114 HARBOUR ENGINEERING.
is sufficiently great, that is, where the depth is at least equal to the length of
the wave from crest to crest, the motion of the particles of water is rotary
along the circumference of a circle, as shown in fig. 94. The wave is one of
oscillation, and each particle complétés a revolution, returning approximately
to its initial position. The profile of the wave, then, is a cycloidal curve traced
out by a generating circle, which constitutes the orbit of the surface particles.
Fig. 95.—Wave in Shallow Water.
Accordingly, the actual momentary direction of motion of each of the
particles is independent and variable. Thus, at the crest, the motion is
horizontally forward ; in the trough, it is horizontally backward ; whilst at the