ON THE ACTION OF WAVES AND CURRENTS. 395 



the estuaries from generator to mouth of tidal river represented about 

 50 miles, and the rivers 54 miles ; in Experiment IV. the estuaries were 

 70, and the rivers 76. 



With the same tide at the mouth the elongation of the estuary would 

 cause the tide to rise higher at the mouth of the river, but as there was 

 only the same quantity of water from the generator the tides with the 

 longer estuaries were smaller at the generators, which would again 

 diminish the tides at the mouths of the rivers. The tides observed at 

 the mouths of the rivers were somewhat higher than in Experiment III. 

 And this fact must be allowed for in considering the results as represent- 

 ing the effect of inci'easing the lengths of the rivers on the distribution 

 of sand. 



In tank E the effect was very remarkable. For the first 5,000 tides 

 the sand rose up the river as far as it was laid, the head of the sand 

 gradually going forwai-d, and the sand falling at the top of the estuary 

 and in the mouth of the river. Somewhat the same appearances 

 appeared in tank F, though it soon became apparent that the advance of 

 the head of the sand was much slower in F, and also the lowering of the 

 sand at the top of the estuary. Sand was going up the river, but it ac- 

 cumulated in the lower reaches. 



In E at 9,000 tides there was an almost sudden change ; the sand in 

 the river was rapidly carried to the top, leaving the lower reaches empty. 

 After 11,000 tides the bottom of the river was swept clean from the 

 mouth to Section 15 (30 miles), and then a steady downward movement 

 of the sand went on all down the estuary until there was deep water all 

 the way down from 10 miles below the head of the river. The clearing 

 of the bottom of the river of sand evidently increased the action of the 

 river, increasing greatly the rise of tide. 



In tank F the result was very different ; instead of the sand shifting 

 suddenly up the river, the sar.d reached Section 15, and then barred the 

 river at Section 11, the river then gradually filling up. At 38,000 tides, 

 when the second survey was made, the tide was still rising at the top of 

 the river, and the head of the sand still proceeding forwards. The 

 experiment was continued to 81,000 tides, and the head of the sand 

 reached Section 19, the tide still rising at the head very slightly. This 

 shows that the conditions of similarity were more nearly fulfilled in the 

 river in tank F in this experiment than in III. The values of the 

 criterion, however, given in the table are lower in IV. than in III. This 

 is because these values are calculated from the rise in the generators 

 which were in these experiments O'llO in tank E and 0081 in F, against 

 0125 and 095 in Experiments III. With the same water going out 

 of the generator there mast have been higher tides at the mouths of 

 the rivers in IV., and as the vertical exaggeration in Experiment IV. 

 waSs/2 times larger than in I. and III., assuming the rise of tide in tanks 

 E and F, Experiments III. and IV., to be as in Experiments I., the values 

 of the criterion in Experiments IV. would be at least 0'261 and 0-103. 

 This is in accordance v/ith the observed results. 



It seems therefore that in order to apply the criterion to the condi- 

 tions of similarity at the top of a long estuary with a tidal river the 

 actual rise of the tide at the mouth of the river should be taken in 

 estimating the value of the criterion for similarity ai these points. It 

 appears however that in no case has the criterion estimated from 

 the tides in the generator exceeded the value '09, but what the condi- 



