88 REPORT — 1856. 



In the Rock Channel the average depth had been diminished, and the 

 average area stationary. 



As it was during this period that the greatest amount of tidal area taken 

 between two surveys was abstracted, the occasion is favourable for consider- 

 ing the influence of works of that kind upon the sea channels. According 

 to the evidence of Mr. Rendel, C.E., House of Commons, 1844- (see 'Ports 

 and Docks of Birkenhead,' by Thomas Webster, M.A., F.R.S., Barrister-at- 

 Law, 1848, p. 77), high water of an 18-foot tide is I'' 25" later at Warring- 

 ton Bridge than it is at the Prince's Pier, Liverpool, where it is 35" later 

 than at the Formby Light-vessel. And from Mr. Joseph Bouit's observa- 

 tions at W^oolston Weir, four miles above Warrington, that on 8th March 

 last, in a 21 -foot tide, high water was 1^ 50" later than was recorded by the 

 tide-gauge at George's Pier, Liverpool. It follows, therefore, that the water 

 which formerly covered the space now enclosed must have passed out to sea 

 on the top of the ebb tide, whilst the flood tide was yet rising in the upper 

 reaches of the river. 



The loss of depth in the Rock Channel appears to indicate that the abs- 

 traction of the tidal area has been prejudicial. The surveys since 1833 

 indicate a progressive, though irregular, tendency towards the silting up of 

 this channel; and there are facts which render it probable that the efl'ects of 

 diminished scour should first be manifiested here. 



The tidal establishment is earlier at the North-west Lightship, or entrance of 

 the western channel, than it is at the Bell Buoy, or entrance of the northern 

 channel ; though the difference is very slight, it is sufficient to give a bias to 

 the stream of tide, as is shown by the experience of bathers on the shore just 

 above the junction of the Rock Channel with the river, who find that with 

 a young flood there is a current out again to sea by the northern channel. 



The same also appears from the experiments of Mr. Enfield Fletcher, C.E., 

 and others with floats. These were liberated at Wallasey Pool, on the ebb 

 tide, for the purpose of ascertaining in what time the water from the pool 

 -would reach the Victoria Bar; but all the floats, without exception, went 

 down the Rock Channel and grounded upon Dove Spit. 



This result may, in part, be due to the attraction of the Cheshire shore. 

 The bias with the ebb would, however, be confined to the upper stratum of 

 the water; the impetus of the current to sea naturally giving to the main 

 bulk the more direct course by the northern channel, in preference to the 

 almost right-angled deflection down the western channel. 



Whilst the Rock Channel has been losing depth, the depth of water in the 

 northern channel, considered in its whole length from the Rock Lighthouse 

 to the Bell Buoy, is almost undiminished since 1833. The loss on the Vic- 

 toria Bar may be due to the diversion to the part of the stream formerly by 

 the Zebra, now by the Queen's Channel. But for the elevation of the banks 

 and of the bottom of the Rock Channel, and of the south part of the Crosby 

 Channel, it is difficult to assign any other cause than the loss of scour at the 

 first of the ebb, and the influence of the prevailing winds in drifting sand 

 from the coast. 



As respects the Rock Channel, the influence of the new north wall in 

 Bootle Bay is very likely to agg^ravate the tendency to silt up, as it tends to 

 impede the advance of the flood tide through that channel by substituting 

 for a shelving shore a nearly perpendicular face almost at right angles to the 

 course of the flood. 



The influence which the direction of the enclosure walls may have upon 

 the course of tide has yet to be considered. 



It appears that between 1846 and 1849, during which these works were 

 in progress, there was no alteration in the direction of any of the channels; 



