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HARBOUR ENGINEERING.
of the greatest fetch, but it is also a matter of experience that heavy rollers
are frequently deflected so as to reach a point on the coast which does not lie
upon their direct path. This is exemplified in the case of a headland and bay
in fig. 98, and the same effect is noticeable at the pierheads of artißcially
sheltered harbours. Islands also act as pivots in many cases, causing the
waves to wheel round and break upon their leeward shores. The result is
due, no doubt, to the retardation produced by shallowing ground upon the
nearer portion of waves approaching a coast obliquely, or running parallel
thereto.
Furthermore, the convergency due to narrowing inlets tends to accentuate,
to a greater degree even, the eccentricities of wave development. Pent
between lateral arms drawing gradually closer together, the volume of water is
raised above the level it would normally assume, and so gives rise to breakers
of a character equally marked.
As a corollary to what has been said, it is evident that waves of great
height cannot reach any coast-line, and, for that matter, any artificial
barrier, unless there be an unbroken extent of deep water penetrating close
into them.
Leng’th of Waves.—The length of waves is a feature which seems to be
independent of the height, though it is connected in some way with the
amount of exposure to wind action, and it influences the force of the wave. In
the Atlantic Ocean waves of from 500 to 600 feet between crests have been
observed, while in the Pacific they are stated to reach anything from 600 to
1000 feet.
The length of waves, however, in the open sea is a difficult matter to
determine satisfactorily, owing to the absence of any reliable linear standard.
Alongside jetties and piers, the obstacles in the way of exact measurement are
not so great, and serviceable computations may be made with the aid of
Bertin’s formula. Observing the length of time in seconds which elapses
between the passage of the same point by two successive crests—in other words,