ON THE ACTION OF WAVES AND CURRENTS. 339 



etream will depend on the slope and quantity supplied, and supposing the 

 quantity to be proportional to /tL^, the depth of the water in the low 

 water channels (not the depth of the channels) will be proportional to 

 the cube root of the slope ; 



2. The size of the grains of sand, which require a certain velocity 

 before they move ; 



3. The fouling of the sand by growth, &c., which increases as the 

 shifting of the sand diminishes ; and 



4. The viscosity of the water, which causes a definite change in the 

 internal motion of the water when the velocity falls below a point which 

 is inversely proportional to the dimensions of the channel. 



The effect of 1 would be confined to the channels ; 2 and 3 would 

 tend to diminish the rate of action ; the 4th might seriously alter the 

 rate of action at different parts of the estuary, and would also affect the 

 appearance of the sand surface. 



The ground so far covered by the experiments has been confined to 

 one initial arrangement and to one height of tide in each tank, these 

 being similar. Two periods have been tried in each tank, the relation 

 between the periods in the different tanks being as the square roots of 

 the dimensions. Six experiments have been started : 



2 in tank A with a period of 53 sees. 



••• >) >> )> ">^ >> 



2 in tank B „ „ 36*5 „ 



'■ » >> j> ^o'o „ 



Of the two experiments started at 53 seconds in tank A the first was 

 continued for 12,697 tides, and then for 3,589 tides at a period of 50 sees., 

 and a survey made (Plan 1). It was then continued 7,815 tides at 65"1 

 sees., and plan 2 marked; it was then continued 17,750 with inter- 

 mittent waves at a period of 60 6 sees., and a survey made. 



It was then continued for 12,705 tides at periods varying from 33 sees., 

 having a mean 43, and Plan 4 made, then continued at a period of 

 333 sees, with intermittent waves, when it was re-surveyed (dotted on 

 Plan 4). 



The second experiment at 53, tank A, was continued to 8,700 tides 

 with the same results as the first. 



Of the two experiments in tank B the first was continued to 11,013 

 tides as in A, then stopped. The second was continued to 12,058 tides 

 at 3G8 sees. ; then at 4,280 tides at 36 sees., and surveyed (Plan 1 B) ; 

 then continued to 6,769 more tides at 36, and again surveyed. 



The experiments started at 33, tank A, and 233, tank B, were 

 continued to 16,603 tides and then surveyed (Plans 7 A, 3 B). 



In all these six experiments the manner in which the water commenced 

 and proceeded to redistribute the sand was essentially the same, the 

 general appearances of the surface being, with the exception of one or 

 two particulars, the same at the same number of tides up to 1,200. After 

 this the two low-speed experiments in B began to present moi-e notice- 

 able differences from the other experiments, which continued to present 

 similar appearances at corresponding tides to the end. 



It thus appeared : — 



1. That the rate of action was proportional to the number of tides ; 



2. That the first result of the tide- way was to arrange the sand in a 



z 2 



