motion, it is now necessary to consider by what means the
movement of water along the shores is caused. The sea is, of
course, subject to the usual law governing the flow of water,
whereby it is constantly trying to find its own level. In a
tidal wave the height of the crest is so small compared with
the length that the surface gradient from crest to trough is
practically flat, and does not lead to any appreciable
movement; but as the tidal wave approaches within a few miles
of the shore, it runs into shallow water, where its progress is
checked, but as it is being pushed on from behind it banks up
and forms a crest of sufficient height to form a more or less
steep gradient, and to induce a horizontal movement of the
particles of water throughout the whole depth in the form of a
tidal current running parallel with the shore.

The rate of this current depends upon the steepness of the
gradient, and the momentum acquired will, In some Instances,
cause the current to continue to run in the same direction for
some time after the tide has turned, i.e., after the direction
of the gradient has been reversed; so that the tide may be
making--or falling--in one direction, while the current is
running the opposite way. It will be readily seen, then, that
the flow of the current will be slack about the time of high
and low water, so that its maximum rate will be at half-ebb and
half-flood. If the tide were flowing into an enclosed or semi-
enclosed space, the current could not run after the tide
turned, and the reversal of both would be simultaneous, unless,
indeed, the current turned before the tide.

Wind waves are only movements of the surface of the water, and