he 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
centres of disturbance intersect each other. Now, no matter how
numerous these waves may be, the law holds good that the motion of
every particle of the water is the algebraic sum of all the motions
imparted to it. If crest coincide with crest and furrow with furrow,
the wave is lifted to a double height above its sinus; if furrow
coincide with crest, the motions are in opposition and their sum is
zero. We have then _still_ water. This action of wave upon wave is
technically called _interference_, a term, to be remembered.

To the eye of a person conversant with these principles, nothing can
be more interesting than the crossing of water ripples. Through their
interference the water-surface is sometimes shivered into the most
beautiful mosaic, trembling rhythmically as if with a kind of visible
music. When waves are skilfully generated in a dish of mercury, a
strong light thrown upon the shining surface, and reflected on to a
screen, reveals the motions of the liquid metal. The shape of the
vessel determines the forms of the figures produc