ON THE ACTION OF WAVES AND CUUEENTS. 343 



tion, and there is nothing surprising that similar ripple marks should 

 appear in the beds of the models. Bat although presenting a very similar 

 appearance, and being about of the same size, the ripple marks seen in all 

 the plans are essentially different in their origin and in the position they 

 take in the regime of the sand in the models from that held by the ob- 

 served ripple marks on the shore sauds. This last is caused by the alter- 

 nating currents produced by the small swell running inshore, while that in 

 model is produced by the alternating action of the tide. There may seem 

 nothing remarkable in this, considering that these currents in magnitude 

 and velocity are not dissimilar — but if the models are similar to the 

 results obtained in estuaries the converse should hold, and the estuaries 

 should be similar to the models. In which case we are face to face with 

 a very striking conclusion, that in the estuai-ies there should be — call it 

 ripple mark or wave mark, produced by the action of the tide, similar to 

 that on the models and on a scale proportional to the height of tide in 

 the estuary. Thus some of the ripples in the models are from hollow 

 to crest as much as one-fourth the mean rise of the tide, the distance 

 between them being 12 times their height. This, in an estuary, would 

 mean 7 or 8 feet high and 80 to 100 feet in distance. 



These ripples in the model are almost confined to the surface of the 

 sand which is below low- water mark, though in places their somewhat 

 eroded ends protrude up the slope from the low-water channels. The 

 existence of these ripples very miich enhances the effect of the water to 

 shift the sand — this was noted in the experiments 2 and 3 on the bars, 

 tank A ; on the smooth walls of the sand the current, which would be 

 about 6 inches a second, did not drift the sand at all except close to the 

 ridge, and then there was no apparent effect till after 1,700 tides, when 

 ripples were just beginning, yet when the ripple once formed in another 

 1,200 tides the top of the bar had spread to 12 inches. 



The ripples also serve to show in which way any shift of the sand is 

 taking place, as they have a steep side looking in the direction of motion, 

 and when the slopes are equal it is an indication of equilibrium. 



Conclusions. — So far as these experiments have gone they have shown 

 that similar results as to the general slope and rate of action of the sand 

 can be obtained by models working according to the kinetic law as low as 

 tides of 



1 inch, with a vertical exaggeration of 100, or 



2 inches ,, ,, „ 64. 



They have not shown, however, that the limit has been reached. Although 

 the results obtained with a tide of 1 inch with a vertical exaggeration of 

 61< in tank B presented peculiarities which appeared in two experiments, 

 it is still open to question whether these might not have been owing to 

 something in the initial circumstances. This first series is therefore yet 

 incomplete ; it should include experiments to show the smallest vertical 

 exaggeration at which similar results can be obtained with tides as small 

 as half an inch and as large as 2 inches. This would give the law of the 

 limits ; this would conclude the first series. Then, if the experiments are 

 continued, another series raitrht be undertaken to determine whether 



• 1 1 



similar effects can be obtained from land water acting on such slopes as 

 have been already obtained ; and again, as to the law of slopes and 

 cress sections on V-shaped estuaries, and then, though this has been 

 already established in my previous experiment, as to the effects of irregular 

 lateral configuration in the shores. 



