§24 REPORT—1890. 
and it was allowed to run on 16,000 more tides and again surveyed. 
Plan 3 shows the result ; the ripple has increased in breadth though rather 
diminished in depth. 
24. Experiments to find the Limits to Similarity. Experiment IV., B, 
Plate III., October 22 to November 27.—In this the rise of tide was 
0-094 foot and the vertical exaggeration as compared with a 30-foot tide 
71. In Experiments I. and II., B, with a rise of tide 0-088 and a vertical 
exaggeration 68, described in the First Report, it had been found that the 
rate and manner of distribution of the sand did not correspond with that 
in the corresponding experimert in the larger tank, indicating that with 
an exaggeration 68 the tide of -088 was somewhat below the limit of 
similarity. The determination of these limits being a primary object of 
the investigation, it appeared desirable to repeat these experiments with 
a slightly higher tide. In IV., B, the character of the action presented 
the same peculiarities as previously observed, but in a smaller degree, 
and the final state, as shown in the plans and in the curve of slopes 
(Plate I1.), is a much nearer approach to the general law, the conclusion 
being that in IV., B, the conditions were still below the limit, but nearer 
than in I. and II., B. 
Hzperiment VIII, A, October 22 to November 14.—This was an ex- 
periment to determine the manner of action with the same horizontal 
scale as the first part of Experiment V., A, but half the rise of tide. 
Experiments I. and II., B, with a rise of tide of ‘088 foot and a period 
of 36 seconds, being a vertical exaggeration of 68, had indicated that with 
this rise of tide a change in the manner of action had already set in, 
but it was none the less desirable to see what would be the character 
of the action and the final state of equilibrium well below this limit. 
The rise of tide in VIII., A, was 0-088 foot and the mean level 0°138 
foot from the bottom, and the period to 70 seconds, the sand being placed 
level at a uniform depth of 15 inch to Section 18 as in the previous 
experiments. The vertical exaggeration would thus be only 34. 
The manner of action of the water on the sand was in this case 
essentially different from that in any previous experiments even in 
I. and II., B, although it presented characteristics which had been in- 
dicated in those experiments. Instead of the sand being in the first 
instance rippled over the whole surface a middle depression was formed, 
extending some way up the estuary, the bottom and sides of which were 
rippled ; the rest of the sand soon became set and yellow. After 16,000 
tides 4 survey was made and the experiment continued to 24,000, when 
another partial survey was made, showing very small alterations, and 
those nearly confined to the rippled channels. It was, in fact, clear that 
the apparent equilibrium was owing to the sand having become set, and 
that to proceed till real equilibrium was established would take an almost 
indefinite time. 
As the setting of the sand, owing to the slow action of the water, 
appeared to play such an obstructive part, it seemed possible that better 
results could be obtained if the sand could be kept alive with waves. 
Accordingly the experiment was stopped, to be repeated with waves. 
Heperiment IX., Tank A, Plans 1, 2, 3, Plate IV. (with Intermittent 
Waves), November 16 to January 4.—The conditions were the same as in 
Experiment VIII., with the addition of the waves. 
This experiment presented the same characteristics as those observed 
in VIII., A. The rate of action did not fall off so rapidly or completely 
ie Oi 
