these be smooth, the direction of 

 the waves which come in upon them facing 

 diagonally, but on the whole down -stream, indicates 

 clearly that there are in the deep water progressive 

 down-stream waves invisible only on account pf 

 their flatness. 



Should I have the opportunity to revisit the 

 Niagara Whirlpool Rapids, I might perhaps detect 

 the existence of down-stream waves there, although 

 I could not do so before I had daily practice in 

 their detection. The fact that waves which have 

 not a length great as compared to the depth con- 

 tinually multiply themselves, would of itself account 

 for the fact that such waves tend to die out rather 

 than to increase in deep streams. Unequal depth 

 of cross -section, as has already been mentioned, 

 also tends to do away with them, causing them 

 to discharge themselves upon the banks. 



