al bed of the river is cumbered with large
boulders, and that the jostling, tossing, and wild leaping of the
waters there are due to its impact against these obstacles. A very
different explanation occurred to me upon the spot. 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 centre of disturbance, cross the river obliquely, and the
result is, that at the centre waves commingle which have really been
generated at the sides. This crossing of waves may be seen on a small
scale in any gutter after rain; it may also be seen on simply pouring
water from a wide-lipped jug. Where crest and furrow cross each other,
the wave 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. The phenomena
observed at the Whirlpool Rapids constitute, in fact, one of the
grandest illustrations of the principle of interference.

Sec. 5. _Analogies of Sound and Light._

Thomas Young's fundamental discovery in optics was that the principle
of Interference was applicable to light. Long prior to his time an
Italian philosopher, Grimaldi, had stated that under certain
circumstances two thin beams of light, each of which, acting singly,
produced a luminous spot upon a white wall, when caused to act
together, partially quenched each other and darkened the spot. This
was a statement of fundamental significance, but it required the
discoveries and the genius of Young to give it meaning. How he did so
will gradually become clear to you. You know that air is compressible:
that by pressure it can be rendered more dense, and that by
dilatation it can be rendered more rare. Properly agitated, a
tuning-fork now sounds in a manner audible to you all, and most of you
know that the air through which the sound is passing is parcelled out
into spaces in which the air is condensed, followed by other spaces in
which the air is rarefied. These condensations and rarefactions
constitute what we call _waves_ of sound. You can imagine the air of a
room traversed