motion of the wave itself, and the motion of the particles which at
any moment constitute the wave.

Stand upon the seashore and observe the advancing rollers before they
are distorted by the friction of the bottom. Every wave has a back and
a front, and, if you clearly seize the image of the moving wave, you
will see that every particle of water along the front of the wave is
in the act of rising, while every particle along its back is in the
act of sinking. The particles in front reach in succession the crest
of the wave, and as soon as the crest is past they begin to fall. They
then reach the furrow or _sinus_ of the wave, and can sink no farther.
Immediately afterwards they become the front of the succeeding wave,
rise again until they reach the crest, and then sink as before. Thus,
while the waves pass onwards horizontally, the individual particles
are simply lifted up and down vertically. Observe a sea-fowl, or, if
you are a swimmer, abandon yourself to the action of the waves; you
are not carried forward, but simply rocked up and down. The
propagation of a wave is the propagation of a _form_, and not the
transference of the substance which constitutes the wave.

The _length_ of the wave is the distance from crest to crest, while
the distance through which the individual particles oscillate is
called the _amplitude_ of the oscillation. You will notice that in
this description the particles of water are made to vibrate _across_
the line of propagation.[10]

And now we have to take a step forwards, and it is the most important
step of all. You can picture two series of waves proceeding from
different origins through the same water. When, for example, you throw
two stones into still water, the ring-waves proceeding from the two