ON TUE ACTION OF WAVES AND CURRENTS. 337 



As the experiment progi-essed the sand, instead of havinj^ a nearly 

 uniform downward slope from the head to the generator, had a uniform 

 slope down the middle of the tank, with two large banks extending from 

 section 8 to section 17 on each side, that on the right being longer. 

 The experiment was continued for 11,013 tides, when it was found that 

 the water was much too low, owing to misadjustment of the scummer ; 

 then as there was no possibility of saying how long this had been going 

 on, the experiment was stopped. 



Experiment 2 (Tank B, Plate VII.), August 28. Plan 1.— In this the 

 conditions of experiment! were repeated, the edge of the float having been 

 replaned. The results from starting were almost identical with those 

 observed in Experiment 1. The sand again came down fastest in the 

 middle, and faster than in tank A. Seven pounds were removed from 

 the generator, and subsequently the condition of the model as regai-ds the 

 lateral banks was nearly the same, except that the longer bank was on 

 the left. The experiment was continued with speeds exactly correspond- 

 ing to those of Experiment 5, tank A, until 16,344 tides had been run ; 

 then Plan I. was taken. The tank was then set running again at 

 35'5 seconds and continued for 0,755' tides, when considerable changes 

 had taken place towards the lower end of the tank. A partial survey 

 was then made and recorded, and the experiment stopped. 



Experiments 3 (Tank A, Plate VIII., and Tank B, Plate IX.), 

 Sept. 2. — The sand in both tanks was arranged as before, a new float 

 straightened to a surface plate being used for B, and the level of the 

 sand in both tanks tested by water, as in experiment 6 A, which tests 

 showed that the sand in A was perhaps •01" highest on the left, while 

 in B it was to something like the same extent highest on the right. 



The tanks were coupled, A being driven from the motor and B from 

 A. Both were set to low tide at starting, and the start made at full 

 speed, 33 seconds tank ^. The progressive appearances simultaneously 

 observed were identical, with the same exception as before noted. 

 Immediately after starting the periods of rise and fall of the generator 

 of A were observed, and the fall being slightly the larger 25 lbs. was 

 removed from the balance weight, which restored the equality. After 

 77 tides it was observed that the sand in A was coming down 

 much faster than in B, and had already begun to come into the generator ; 

 the periods of rise and fall were noted, and it was found that the rise was 

 17 seconds and the fall 15 seconds. The weight was replaced, the tanks 

 stopped, and 50 lbs. of sand removed from the generator and lower end 

 of the trough of A which left the end of the sand the same in both tanks. 

 The tanks were then started, and the rise and fall in A were equal. 



It may be well to remark that though the tank B is driven from A, 

 the periods do not .synchronise, so that the unequal motion caused by 

 imperfect balance of A eventually affects all stages of the tide in B 

 equally, while the resistance of B is so small compared with that of A, 

 that any want of balance hardly affects the motor when driving both 

 tanks. In starting there would be the .same disturbance of balance in 

 both tanks owing to the slow descent of the water from the flat sand, but 

 it would be only that of A that would efiect the balance. 



After running 1,653 tides, tank A, it was seen that the sand had come 



into the generators of both tanks, so a stop was made, and all sand below 



section 20 again removed from both tanks — 120 lbs. from A and 12 lbs. 



from B, making altogether 176 lbs. from A against 1'2 lbs. from B. 



1889. z 



