Hunt — On the Action of Waves on Sea-beaches, 8fc. 273 



We have here eighteen full currents in 31*50 minutes, giving the 

 average duration of each current as 1*75 minutes, and thea verage 

 interval between the waves 3*50 minutes. The outrushes quite ob- 

 literated any in-current on the surface due to the wind and rising 

 tide. 



These observations made in the harbour harmonize with another 

 set made at Oddicombe on the 4th of November previous. Wind 

 N.W. ; very strong off shore. Tide, neaps. Nearly high- water. 

 Eeach-slope steep. Sea surging up and down some two or three 

 feet vertical, yet not breaking on the shingle. The run on the 

 beach seemed rather owing to the water surging backwards and for- 

 wards than to any very definite swells, though swells could be dis- 

 tinguished sometimes. The water-mark against the eastern rocks 

 seemed nearly three feet between rise and fall ; but the change of 

 level caused no break on the beach continually. In this case we 

 see that waves of some kind, some three feet in vertical height be- 

 tween trough and crest, were rolling in upon the beach ; but, not- 

 withstanding their considerable height, the rise and fall of the 

 water was so gradual that there was no break or plunge. It will 

 be seen at once that the plunge being a potent auxiliary of the for- 

 ward current in the accumulation of shingle, and its loss to the 

 forward current being equivalent to a corresponding gain to the 

 backward current, if in any case an accumulation of shingle has been 

 in progress under the action of plunging waves, the occasional inter- 

 vention of non-plunging waves of equal or greater power must tend 

 towards the removal of such accumulated shingle. 



The question arises, what is the cause of these irregular waves 

 of long period ? It seems probable that they result from the dis- 

 turbance of the equilibrium of the water by wind pressure, whereby 

 the water is unduly raised at one point and lowered at another. 

 This phenomenon is well observed in narrow straits between two 

 seas, such as the Kattegat, the Straits of Dover, and the Bos- 

 phorus. 



In the Kattegat, according to Vice-Admiral Zhartmann (quoted 

 by Mr. Thomas Stevenson), "where the tides have a velocity of 

 1 to 2 knots, and the common rise is one foot, ' the currents may 

 sometimes, in boisterous weather, continue to run for three weeks 

 the same way, and even to attain the velocity of four knots ; and in 

 a furious gale of wind, on the 15th of January, 1818, the water rose 



SCIEN. PROC. B.D.S. — VOL. IV. PT. VI. 2 C 



