40 HARBOUR CONSTRUCTION. 



A similar effect may be produced by the convergence of sub- 

 merged rocks, or contours. It is believed that the height of the 

 waves at the Dhu Heartach Light, off the west coast of Argyle, 

 is greatly increased by such a formation of the sea-bed. 



Impact of Waves. The intensity of the stroke of waves is 

 due, in a great measure, to the non-elasticity of the water. 



The difference between hydrostatic pressure and hydro- 

 dynamic force may be illustrated by taking a high-pressure 

 water service as an example. So long as the water in the pipes 

 is at rest, only the pressure proportionate to the head of water 

 has to be considered ; but set the column of water within the 

 pipes in motion by opening a sluice-valve, and a new force at 

 once comes into operation. If such force be suddenly arrested 

 by the too rapid closing of the sluice, a burst pipe will probably 

 be the result, unless, as is customary, a suitable relief-valve has 

 been placed on the pressure side of the sluice. Hence, in a high- 

 pressure water service, screw-down cocks take the place of plug 

 taps, on account of the suddenness with which the latter close. 



If such precautions are found to be necessary in arresting 

 the progress of so comparatively insignificant a moving column 

 of water as that above referred to, some idea may be formed of 

 the resistance which must be offered in order to suddenly stop 

 such moving columns of water as are represented by translatory 

 storm-waves. 



The dynamical effect of waves striking a plain surface is not 

 easy to estimate. In the case of one non-elastic body striking 

 another similar body the blow is infinite, and one or both of the 

 bodies must be shattered. 



The greater the elasticity of one or both of the bodies so 

 coming into contact, the more is the effect of the blow reduced ; 

 hence the results obtained from dynamometers arranged with 

 springs and plungers are liable to variation according to the 

 strength and range of the springs used. For this reason it is 

 probable they do not faithfully represent the force of a blow 

 struck by a wave upon a non-elastic surface, such as that pre- 

 sented by the upright face of a breakwater or rock. 



The error arising from this cause may be reduced, in some 

 measure, by the wave remaining in contact with the surface 

 struck for an appreciable time. I apprehend, however, that after 

 the stroke has been delivered, only hydrostatic pressure, as dis- 

 tinguished from hydrodynamic force, is exerted. 



