560 REPORT — 1887. 



cities in the channel would be as the square roots of the vertical scales — 

 about glj — and the ratios of the periods would be the ratio of horizontal 

 scales divided by this ratio of velocities, or 



33 1^ 



31800—960' 



Hence, taking 11-25 hours 40,700 seconds as the tidal period, the period 

 of the model 



= — — - =42 seconds (about). 

 950 



This period was adopted for working the model from the shaft. 



It was then found that the circulation at the top of the flood, which 

 was very evident while the bottom was flat, caused a general rise of the 

 sand on the Cheshire side and lowering on the Lancashire, which Avent 

 on for about 2,000 tides. That during this time, owing to the increase 

 of flood up the Lancashire side and the diminution of that on the Cheshire 

 side which followed from the deepening of the one and the shoaling of 

 the other, the circulation steadily diminished until its character was so 

 changed that it could no longer be called a general circulation, and that 

 after this, although there were further changes in detail going on in the 

 estuary, the two sides maintained a steady condition as regards depth for 

 low tides. 



During this time banks were formed and low-tide channels, which 

 resembled in all the principal features those actually in the Mersey ; the 

 eastern bank, with the deep sloynes on the Cheshire side, the Devil's 

 Bank and the Garston Channel, the Ellesmere Channel and the deep water 

 in Dungeon Bay and at Dingle Point — all these were very marked in 

 character and closely approximate in scale. 



And, wliat is as important, the causes of these as well as all minor 

 features could be distinctly seen in the model. 



The eastern and Devil's Bank are seen during the process of their 

 formation to be simply an internal bar formed hy carrying the sand 

 brought down by the ebb out of the narrows and sloyne, until debouching 

 into the broad estuary ; its velocity is so far diminished that it can no longer 

 carry its charge, just as happens at the mouth of every river. The pecu- 

 liar configuration of these banks is explained by the existence of two 

 lines of eddies from about half-tide to the top of tlie flood : the first of 

 these is caused by the sharp corner at Dingle, and lies between Dingle 

 and Garston, the eddies having their centres over the Devil's Bank ; and the 

 second, caused by the divergence of the Cheshire Bank towards Eastham, 

 having the lines of centres over the Eastham Bank. These eddies, which 

 during the most rapid part of the flood only efiect a diminution of the 

 velocity of the flood, cause, as the velocity slackens toward the top of the 

 flood, back water to set in along both shores, which back waters, starting 

 the ebb, cause this to be strongest over the Garston and Eastham Channels, 

 which are thus kept open. 



The lateral configuration of the shores at Dungeon Bay and at 

 Ellesmere is seen to cause back waters to exist in these bays during 

 the whole of the flood in the latter, and from one to two hours before 

 the top of the flood in the former, which fully accounts for the deep 

 water at these points. The existence of these back waters in the actual 

 channel has been verified. There are many other circumstances brought 



