ed. In a circular
dish, for example, a disturbance at the centre propagates itself as a
series of circular waves, which, after reflection, again meet at the
centre. If the point of disturbance be a little way removed from the
centre, the interference of the direct and reflected waves produces
the magnificent chasing shown in the annexed figure.[11] The light
reflected from such a surface yields a pattern of extraordinary
beauty. When the mercury is slightly struck by a needle-point in a
direction concentric with the surface of the vessel, the lines of
light run round in mazy coils, interlacing and unravelling themselves
in a wonderful manner. When the vessel is square, a splendid
chequer-work is produced by the crossing of the direct and reflected
waves. Thus, in the case of wave-motion, the most ordinary causes give
rise to most exquisite effects. The words of Emerson are perfectly
applicable here:--

[Illustration: Fig. 10.]

'Thou can'st not wave thy staff in the air,
Or dip thy paddle in the lake,
But it carves the brow of beauty there.
And the ripples in rhymes the oars forsake.'

The most impressive illustration of the action of waves on waves that
I have ever seen occurs near Niagara. For a distance of two miles, or
thereabouts, below the Falls, the river Niagara flows unruffled
through its excavated gorge. The bed subsequently narrows, and the
water quickens its motion. At the place called the 'Whirlpool Rapids,'
I estimated the width of the river at 300 feet, an estimate confirmed
by the dwellers on the spot. When it is remembered that the drainage
of nearly half a continent is compressed into this space, the
impetuosity of the river's escape through this gorge may be imagined.

Two kinds of motion are here obviously active, a motion of translation
and a motion of undulation--the race of the river through its gorge,
and the great waves generated by its collision with the obstacles in
its way. In the middle of the stream, the rush and tossing are most
violent; at all events, the impetuous force of the individual waves is
here most strikingly displayed. Vast pyramidal heaps leap incessantly
from the river, some of them with such energy as to jerk their summits
into the air, where they hang suspended as bundles of liquid pearls,
which, when shone upon by the sun, are of indescribable beauty.

The first impression, and, indeed, the current explanation of these
Rapids is, that the centr