ON THK ACTION OF WAVES AND CUUUENXS. 391 



up oiitside the walls and also a slight silting up in the river above its 

 mouth, which effects were very much increased when the walls were 

 raised to half-tide. On the walls being removed in tank E and then 

 gradually carried forward, the silting up behind the wall and deteriora- 

 tion of the river increased, but there was no improvement in navigable 

 depth between the walls. 



The dredging in tank F, so long as it was continued, added about 20 

 feet on a 30-foot tide or 10 feet on a 15-foot tide, to the navigable depth 

 between the walls, but there was the same silting up behind the walls 

 and the same deterioration in the river. 



It thus appears that the similarity of the results in both tanks 

 supports the conclusion that vertical zvalls having the horizontal direction 

 of the current in a straight tideway and terminating ivell below high luater, 

 produce hut little effect on the distrihution of the sand between them, so long 

 as the passage is freely open at both ends, but that if the passage be blocJced 

 at one end they form a bay in luhich the sand rises at the head. 



9. The effects of the tide in estuaries not symmetrical. — Having so far, 

 in accordance with the original scheme of this investigation (First Report, 

 1889, p. 5), simplified the circumstances which influence the distribu- 

 tion of sand by maintaining the lateral boundaries perfectly sym- 

 metrical, and as nearly rectilinear as practicable, and haA'ing found 

 definite laws connecting the distributions of sand in the beds of the model 

 estuaries with the period and rise of the tide and the length of the 

 estuary, besides the laws connecting the period of the tide with the 

 horizontal and vertical scales under whicli the models give similar results, 

 there remained two questions : 



(1) How far such discrepancies as appear between the general distri- 

 butions of sand found in the models and those observed in actual estuaries 

 are attributable to irregularities in the boundaries of the latter ? 



(2) How far the influence of these boundaries is subject to the same 

 laws of similarity as those already obtained ? 



The original experiments of the author in models of the Mersey which 

 led to the appointment of the Committee (B.A. Report, 1887) had to a 

 great extent answered these questions, showing that similar irregulari- 

 ties in the lateral boundaries exercise similar and predominating influences 

 on the lateral distributions of the sand in the models and in the estuaries. 



It seemed, however, desirable, so far as time allowed, to confirm these 

 results of the author's and make this investigation complete in itself by 

 carrying out experiments in both models similar to those already carried 

 out, except that the boundaries should be boldly irregular. 



Such experiments also afforded opportunity for studying some general 

 effects of great importance. The relation between the depths of water 

 and the rise of tide had come out very definite in the symmetrical experi- 

 ments, and it was desirable to see how far these relations would be 

 disturbed by lateral irregidarities. For instance: (1) Would bold irregu- 

 larities in the bounda^'ies of the estuary alter the depth of water in the 

 river ? Bold irregularities in the boundaries, causing the water to take a 

 sinuous course, would have the effect of virtually narrowing and in- 

 creasing the length of the estuary, and by causing eddies would obstruct 

 the passage of the water to some extent. Lengthening the estuary would 

 tend to increase its depth at corresponding points, and obstructing the 

 •water would tend to diminish the tidal action in the river ; at all events, 

 until the estuary had increased in depth. 



