ON THE ACTION OF WAVES AND CURRENTS. 333 



of the tide running straight up and down such an estuary might tend 

 to shift the sand up or down according to the slope at each point, and 

 the period and height of the tide, or until some definite relation between 

 these three quantities was attained. If such a relation exists, its elucida- 

 tion would seem to be fundamental to a full understanding of the regime 

 of estuaries. 



Further, there was the very important question how far such a tidal 

 action would leave tlie bed beach-like, with uniform slope and straight 

 contours, or groove it with low-water channels as in the mouths of 

 estuaries, i.e. whether a parallel estuary without land water, having a 

 uniform slope and straight contours, would be stable under the action of 

 a tide of which the general motion was straight up and down ? 



Considering that the new rectangular tanks with their clean paraffined 

 sides were admirably adapted for such experiments, and that any internal 

 modelling would have required further time, which was already very shorty 

 if a report was to be presented at the Newcastle meeting, it was decided 

 to commence with a series of experiments on the general slope and con- 

 figuration of the sand with parallel vertical sides, after making some 

 preliminary experiments while the tank A was having a preliminary run 

 to test the working of the motor. 



Following is an abstract account of these experiments and the results 

 obtained. It has not been thought desirable to introduce into this report 

 a complete copy of the note-book. The initial conditions of each experi- 

 ment are given, together with the date, the number of tides run, and the 

 mean period of the tide ; also notes made during the running on circum- 

 stances which are likely to have affected the general results. The final 

 results are contained in the charts (or plans as they are headed), the- 

 longitudinal and cross sections which have been taken from the charts 

 and the diagram of mean slope obtained from the areas of contours. 

 These are all appended to the report. 



Preliminary Experiments with Balls. — Little balls of parafiin the size of 

 peas were prepared, colouring matter having been first mixed with the 

 parafi&n to distinguish the balls, and to so load some that they would 

 just sink while others floated. Then, before the motor was started, the 

 water being quite still, the balls were placed in rows across the tank at 

 definite distances down the tank, and from the centre line — one set of 

 balls on the bottom and another set floating above. The motor was then 

 startfid, and the change in position of the balls noted. 



It was supposed that the floating balls would move with the water 

 and show by any change of their mean position if there was any circulation 

 in the water. This was what they did when the surface of the water 

 was perfectly clean, but the slightest scum very greatly diminished the 

 range of the motion of the floating balls. This matter of scum, if it can 

 be so called, when it is entirely unperceivable by the eye, is very im- 

 portant in these model experiments ; for, however slight it is, it tends to 

 prevent the horizontal motion of the immediate surface, and indirectly t» 

 modify the internal motion of the water ; the only test of perfect freedom 

 from surface impurity is that small drops caused by a splash falling on 

 the surface float along. When the surface was in such a state the 

 floating parafiBn balls oscillated up and down with the water, and kept 

 the position for many oscillations both up and down and across the 

 channel. 



The sinking balls are subject to the constant resistance of the bottom. 



