218

HISTORY OF ENGINEERING.

Book I.

for this purpose was at the end of the canal of Bergues , in width 26 feet, with a double pairof gates ( Partes Busquees ); one of these supported the waters of the canal, the other that ofthe sea at high water, so that vessels could pass from the canal to the port, and from theport to the canal at certain times of the tide ; this arrangement was one of the earliest of itskind. The sluice had others attached to it, which turned when the sea was low and theharbour dry, to allow the waters which served as a reservoir in the canals to How out withso much violence and velocity that they thoroughly scoured the harbour and the channelbetween the jetties; its effects were felt to the extent of 1600 toises, the distance ofthe sluice from the head of the jetties. At the mouth of the canal of I-a Moerc, a sluiceof a different construction was established, which served a similar purpose; when theyacted singly or together, they produced a force which perfectly answered the desiredpurpose. . '

A third sluice at the canal of Furnes, within the town, contributed to clear the channel,and from 1701 to 1710, the port was cleaned and scoured out to the depth of 15 feet bythese contrivances.

The grand sluice to the basin at Dunkerque , constructed by Marshal Vauban, in the year1684, was considered throughout Europe the masterpiece of its kind. Its width was 42feet, and the gates supported a weight of water, 20 feet in height.

The foundations were laid on a moving sand, into which were driven eight rows of piles;these carried as many pair of binding pieces, through or between which were driven a doublerow of sheet piles, care being taken that those of one row covered the joints of the other.Between the heads of the piles was laid a foundation of brick, carried up with mortar madeof Dutch tarras, and the whole was brought up 18 inches in height, and made level withthe tops of the binding pieces; other piles were driven under the walls. Between the firstand second row’ of sheet piles were laid ten cross timbers; between the second and third,fifteen ; between the third and fourth, two; between the fourth and fifth, six ; between thefifth and sixth, two; between the sixth and seventh, eleven ; and between the seventh andeighth rows, ten; these cross timbers extended throughout the whole width of the founda-tions that were to be occupied by the sluice. Between these timbers the spaces werefilled in with masonry, and the whole brought to a level; a ffoor of oak plank was thenlaid over, dowelled, pinned, and securely caulked at the joints.

On this floor was a second row’ of cross timbers immediately over those below, exceptingin that part occupied by the four principal pieces which came under the base of the framingof the pointing sills, which were 54 feet in length, entering at each end 3 feet intothe walls, and their scantling was 24 inches square, or a double thickness. Forty-three lon-gitudinal timbers at right angles with the tw-o row’s of transverse timbers w’ere now halveddoum, and securely pinned. On these were laid a third range of transverse timber, whichformed a second grillage, ami after the intervals were filled up with masonry, a second floorof planking was laid over the whole, and pinned and caulked down as the other. Thequantity of timber employed in this construction seems, however, to have been more thanwas necessary.

The masonry of the side w’alls was then carried up, leaving in each a small aqueduct,arched with stone; they were 3 feet in width internally, and lined throughout with Jiardand durable stone, to resist the violence of the water; these were the more necessary as thegates, which were curved, had no small sluices for the passage of the w’ater.

'Die counterforts were made equal to sustain any pressure to w’hich the side walls mightbe subjected ; and two others were placed to receive the iron ties that secured the collarsof the gates. The aqueducts were closed by paddles, 3 feet 6 inches wide, each of whichsustained a considerable pressure.

In the construction of the side walls, four iron uprights were introduced 6 feet below thetop, at the groove where the paddles worked, two being placed on each side, and the wholefour at an equal distance, occupying a square of 3 feet each way. Each of these iron up-rights had at the lower end an eye, with an iron key passed through it, 8 feet in length and3 inches square ; these were attached to iron anchors, 7 feet in length and 3 inches square,the whole for the purpose of rendering it solid. On the top of the masonry were foursolid blocks of timber, 18 inches square; on these was placed the box with a groove inwhich was the nut of the screw. The nut was turned by means of levers, and the screwmounted, which carried with it the paddle below.

The Sluice at the Mouth of the Canal of Bourbourt /, used for scouring the harbour ofDunkerque , had a clear width of 14 feet, and the side walls were sufficient in length toallow of two sea and two land gates, with a swing bridge between.

After the several rows of piles which carried the binding pieces that confined the sheetpiling were driven, the cross timbers were placed under the sills, their extremities beingallowed to pass through the side walls. On a series of longitudinal timbers was laid acourse of transverse ones; after the intervals were closely filled with masonry, there waslaid a floor of plank. Above this and immediately over the first transverse timbers, wasanother, which was doubly floored ; the upper planks crossing the lower at right angles.