ON THE ACTION OF WAVES AND CDKRENTS. 399 



showed very little inequality in the rise and fall. On attempting to 

 increase this by farther altering the balance, it was found that this could 

 not be done. To continue this part of the investigation it would have been 

 necessary to introduce complex gearing. Time did not suffice for this, 

 and the study was not carried further. 



20. Experiments loith Tides varying from Spring to Neap, Tank E, 

 v., VI., VIL, rilL, Tanl: A, XIII. ^Plates X., XL, XII., and XIV., 

 March 20 to August, 1891. — The gearing for tank E having been modified 

 so as to cause a rise in the generator, varying to over an interval of 

 29 tides, the variation being harmonic and adjustable, so as to admit of 

 any relation between the maximum and minimum rise. 



These were adjusted so that the mean rise was the same as the rise in 

 Experiment V., the spring and neap rises being in the ratio 3 to 2. A 

 drain with an adjustable orifice was put in the bottom of the tank to 

 drain off neai'ly all the fresh water, and the scummer adjusted so as to 

 draw off the excess of land water at low spring tide level ; this being 

 adjusted by trial until when running the mean tide level was the same as 

 before. 



Experiment V. was then restarted without the sand having been 

 disturbed to afford a preliminary trial of the apparatus, the period 

 being that of Experiment V., 36 seconds. This was continued 18,000 

 tides, till the apparatus was completely in hand ; then the sand was 

 relaid for Experiment VI., Plan 2, Plate X., in which the conditions 

 were the same as V., except the tide. The mean rise in the generator was 

 the same in VI. as in V., and the ratio of the spring and neaps 3 to 2. 

 This brought the rise in the generator at spring tides in VI. greater than 

 that in Experiment IV., in the ratio of 1"1 to 1. The action on the sand 

 was miich more rapid than in Experiment V. with the uniform tide being 

 nearly as quick as in IV. The sand reaching the top of the river in 

 13,000 tides, as against 10,000 in IV. and 25,000 in V., and the bottom 

 of the river being swept as clean in 17,000 tides in VI., as in 14,000 in IV. 

 In other respects the action in VI. very closely resembled that in IV. 

 The rate of action was a little slower, but the action itself seemed rather 

 stronger, as corresponding to a higher tide. Surveys were taken at 

 20,000 and 34,000 tides. The experiment was then stopped, in order to 

 make the conditions comparable with those of Experiment V ; it being 

 quite clear that the action of spring and neap tides, having a mean rise 

 equal to that of a uniform tide, was not only much more rapid, but led to 

 a different state of final equilibrium. 



Experiment VIL, Plan 1, Flate XL. — In this the tide was adjusted 

 until the rise of the generator at spring tide was the same as that for the 

 uniform tide in V., the other conditions being all the same. 



The character of the action now became identical with what it had 

 been in Experiment V., but the rate was decidedly slower. Thus the 

 sand moving up the river reaches : — 



Section 19 after 13.000 in V. and 39,000 in VI. 

 „ 27 after 20,000 „ „ .'51,000 iu VI. 



The survey taken after 



18,000 

 51,000 



are almost identical, the latter being a little the forwardest. 



18,000 tides in Experiment V , Tank E, and 

 51,000 „ „ VII., „ 



