merely alters the form of the wave raised by the moon_; or, in other
words, the greater of the two waves (which is due to the moon) is
modified in its height by the smaller (sun's) wave. When the summit of
the two happens to coincide, the summit of the combined wave will be
at the highest. When the hollow of the smaller wave coincides with the
summit of the larger, the summit of the combined wave will be at the
lowest.

It is necessary to have a clear and distinct conception of the
difference between the _motion_ of a _wave_ and that of a _current_.
In the current there is a transfer of water; in the wave the transfer
is no more than would be brought about by a particle of water
impinging on another where that particle has a motion perpendicular to
the surface, and a rising and falling results. The onward movement of
the wave itself is always perceptible enough. That the water is not
moving with the same velocity is also evident from watching the
progress of any light body floating on its surface. This fact may be
practically illustrated in the case of a ship at sea, sailing before
the wind in the same direction as the waves are moving. When the crest
of a wave is near the stern, drop a piece of wood on it. Almost
instantly the wave will be seen shooting ahead of the vessel, while
the wood is scarcely removed from the position where it fell on the
water. The wave has moved onward, preserving its identity as a wave,
the water of which it is formed being constantly changed; and thus the
motion of the wave is one thing, that of the water in which the waves
are formed is quite another thing.

Again, waves are formed by a force acting horizontally; but in the
case of the tide wave, that force acts uniformly from the surface to
the lowest depths of the ocean, and the breadth of the wave is that