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 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