ON PHYSICAL AND ENGINEERING FEATURES OF THE MERSEY. 550 
works more than the natural depth of water on the Bar, except that about 
the year 1838, when the condition of the sea channels for navigation was 
below the normal efficiency as regards depth and otherwise, Captain 
Denham, the Marine Surveyor of that day, was authorised to harrow or 
rake across the Bar in the channel then forming in the course of natural 
changes at the outer end of the main channel. A sum of between 3,000/, 
and 4,0007. was spent in this work, the precise effect of which is uncertain, 
as in the course of nature a channel having the normal depth was formed 
in this position and was adopted for navigation. 
It must not be inferred that the subject of the Bar obstruction was 
lost sight of through the period intervening between Captain Denham’s 
experiment and the commencement of practical work. On the contrary, 
it had never ceased to be a source of anxiety to the authorities, and more 
particularly, in recent years, to the Dock Board, and the author as their 
engineer, and suggestions for its amelioration had from time to time been 
under consideration. 
There was, however, a natural and wise hesitation to tackle a question 
that presented such formidable difficulties and responsibilities, both physi- 
cal and financial, at all events unless and until there appeared a fair 
prospect of obtaining successful and satisfactory results within reasonable 
limits, both as regards time and expenditure. 
At New York, the western terminus of the great Atlantic ferry, in- 
convenience arising from a similar cause had been felt, and after failure 
of certain expedients, experiments by dredging the obstructed channel, 
undertaken in 1885 and subsequent years, met with a considerable degree 
of success. Although the problems were by no means the same, the 
difficulties at Liverpool being infinitely greater than those at New York, 
the success at the latter port appeared to warrant an experiment on the 
Bar at Liverpool, and it was accordingly decided to undertake an experi- 
ment of some magnitude in dredging. 
Had the lowering of the Bar been dependent on the old-fashioned 
bucket system of dredging, excellent as it is for some positions, experience 
teaches that in this instance costly failure would have been inevitable. 
The employment of the centrifugal pump as a dredger, which is a com- 
paratively recent application, offered the best, and practically the only, 
means of removing the Mersey Bar, which consists of sand of various 
degrees of fineness. The author had made early experiments with the 
centrifugal pump as a dredger, these being carried out after an examina- 
tion which he made in 1876 of a plan he saw in practice in the sandy bed 
of the river Loire in France, where he found a suction dredger at work 
in clearing out the foundations for a bridge oyer that river. His first 
attempt to adapt this principle to the work on the Dock Estate at Liver- 
pool was by fitting up an old mud hopper barge with a centrifugal pump 
and trailing suction-pipe, for the purpose of testing its ability to remove the 
silty accumulation from the docks, and thus supersede the clumsy bucket 
system which was found very inconvenient to work in such confined spaces, 
and was costly. 
This experiment, for the most part, failed by reason of the light 
flocculent character of the material to be dealt with, and consequent im- 
possibility to retain it within the hoppers, as also from the frequency of 
foreign substances which had fallen into the docks, such as ropes, baskets, 
bags, mats, and the like, choking and breaking down the suction pipes, 
