ON THE ACTION OF WAVES AND CURRENTS. 531 
has been raised and a slope formed that the water runs down freely at 
low water, so that during the early part of the experiment not only is 
the rise of tide at the head of the estuary high, but also the low tide and 
the mean level of the tide. The result is that the mean level of the water 
at the head of the estuary is higher during the early part of the experi- 
‘ment. These changes in the tide at different parts of the estuary and at 
different stages of the tide are well shown by the automatic tide curves, 
Plate XVIII. As the sand is rising at the top of the estuary the result 
of the high water is to raise the first banks above the level to which the 
tide finally rises. 
As these banks come out and the ripple is washed off, leaving smooth 
surfaces and channels from which the water runs, leaving clean dry 
banks, the mean level as well as the rise of tide falls, leaving the tops of 
the bank, which were at first covered, high and dry. 
These effects were much greater in Experiments C and D than in A 
and B, and still more marked in K, F’,and F. In EH, F, EF’, the plans 
land 2, taken at 16,000 and 33,000 tides respectively, show the differ- 
ence in the level of the sand at the mouths of the respective rivers. In 
Tank E the rise of tide at the mouth of the river was observed to be 
0:02 higher at 16,000 than at 33,000 tides, and in Tanks F and EF” at 
16,000 tides there was a bore which ran up to the top of the river, while 
at 3° ,000 tides the sand at the mouth was not covered at high water. 
t thus seems that the condition of things which follows from starting 
wii. the sand level and a constant height of low water is to institute a 
distribution of sand at the top of the estuary corresponding to a state of 
equilibrium with a higher tide than that which ultimately prevails; and 
the greater the initial height of the sand relative to the mean level of the 
water the greater will be this effect. That this action tends to explain 
the closing of the mouths of the rivers in Tanks I” and F and not in E is 
clear. But it is not clear that this is the sole explanation ; the conditions 
in F’ and F were not far removed from the limits of similarity obtained 
in the rectangular tanks, and it is not clear that these limits may not be 
somewhat different in the long estuaries with tidal rivers. This is a 
matter which requires farther experimental examination, for which there 
has not been time. 
30. Experiment IT. in E and F, Plates XV. and XVI, without Land 
Water, August 5 to September 1.—These experiments have been made 
under the same conditions as in E and F, 1, except for the landwater. The 
general appearance of the progress of the experiments was nearly the same, 
and Pian 1 shows little difference. But as the experiment in E proceeded 
it became clear that the river was going to fill up gradually from the end. 
The bore no longer reaches the end at 16,000 tides, while it had ceased 
to exist and the tide had ceased to rise at Section 11 in the river at 
32,000 tides, the end of the estuary also having filled up, the action in F 
being nearly the same. Thus we have evidence similarly shown by both 
estuaries that, although the fresh water produces little effect on the 
condition of equilibriam of a broad estuary, the existence of a long tidal 
river above the estuary does produce a great effect on the level of the 
low-water channels in the upper portions of the estuary, and that land 
water, even in such small quantities, is effective to keep open a long tidal 
riyer emptying into a sandy estuary or bay. 
