NIAGARA. 189 



the river is cumbered with large boulders, and that the 

 jostling, tossing, and wild leaping of the water there, 

 are due to its impact against these obstacles. I doubt 

 this explanation. At all events, there is another suffi- 

 cient reason to be taken into account. Boulders de- 

 rived from the adjacent cliffs visibly cumber the sides 

 of the river. Against these the water rises and sinks 

 rhythmically but violently, large waves being thus pro- 

 duced. On the generation of each wave, there is an 

 immediate compounding of the wave-motion with the 

 river-motion. The ridges, which in still water would 

 proceed in circular curves round the centre of disturb- 

 ance, cross the river obliquely, and the result is that 

 at the centre waves commingle, which have really been 

 generated at the sides. In the first instance, we had a 

 composition of wave-motion with river-motion; here 

 we have the coalescence of waves with waves. Where 

 crest and furrow cross each other, the motion is an- 

 nulled; where furrow and furrow cross, the river is 

 ploughed to a greater depth; and where crest and crest 

 aid each other, we have that astonishing leap of the 

 water which breaks the cohesion of the crests, and 

 tosses them shattered into the air. From the water 

 level the cause of the action is not so easily seen; but 

 from the summit of the cliff the lateral generation of 

 the waves, and their propagation to the centre, are 

 perfectly obvious. If this explanation be correct, the 

 phenomena observed at the Whirlpool Rapids form one 

 of the grandest illustrations of the principle of inter- 

 ference. The Nile ' cataract/ Mr. Huxley informs me, 

 offers more moderate examples of the same action. 



At some distance below the Whirlpool Rapids we 

 have the celebrated whirlpool itself. Here the river 

 makes a sudden bend to the north-east, forming nearly 

 a right angle with its previous direction. The water 



