SCOURING AND SLUICING. 275 



It was proved by experiment that with a head of 25 feet, 

 the force of water was sufficient to scour the sand away to a 

 depth of 16 feet below the old dock sill datum in an hour and 

 a half. The whole of the sand-bank affecting the landing-stage 

 could therefore have been readily removed by scouring, had not 

 the stage intervened. 



It was, however, found impossible to work the sluices to 

 advantage with the pontoons high and dry aground, presenting 

 an almost solid barren immediately in front of the outfalls at 

 low water, and had it been possible to do so the effect would 

 have been disastrous to the stage ; for the river bed would have 

 assumed the form of a huge step on the longitudinal section, the 

 tread being 400 feet in length and the rise 7 feet in height ; the 

 consequence of which would have been an inevitable collapse 

 of the pontoons where the step occurred. The only prudent 

 course, therefore, to be followed was to work the sluices with 

 great caution so as to avoid the formation of abrupt stepping, 

 ridging, and furrowing; and, in order that such irregularities 

 might be instantly detected, soundings were carefully taken 

 every day until the stage became properly water-borne from 

 end to end. 



A partial trial of the sluices, continued for several days, 

 removed a large quantity of sand from between the outlets of 

 the culverts and the stage, as well as from under it at the 

 extreme south end ; but further north the bottom was not affected 

 beyond the centre line, the reason being that the sluices could 

 not be brought into use until the pontoons were off the ground, 

 by which time the tide had risen sufficiently to obstruct the 

 sluicing current, rendering it inoperative beyond a distance of 

 90 feet from the outlets. 



Under these circumstances, it was found necessary to resort 

 to mechanical appliances to assist in putting the sand in suspen- 

 sion, so that it might be in a better condition to be acted upon 

 by the sluicing current. 



These appliances took the form of scrapers, as commonly 

 used for clearing the dock sills, but with a special form of tooth. 

 Flat harrows, provided with tines or spikes (Figs. 277, 278), and 

 drag-chains armed with plates (Figs. 279, 281), all of which 

 were dragged over the bottom by steam-crabs placed on barges 

 (Fig. 274). 



For the purpose of reducing ridges in the sand, the formation 



