S REPORT 1856. 



neers will show how their predictions have been corroborated, and how 

 necessary it was to fi-ame and constitute a Commission of Conservancy. This 

 was done upon the principles laid down by Messrs. W. C. Mylne and George 

 Rennie, in their report of 1837, as also from the assistance of the marine 

 surveyor. Lieutenant Lord. 



The general principles laid down by Messrs. Whidbey, Chapman and 

 Rennie, in their report of 1822, to the Corporation of Liverpool, were — 



" That all channels through which water flows must be of a magnitude 

 proportional to the quantity which passes them ; and any increase or dimi- 

 jiution of that quantity will enlarge or diminish the channel, unless where 

 formed of material so hard that the strength of the current is not able to 

 remove them." 



Mr. Whidbey says, in his report of 1818, "Tide harbours are deep or 

 otherwise, in proportion to the quantity of water that flows into them from 

 the esa, and the fresh water that comes down from the interior. The greater 

 the quantity of water, the greater will be the depth, from the eff'ect which 

 the increased body of water will have in scouring the bottom at the time of 

 the ebb tide, and carrying out the sullage." 



Again, with reference to embankments, Mr. Whidbey says, — 



" It is evident that if a certain portion of either side of a river or harbour 

 be embanked, and the tide be prevented from flowing over it in its usual 

 way, a diminished quantity of water will flow in from the sea equal to the 

 cubic contents of what has been embanked, and consequently there will be a 

 less quantity to ebb out ; and the scouring effect being thereby lessened, it 

 will be rendered incapable of carrying out to sea the sullage and alluvious 

 matter washed down from the country, with the same force as before the 

 embankment was made." 



The same principle was advocated by Messrs. Chapman, Rennie, Walker, 

 Giles and Stevenson, in all their subsequent reports relative to encroachments, 

 and to obstructions made to the free flow of the tide by piers and jetties. 



The very accurate survey and maps of the estuary made by Mr. Giles for 

 the Corporation, by the recommendation of the late Mr. Rennie, is one of 

 the most valuable records of any harbour in existence. It forms, in fact, the 

 standard for all future surveys, with reference to any changes which may 

 take place. 



The annexed is a catalogue of the reports which have been made by the 

 engineers and surveyors of the Mersey. The calculation of the area and 

 contents of the estuary of the Mersey between the Blackrock at entrance, 

 and Woolston Weir above Warrington, as shown by the annexed tables, 

 No. 2, are taken from Mr. G. Rennie's report of 1838. 



Captain Denham, the surveyor to the port, in his report of 1836, gives his 

 opinions on the causes of variations of the Devil and Pluckington Banks, and 

 expresses considerable doubt how far their removal could be effected by 

 jetties projected from the Cheshire shore. 



Lieutenant Lord, who succeeded him as surveyor, in his report of 3rd April, 

 1840, proposed a similar remedy. The question had been previously dis- 

 cussed, and remedies proposed, by former engineers. Lieutenant Lord's 

 report of the 23rd March, IS^O, entirely coincides with the opinion of former 

 engineers in the necessity of preserving the whole of the estuary and its 

 tributary streams from encroachments, and the necessity of guarding the 

 shores from the action of tlie winds and waves by defences of stone, and that 

 the limits of high-water margin should be accurately defined. 



As regards the tides, these have been accurately defined for a long period 



