276 Scientific Proceedings, Royal Dublin Society. 



It may be observed, that Sir George Airy instances the follow- 

 ing example of a negative wave : — " The phenomenon of a negative 

 wave is given in great perfection by the paddles of a steam-boat : 

 the first wave which passes away from it being a hollow of consider- 

 able depth. We were first made aware of this by observation of 

 the traces made by Mr. Bunt's excellent self-registering tide- 

 gauge on the banks of the Avon, at a short distance below Bristol. 

 . . . ." — [Tides and Waves, 398.) It may be worth noting, that 

 the waves generated by a bluff-bowed paddle-steamer of the old 

 type are very complicated. While the paddles are making one set 

 on either side, the bows of the vessel are throwing off another set 

 on either side. The latter would resemble those thrown off by a 

 sailing vessel, being independent of the cause of motion, whether 

 steam or wind. The bow- waves being generated in advance of the 

 paddle-waves would precede them to the shore ; at any rate, in the 

 case of so narrow a river as the Avon. It seems possible that the 

 traces in question may have been the records of a positive wave of 

 translation, partially or wholly transformed into a free rolling wave 

 in its passage to the river's bank. 



It would appear that waves generated by the general equili- 

 brium of the water being disturbed by wind-pressure will reach the 

 shore as waves of translation, subject to modification, if the dis- 

 turbance takes place at a sufficient distance from the shore. Their 

 peculiar action on the beach will depend on the extent to which 

 they plunge. I have said that, on November 4th, 1882, a vertical 

 rise and fall of the water-level to the extent of nearly three feet 

 caused no break on the shingle at the point I was observing at 

 Oddicombe. A simple backwards and forwards swing of the water 

 of this character, however caused, cannot fail to cut out material 

 accumulated by previous plunging waves. On a subsequent oc- 

 casion, I made the following note: — Oddicombe, 25th Nov., 1883. 

 " Wind off shore. Disturbed swell cutting out the beach. Angle 

 of one pebble slope 41°, of another 41° ; both in the small cove. 

 This was the maximum angle of repose, as an umbrella drawn 

 lightly across the face of the slopes set the pebbles freely in motion. 

 Moreover, the swells surging up would set dry pebbles free high up 

 the slope I was observing." The two pebble slopes referred to 

 were not curved, their profiles being perfectly straight ; and they 

 were clearly ridges or banks of denudation, and not of accumulation. 



