ON EIVEES AND ESTUAKIES. 559 



nothing further wa? done until 1885, when my attention was directed to 

 the inner estuary of the Mersey. 



This estuary may be described as a crescent-shaped shallow pan, 

 eleven miles long by three broad, lying north-west and south-east, having 

 its upper horn pointing east and its lower horn north ; the northern horn, 

 being prolonged for five miles into a narrow deep channel, runs north to 

 the outer estuary or sandy bay of the sea. One of the most marked 

 features presented by the configuration of the bed of this inner estuary 

 is the invariable preference of the low-tide channels for the concave or 

 Lancashire side ; whereas, were the estuary acting merely the part of a 

 river, whether during flood or ebb, it would be expected to follow the 

 nsual law, and have the deepest water on the convex or Cheshire side. 



That this prevalence of the deepest water on the concave side must be 

 the result of the momentum left in the water by the flood at once seemed 

 to me probable ; for if the bottom were level or deepest on the Lancashire 

 side the effect of the curved shape would be to cause the flood entering at 

 the northern horn to follow the south-eastern or Cheshire shore, and the 

 momentum of this water would tend to carry it round the head of the 

 estuary and back along the Lancashire side ; would, in fact, tend to set 

 np a circulation before the top of the flood was reached ; so that on the 

 Lancashire side the water would be moving down the estuary before the 

 ebb commenced ; whence, considering that the flood tends to raise the 

 bottom and the ebb to lower it (for the reasons already pointed out), it 

 seems that the stronger flood on the Cheshire side would raise this side, 

 Avhile the stronger ebb on the Lancashire side would lower this. This is 

 supposing the bottom to be level. 



In order to verify these conclusions a vessel was constructed havino- a 

 flat bottom and a vertical boundary of the same shape as the high-tide 

 line of the inner estuary from the rock to the same distance above 

 Runcorn. The horizontal scale was 2" to a mile, and the vertical scale 

 1 inch to 80 feet, -j-iioTj- 



A shallow tin pan was hinged on to the otherwise open channel at the 

 rock, by raising and lowering which, when full of water, the m.otion of 

 the tide could be produced throughout the model through the narrows ; 

 the true form of the bed of the channel was given to the model by means 

 of paraffin. And in order to obtain approximately the proportional 

 depth in the inner estuary, sand was placed level on the bottom so that 

 the high-tide depth was reduced to the equivalent of about twenty feet. 

 The idea in making this model was not so much to obtain a shifting of the 

 sand as to show the circulation of the water as resulting from the flood 

 tide with a level bottom. In the first instance the tide pan was raised 

 and lowered by hand, but as at the first trial it became evident that the 

 model was not only going to show the expected circulation, but was also 

 capable of showing, by the change in the position of the sand, the e9"ect 

 of this circulation on the configuration of the estuary and other important 

 efiects, it was arranged that the model should be worked from a con- 

 tinuously running shaft. The working of the model by hand at once 

 showed that there was only one period of working at which the motion of 

 the water in the model would imitate the motions of the actual tide in 

 the Mersey, which period was found to be about forty seconds ; a result 

 that might have been foreseen from the theory of wave motions, since the 

 scale of velocities varies as the square roots of the scales of wave heights, 

 so that the velocities in the model which would correspond to the velo- 



