ON THE ACTION OF WAVES AND CUIIUEXTS. 389 



what it is ia the large tauk E, although the value of the criterion of 

 similarity /i'e ' may be but little below 0'09. 



It was not found practicable to get the value of the criterion any 

 greater in tank F, but it was found on diminishing the rise of tide in the 

 large tank B until the criterion had a value 0-0.9, that the results were still 

 similar, although the rate of action and the increase in the size of the 

 ripple indicated that the limit was being approached. That the dissi- 

 milarity in tauk F was only the result of a phase in the formation of the 

 estuarj was also definitely shown by the effects of dredging out the sand, 

 which was above the initial level in the river during the early stages of 

 the Experiments V. and YI., after which the action in tank F resumed 

 the same coui'se as that in E, and led to the same final condition of 

 equilibrium, showing by the rate of action and size of ripple that the 

 limit of similarity was approached. 



It thus appears that with such arrangements as these tanks represent 

 there are two possible conditions of final equilibrium. 



The one is that which has uniformly been presented by tank E, and 

 in Experiment V. in tank F after dredging ; namely, the tide rising up to 

 the top of the river and keeping the sand low in the estuary. The other, 

 that which was presented in Experiments I., II., III., and IV., in ta.nk F ; 

 namely, the sand at the top of the estuary rising to high water level, as it 

 would do if there were no river, choking the mouth of the river except 

 so fixr as necessary to allow the land water to pass, and so preventing any 

 tidal action from the river. 



Which of these two conditions the river will assume during the process 

 of forming the estuary appears to be a critical matter, decided by whether 

 the tidal action of the river in lowering the sand at the head of the estuary 

 predominates over the tendency of the tide in the estuary to raise the 

 sand at the mouth of the river. 



There is a possible condition of instability between the river and the 

 «stuary. The emphatic difference in the action of the long tidal river and 

 mere tidal capacity at the head of the estuary in keeping down the sand 

 at the head of the estuary ; and, further, the very great effect which an 

 increase in the length of the river has on the depth of water in the estuary 

 and in the river are clearly shown.^ In Experiments III. and V. in tank 

 E, an increase of from 50 to 70 miles in the length of the river in V. 

 causing the depth of water to increase from by 40 to 30 feet all down the 

 river and estuary, lowering the sand in the lower river and upper estuary 

 from the level of half-tide to 28 feet below low water. In neither of these 

 experiments was the condition of instability reached, but 50 miles was 

 very near the limit. 



In such a state any diminution of the upper tidal waters of the river, 

 "by shm-tening the river or by land reclamation, might well have caused 

 the critical stage to be passed and caused the river to silt up — ^just as in 

 the other way the increasing of the tidal capacity high up the river by 

 dredging in Experiment V., tank F, caused the critical stage of silting 

 up to be passed and the i-iver to open out. The sand actually removed 

 in this experiment by dredging was 8 per cent, of the tidal capacity, 



h is the actual rise in feet, e the vertical exaggeration as referred to a 30-foot 

 tide. 



' See Plate IV. in which the sections of the rivers and estnaries in tank C, 

 E.xperiment II., and tank E, Experiments III. and IV. are plotted to the same 

 vertical and horizontal scales. 



