NIAGARA. 143 
the spray, carrying away the lighter drops, and leaving the 
heavier ones behind. Wafted in the proper direction, 
rainbows appeared and disappeared fitfully in the lighter 
mist. In other directions the common gleam of the sun- 
shine from the waves and their shattered crests was exqui- 
sitely 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 center, forming around it a kind 
of halo. 
The first impression, and, indeed, the current explana- 
tion of these rapids is, that the central bed of the river is 
cumbered with large boulders, and that the jostling, toss- 
ing, 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 sufficient reason to be taken 
into account. Boulders 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 center of dis- 
turbance, cross the river obliquely, and the result is that 
at the center 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 annulled; where 
furrow and furrow cross, the river is plowed to a greater 
depth; and where crest and crest aid each other, we have 
that astonishing leap of the water which breaks the cohe- 
sion 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 gener- 
ation of the waves, and their propagation to the center, 
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 interference. 
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, 
