ON THE ACTION OF "WAVES AND CURRENTS. 335 



in a steadily diminishing manner, while at the same time it rose towards 

 the head of the tank at a steadily diminishing rate, until both these 

 changes ceased to be observable. The configuration of the surface also 

 changed at a steadily diminishing rate. The chief features in this con- 

 tigaration were the banks, which gradually formed at the head of the 

 tank in a very symmetrical form, and then extended down the tank past 

 low- water level, losing their symmetry as the low-water channels between 

 them began to take effect. These banks and channels appear clearly in 

 the plan. The minor features were numerous minor channels caused 

 by the water running off the banks. These covered the surface with 

 very beautiful detail, which, however, it is quite impossible to record. 

 Also ripple bars across the channel below low water. 



After the first survey was taken the tank was started again July 17 

 at a somewhat slower period, viz., 66' 7 seconds, and ran for 7,815 tides, 

 when the second survey was made. The daily observation taken as before 

 showed considerable changes of detail — so much so that it was a matter of 

 surprise to find that Plan II. corresponded almost exactly with Plan I., 

 the only difference being slight divergences and deepenings of the de- 

 pressions and raising of the banks. 



The tank was again started on July 25 at a period of 606, to keep 

 the sand in motion until the agitator for producing waves could be put in 

 action. This was accomplished after 7,780 more tides, from which no con- 

 siderable change was observed. The agitator made 200 beats in the tide 

 generator for every tide. At first the agitating bar was straight, 3' 6'' 

 long, with a section 6' broad and 1^" deep. This caused parallel 

 waves '06' high, '8 long. The effect of the waves was at once apparent 

 in the destruction of all the beautiful tracery on the banks, which soon 

 presented a smooth washed-out appearance. After 4,000 tides a -shaped 

 agitator, as shown in the drawing, was substituted for the first. This 

 sent oblique waves of much the same size as before. The waves were kept 

 going during the day and stopped at night ; and after 6,000 further tides 

 the tank was stopped to survey for Plan III. This shows that at low water 

 the waves had to some extent levelled the sand; they had also washed off 

 the ridges of the banks and filled the narrower channels ; yet on the whole 

 the depressions are deeper, and at the head of the estuary there is a 

 marked change in the arrangement on the left side. 



The model was (August 8) set to run at 33" with the wave agitator 

 going during the day. On the 19th it was found that so much additional 

 sand had come down into the generator as to disturb the balance so as to 

 require 100 lbs. additional weight to equalise the period, this was added, 

 and again on Monday the 12th it was found that more sand had come 

 down, requiring 50 lbs. more weight to re-establish the balance. On the 

 13th the sand was removed from the generator and the balance restored 

 by removing the 150 lbs. It had also been found that from some cause 

 the speed fell off considerably ; at one time the speed was 70''. The 

 cause of this was not at first perceived, but somehow the speed was re- 

 stored, though it was subsequently found that the belt on the motor was 

 slipping ; this having been put right, the running at 33" was continued 

 till 12,705 tides had been run since this speed was commenced. Survey 

 IV. was then taken. It was found that the mean period over the interval 

 had been 432" instead of 33". It being uncertain how far the irregular 

 speed and disturbed balance had affected the results, the model was 

 started again August 10 to run at 33" with a mean speed of 33", and 



