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BREAKWATER DESIGN.
123
wave by 30 per cent., so that, if we give effect to this modification, his co-
efficient becomes raised to 2’33. This value, though much higher than any
of the values of the other experimentalists quoted above, is not without
support from the observations of Bidone, who obtained coefficients ranging
from 1’5 to 2'3 for the pressure of water-jets.
The values assigned to k in the foregoing equation are all based on the
assumption that the line of action of the wave is perpendicular to the surface
on which it impinges. When the line of incidence makes an angle a with
the surface, the coefficient undergoes further modification, and, according to
Lord Rayleigh, k becomes
‘ig-rrsin a
2 +j/7r sin a
When a = 90° it will be noticed that this expression becomes approximately
2, which, to a certain extent, coincides with the value of 7c given previously.
One point of interest about the fundamental equation p=kwh is that it
may be written
p=^x2kgh;
and since w, the weight of salt water in Ibs. per cubic foot, differs imper-
ceptibly from the value of 2g, the equation becomes practically
p=2kgh; ..............................©
or, giving k its mean value of say 1’6,
p=^gh ..............................(g)
which is also transformable into
p=l'6w2 ..... (0)
Measurement of Wave-stroke. —It is a matter for regret that few
or no appliances are available for satisfactorily comparing the results of
theoretical calculation with actual pressures. It is true that various kinds
of apparatus have been contrived for the express purpose of registering the
compressive force of the wave-stroke, but for certain reasons these records
cannot be considered an absolutely reliable criterion. The recoil of a spring
is far from being a satisfactory niethod of gauging the colliding force of
incompressible bodies. The very elasticity of the spring robs it of one of
the most characteristic features of the ideal breakwater, and the retreat of
the surface plate before the impulse of the wave is not in accordance with
actual conditions. The real intensity of the blow, in fact, lies in the absence
of yielding in either body. Theoretically, the effect of such impact is infinite,
and in practice it must often far transcend the imperfect records of a none
too sensitive spring dynamometer.
Furthermore, the assumption of uniform distribution of pressure involved
in such means of measurement is untenable. Wave power is at least as
subtle and irregular as wind pressure. Waves strike hardest in isolated