392 REPORT— 1891. 



(2) At the mouth of the estnary the flow of water had so far been 

 straight up and down, and equal all across the estuary. By rendering 

 the mouth unsymmetrical, circulation would be set up which would render 

 the up-currents stronger at one part and the down-currents stronger at 

 another, an efi'ect which would correspond to some extent to that of tidal 

 currents across the mouth of the estuary. 



(3) The large tidal sand ripples below low water in the model estuaries, 

 with the flood and .ebb taking the same course, constitute a feature which 

 it is impossible to overlook, yet the existence of corresponding ripples 

 had been entirely overlooked in actual estuaries until they were found to 

 exist when they were looked for, having been first seen in the models. 

 The reason that they were overlooked before is, no doubt, explained by 

 the fact that the bottom is not visible below low water in actual estuaries ; 

 but this is not all. In the estuaries these ripples, where found, have been 

 confined to the bottoms and sides of the narrow channels between high 

 sand banks, and they do not occur on the level sands below low water 

 towards the mouths of estuaries to anything like the same extent as in 

 the models. By rendering the estnary unsymmetrical and so causing the 

 ebb and flood to take diSerent courses, this effect, as explaining the 

 greater prevalence of ripples with symmetrical estuaries, would be tested. 



These considerations led to the repetition of Experinipnt V. in tank F, 

 at first with a single groin extending from the right bank into the middle 

 of the estuary at the mouth, and subsequently to the introduction of 

 .three more groins from alternate sides of the estuary to the middle, up 

 the estuary, and then to the introduction of similar groins into tank E, 

 during Experiment VII., with spring and neap tides. 



The result of these experiments is to show conclusively : 



(1) That the laius of similarity found for symmetrical channels v:ith 

 uniform tides hold with sinuous channels for uniform or varying tides. 



(2) That the greater uniformity of the depth of sand on cross sections of 

 models tvith symmetrical boundaries than toith actual estuaries, does not 

 •exist when the hanlcs are equally irregular. 



(3) That the circulation caused by the unequal flow of the tide in model 

 ■estuaries tends greatly totalce the snndout, and that the natural tendency in 

 •an estuary to scarp the boundaries so as to increase its sinuosities tends 

 (jreatly to the deepiening of the channels. 



(4) Tliat in the models ivith boldly irregular boundaries the tidal ripples 

 are much less frequent than in the symmetrical models, being confined to 

 places ivliere there are n.n cross currents, as in actual estuaries. 



10. Conclusion of the Investigation. — It seems that the objects of this 

 investigation have now been accomplished. 



The investigation of the action of tides on the beds of model estuaries 

 has been found peifectly practicable. Two tanks have been kept running 

 night and day from June 22, 1889, to August 1891, and have each ac- 

 complished upwards of 1,200,000 tides, representing the experience of 

 2,000 years. Such difliculties as protecting the sand from extraneous 

 disturbance and keeping it free from fouling, regulating the levels of the 

 water, the tidal periods, the rise of tide, forms of the tide cui-ve and the 

 supply of land water, observing and recording the I'esults, have all been 

 fairly overcome, though none of the precautions taken could have been 

 safely dispensed with. 



The limits to the conditions under which the results will conform to 

 the simple hydrokinetic law of similarity have been fairly established j 



