SCOURING AND SLUICING. 285 



bottom of the basin, and the sluices were intended to operate in 

 such a manner as to discharge a sheet of water that could be 

 regulated to any degree of force by sluicing- valves. 



The low-water basin was ultimately constructed to the 

 following dimensions 1 (Fig. 284). 



Length 1750 feet 



Width at western end 400 



Width at mouth or east end 300 



Depth below old dock sill 21 



Area 14 acres 



The arrangements 2 for sluicing (Figs. 285, 286) consisted 

 of two chambers or passages leading from the float, each 30 feet 

 in width, and placed one on each side of the entrance-lock. 

 These chambers were furnished with gates similar to a dock- 

 entrance, and provision also made for closing the end of each by 

 means of a caisson. The inner sills were laid at a level of 8 feet 

 below the old dock sill. The floors were inverted, and fell away 

 from the float at an inclination of 1 in 33 for a length of about 

 200 feet, where a level of 1 5 feet below the old dock sill was 

 attained ; they then dropped rapidly, and in a length of 25 feet 

 reached the 18 -feet level, at which they continued for a distance 

 of 45 feet to within 15 feet of the mouths of the sluices. In the 

 latter length there was a slight fall of 6 inches. In the cham- 

 bers, the width of 30 feet was continued for a distance of 30 

 feet from the commencement at the float ; they then began to 

 open out by a gradually enlarging width, until each finally 

 closed at the back of the respective sluices, wide enough to 

 embrace the whole group on one side. 



The sluices were twenty in number, arranged in groups of 

 ten on each side of the lock. The leading channel of each 

 sluice measured 5 feet 2 inches by 8 feet, equal to a sectional 

 area of 41 feet 4 inches for each sluice, or a total area of 826 

 feet 8 inches for the twenty sluices. At the chamber end each 

 sluice widened out horizontally, and to a slight degree vertically ; 

 but towards the basin the shape altered considerably, until the 

 aperture became 4 feet 6 inches high, by 12 feet in horizontal 

 width. The sills of the sluice openings were set at the level of 

 18 feet 6 inches below the old dock sill, and immediately outside, 

 and in the basin a stone apron was laid with its surface form- 

 ing a continuous slope with the sills for a distance of 30 feet, 



1 M.P.I.C.R, vol. xxviii. p. 521. * Ibid., vol. xxviii. p. 522. 



