256 Scientific Proceedings, Royal Dublin Society. 



plunge of the waves was more remote from tlie water margin on 

 the one side than the line of furthest reach was on the other. 



With the eye trained by observing the plunge of waves, where 

 the margin of repose is known, it is not difl&cult to make a fair esti- 

 mate of that margin when heavy waves are rolling on to a sea- 

 beach, and to assure oneself that in no case do heavy waves plunge 

 anywhere near the margin of repose of the water for the time being 

 — I say advisedly, heavy waves, for the action of short wind- waves 

 is at times completely masked and rendered unintelligible by the 

 presence of long, low swells, whose concurrent action may occa- 

 sionally pass unobserved. 



In investigating the action of an oscillating wave approaching 

 the shore it will be well to select an ordinary swell as being no 

 longer under the influence of the wind or other force that may 

 have brought it into existence. 



In the case of such a wave, authorities seem agreed that the 

 motion is one of oscillation solely, and that there is no translation 

 of the water in the direction of propagation of the wave. In a 

 passage already quoted, Mr. Eussell, referring to a wave of this 

 kind, says that "a body floating near the surface is alternately 



carried forward and backward by the waves " Lord Eayleigh, 



describing " periodic waves in deep water," says that " the motion of 

 each particle of the fluid is in a circle, which is described with uni- 

 form velocity." [Phil Mag., s. 5, vol. i., 1876, p. 267.) The direction 

 of this motion we may learn from Sir Greorge Airy : — " The 

 particle revolves uniformly in the circle of which we have spoken ; 

 moving forwards when at the highest part of the circle, and back- 

 wards when at the lowest part." ( Tides and Waves, 182.) 



From the above descriptions of the motion of water under the 

 influence of an oscillating wave it will be seen that the motion im- 

 parted to a floating body by such a wave will be more accurately 

 described as backward and forward than as " forward and back- 

 ward." It is important to bear in mind that the primary motion 

 of a floating object is towards the advancing wave, and therefore 

 backwards as referred to the direction of advance of the wave ; and 

 that the wave draws its supplies entirely from the front, and not 

 from the rear. The truth of this can be shown from the facts that 

 the ridge of an oscillating wave is preceded by a depression, and 

 that the motion of the wave is much more rapid than the motion of 



