ON THE ACTION OF WAVES AND CURRENTS. 523 
harmonic curves are at all parts of the tank comparable with a simple 
harmonic motion. But it is important to notice that they are not essen- 
tially so, being merely comparable with the motion of the generator, so 
that if the generator were given a compound harmonic motion, such as 
that of the tide in the mouths of most estuaries, these curves would have 
a different dynamic significance. These curves would still be valuable as 
showing the state of progress and final similarity of the tidal motion at 
the same parts of the estuaries, but to bring out their dynamical signifi- 
cance it would be necessary to substitute a simple harmonic motion with 
the same period for that of the generator. 
§ IV.—Descrirtion or tHe Hxpurmments on THE Movement or Sand 
IN A 'TIDEWAY FROM SEPTEMBER 9, 1889, ro Sepremper 1, 1890.! 
23. Continuation of Experiments VII., Tank A, and III., B, Plate IIT., 
September 7 to October 11.—These experiments were in progress at the 
time of the Newcastle Meeting of the British Association, and had so far 
advanced that tracings of the first surveys were exhibited and included 
in the First Report. So far as they went, they took an important place 
in the conclusions arrived at in that report, showing that with a vertical 
exaggeration of 100 the results obtained in the small tank (B) with 
rectangular estuaries without land water as to rate and general distribution 
of the sand were closely similar to those obtained in A, and that the 
mean slopes reduced to a 30-foot tide in these experiments agreed with 
those obtained in A, with vertical exaggerations of 64, It was desirable 
to continue these experiments to see how far a state of equilibrium had 
been arrived at. This was accomplished by the assistance of Mr. Foster, 
who kindly looked after the running of the tanks till the return of the 
author and Mr. Bamford in October, and thus enabled a month which 
would otherwise have been wasted to be utilised in obtaining an experi- 
ence of the effect of about 100,000 tides after apparent equilibrium had 
been obtained in each tank. Daily records of the counters were taken, 
and although there were several stops the intervals of running gave the 
periods very constant. 
The plans show but little alteration, except that the sand, particularly 
in B, had shifted upwards and accumulated somewhat at the head of the 
estuary, leaving the slope the same ; a circumstance which would be ac- 
counted for by a difference in the level of the water, and which is also 
indicated by the mean slope reduced to a 30-foot tide shown in Plate II. 
The agreement of the slopes here shown as compared with the mean 
slope in the case of Experiment V., A, which has been introduced in 
this, diagram for the sake of comparison, is quite as great as could be 
expected, considering the difficulties of the experiments, and affords very 
valuable evidence of the permanence of these slopes when once a condition 
of equilibrium has been attained. 
In respect of the ripple the two tanks presented a very different ap- 
pearance, which is clearly shown in the plans and sections. While the 
ripple in A was comparatively small and shallow, in B it was larger and 
deeper than anything previously noticed; that this was a symptom of 
the condition of B being on the verge of dissimilarity seemed probable, 
and to test this the period of B was increased from 23°85 to 26-5 seconds, 
* In the published report of these experiments it is not thought desirable to give 
the daily records of progress in the notebook. 
