ENGINEERING STRUCTURES 131 



jet of water on a normal plane is no greater than that of a 

 continuous jet of the same velocity ; that waves therefore 

 exert a continuous pressure over a definite period of time and 

 not a sudden momentary blow, and that as a result of this 

 continued pressure on a wall with open joints the wave forces 

 water through the joints, and compresses the air in the 

 interior. As the recession of a wave is sudden compared with 

 its advance, the pressure is suddenly withdrawn from the 

 outside, and the excess internal pressure results in outward 

 displacement of the face blocks. 



Experiments at Roorkee x on the impact of jets on a normal 

 plane, also led their authors to the conclusion that the maxi- 

 mum pressure produced by sudden impact is the same as is 

 exerted by a steady jet, and is given, within about 1 per cent., 

 by ' H ' or v 2 + 2g feet of water where ' H ' is the effective 

 head producing flow, measured above the point of impact. 

 In these experiments the pressures were measured by a water- 

 column, in communication with small orifices in the plane. 



The present investigation has been carried out in the 

 engineering department of University College, Dundee, with 

 a view of determining the magnitude of the effects which such 

 actions as have been outlined, may produce, and of showing 

 that still another phenomenon may be called into play during 

 wave impact, and may produce much greater effects than are 

 compatible with these theories of simple hydrostatic trans- 

 mission of pressure, or of air compression. 



Pressure following impact When a mass of water is hurled 

 with velocity 'v* against the face of a breakwater, the 

 pressure on the face rises almost instantaneously to a value 

 approximating to v 2 -f 2g feet of water. Substantial confirma- 

 tion of this is afforded by the results of experiments by the 

 late Thos. Stevenson, who, measuring these impact pressures by 

 means of a dynamometer with spring control, 2 fixed at about 



1 Proc. Inst. C.E., vol. Ix. p. 436. 



a Trans. Roy. Soc. Edinburgh, vol. xvi. p. 23. 



