530 REPORT—1890. 
below this the sand was still moving up when the experiment stopped. 
The bore still formed at the mouth but very much diminished, and was 
very slowly diminishing. The final condition of the estuary shows the 
contour at low water in the generators extending up to Section 9, and the 
contour at high water in the generator to Section 11. 
In tank IF” with the sharp turns in the river the action of the sand at 
the bottom of the tank was at first sluggish, as in Hxperiment IV. In 
the top of the estuary and river the appearance of things for the first 
10,000 tides was much the same as in Tank E, except that the ripple ot 
the sand did not extend more than half-way up the river and deep holes 
were formed at the bends, banks being formed between them. The bore, 
however, ran up to the end of the river until some time after the first 
survey was taken, and the tide still rose very slightly when the second 
survey was made, though the river was barred by a bank between the 
bends by which the flood just passed in small channels at the sides, 
The sand had risen in the top of the estuary until it virtually closed the 
mouth of the tidal river, and the condition of the estuary resembled that 
obtained in Tank D. This virtually ended the experiment, but oppor- 
tunity was taken to try the effect of a larger quantity of land water, which 
was increased to one quart in two minutes—7.e., nearly three times—and 
the experiment continued for 20,000 more tides without any material 
effect. oi 
In Tank F the action at the lower end of the tank was again slug! ‘sh. 
At the top of the estuary and in the river the conditions of the sand Were 
as near as possible similar to those in Tank H, but, as it came out, the 
mean level of the water relative to the level of the sand was some 5 feet 
(reduced to a 30-foot tide) lower in F than in E. 
The appearances for the first 16,000 tides were the same as far as was 
observed ; the ripple now extended up to the top of the river and no 
banks formed at the mouth. Nevertheless, before the second survey was 
taken, the tide ceased to rise above the mouth of the river, proving that 
the previous failure to realise the same state in the small tank as in the 
larger had not been entirely due to the bends in the river. The question 
remained whether it might not be owing to the higher level of the sand 
relative to the mean level of the tide. 
This question brings into prominence a fact observed during all the 
experiments, but which had not previous to the experiments on E and F 
assumed a position of importance. This is the gradual diminution of the 
rise of tide owing to the lowering of the sand. 
29. The rise of the tide depends not only upon the rise of the generator, 
but also upon the tidal capacity of the tank. This capacity is the product 
of the area of the surface at high water multiplied by the rise of tide less 
the volume of sand and water above low water in the generator. Now 
in starting the experiments with the sand at the level of mean tide, not 
only is there much more sand above the level of low water in the generator 
than when the final condition of equilibrium is obtained, but the quantity 
of water retained on the top of the level sand is considerable, so that the 
tide rises considerably higher in the generator at starting than when the 
condition of equilibrium is obtained, which excess of rise gradually 
diminishes as the sand comes down at the lower end of the estuary. 
Although the foot of the sand comes down pretty rapidly at the com- 
mencement of the experiment, owing to the surface being rippled the 
water runs off slowly, ard it is not till the sand at the head of the estuary 
