218 THE POPULAR SCIENCE MONTHLY. 



utes. At times, the wind, coming up the river, searched and sifted the 

 spray, carrying away the lighter drops, and leaving the heavier ones 

 behind. Wafted in the proper direction, rainbows appeared and dis- 

 appeared fitfully in the lighter mist. In other directions the common 

 gleam of the sunshine from the waves and their shattered crests was 

 exquisitely beautiful. The complexity of the action was still further 

 illustrated by the fact that in some cases, as if by the exercise of a 

 local explosive force, the drops were shot radially from a particular 

 centre, forming around it a kind of halo. 



The first impression, and, indeed, the current explanation of these 

 rapids is, that the central bed of the river is cumbered with large 

 bowlders, and that the jostling, tossing, and wild leaping of the water 

 there are due to its impact against these obstacles. This may be true 

 to some extent, but there is another reason to be taken into account. 

 Bowlders derived 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 produced. 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 disturbance, cross the river ob- 

 liquely, 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 coa- 

 lescence of waves with waves. Where crest and furrow cross each 

 other, the motion is annulled ; 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 explana- 

 tion be correct, the phenomena observed at the Whirlpool Rapids 

 form one of the grandest illustrations of the principle of interference. 

 The Nile " cataract," Mr. Huxley informs me, offers examples of the 

 same action. 



At some distance below the Whirlpool Rapids we have the cele- 

 brated whirlpool itself. Here the river makes a sudden bend to the 

 northeast, forming nearly a right angle with its previous direction. 

 The water strikes the concave bank with great force, and scoops it 

 incessantly away. A vast basin has been thus formed, in which the 

 sweep of the river prolongs itself in gyratory currents. Bodies and 

 trees which have come over the falls are stated to circulate here for 

 days without finding the outlet. From various points of the cliffs 

 above, this is curiously hidden. The rush of the river into the whirl- 

 pool is obvious enough ; and, though you imagine the outlet must be 

 visible, if one existed, you cannot find it. Turning, however, round 



