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 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 produced.  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