258 Scientific Proceedings, Royal Dublin Society. 



the first place, a motion of translation towards the advancing wave- 

 crest, and in the second, a motion of translation in the same direc- 

 tion as that crest, the said object, if the bottom be level, being 

 ultimately deposited at or about the same spot whence it set out. 

 As the water shoals, and the depth in proportion to the size of the 

 wave diminishes, the currents increase in amplitude and intensity ; 

 but, so far as I can ascertain, remain equal and opposite, so long as 

 the wave does not plunge. The action of these currents can be ob- 

 served either from a boat at sea, when the water is clear and the 

 waves of convenient dimensions ; or, in the case of artificial waves, 

 in a suitable tank. 



When a wave of oscillation is passing over and through a prac- 

 tically frictionless fiuid, such as water, it may travel for hundreds of 

 miles from its point of origin, drawing its supplies in advance, with- 

 out encountering appreciable resistance ; but the moment such a 

 wave begins to feel the bottom, the two currents to which it gives 

 rise encounter resistance ; the wave has begun to expend itself in 

 doing appreciable work, which work, if continued, must sooner or 

 later reduce its motion to nothing. The action of the bottom on a 

 wave may be compared to that of a friction-brake on a machine : if 

 applied gradually it will by insensible degrees bring the machine to 

 a standstill ; if applied too suddenly it may produce the same result 

 by a disruption of the machine itself. When, in the case of a wave, 

 the break is suddenly applied in the form of a steep beach, the wave 

 plunges, and is destroyed almost instantaneously. When the brake 

 is applied very gradually, by the bottom shoaling slowly, the wave 

 may be reduced to very small dimensions without breaking at all. 



The dissipation of waves without breaking may be occasionally 

 witnessed in the shallow water over mud-banks in estuaries, where 

 the distance the wave has to travel in shallow water is great in pro- 

 portion to the length of the wave. On the sea-shore, the distance 

 a wave has to travel after feeling the bottom is commonly so short 

 compared with the size of the wave, that the reduction of the wave 

 by the friction of the bottom cannot keep pace with the rate of 

 shoaling of the water. 



If the wave survive to reach the shore, as is usually the case, 

 the primary and secondary currents will be found in their relative 

 sequence, the water being drawn off from the shore as the wave-crest 

 approaches. 



