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HISTORY OF ENGINEERING.
Rook I.
level of low water. Eighteen embrasures were formed for cannon, ami so arranged, thatsixteen could be brought to bear upon vessels in the roadstead : on the other side it com-manded all the approaches to the citadel.
The lower floor, which was entirely below the ground, served as a deposit for stores; theupper for the use of the garrison, above which was the magazine for powder. On the firstfloor was a chapel, and on the second a room for the deposit of biscuit.
Over the roof was contrived a lighthouse or beacon, 8 feet in diameter, with a smalldome covered with lead.
On one side of the fort was the lodging for the commandant, consisting of kitchen,dining-room, and two chambers, with all the necessary offices, from which was a readycommunication with the whole of the ramparts. On the opposite side was a lodging forsix artillery men, a covered shed for the cannon, and the magazine, in which ammunitionwas kept: adjoining these were the barracks, consisting of twelve rooms capable of accom-modating an hundred men. Three staircases conducted to the several parts of the ramparts,and the whole was well provided with water, which was kept in two large cisterns, placedopposite the governor’s house.
Vauban executed this work without a coffer-dam, at the time of low water, the tide risingabout 13 feet. Piles were first driven round the site to be built upon, and four-inch plankin six feet lengths secured to them, and further kept in their places by binding pieces oneach side. These were surrounded by sheet piles.
The Canal of Marclich, cut after the demolition of the fortifications at Dunkerque, todischarge the superfluous w'aters, was completed in the year 1715, under M. Le Blanc; andthe lock he constructed was considered the finest in Europe . It was divided into twopassages, one 47 feet in width, the other 27 feet; its length was 295 feet; that of themiddle wall 32 feet, and each of the side walls without the buttress, 25 feet. It was distantfrom the sea 3384 toises: the width of the canal was 50 toises, and its depth 21 feet.
The turning gates employed at this sluice were remarkable for some ingenious con-trivances : in particular, that placed in the widest passage; that in the smaller resembledone already described in the lock at Borgues.
When required to be closed, one paddle was lowered, and the other raised: each openingbeing made a little more than 6 feet square. The weight on the fresh water side beinggreater than that towards the sea, it was necessary to provide an additional contrivance tokeep the turning gate secure in the reveals against which it lodged. For this purpose,two locking bars, moved by a perpendicular rod, were introduced; a simple jack at topelevated the rod to which the latches or locking bars were attached, and which being raisedrelieved the gates.
To raise the paddles, the lock-keeper descended by a ladder, placed on the land side,against the gate, and by means of a rack and pinion lifted it up ; the other paddlebeing previously lowered, that the sea might close the gate on that side when the canal hadreceived the quantity of water required. Capstans and cables were attached to the lowerhalf of the gate, and facilitated both its opening and shutting. A chain secured to eachside by means of rings prevented the gate from closing, and supported it against the violenceof the water as it rushed through.
Gravelines is situated on the river Aa , which runs through a fertile portion of Artois, andempties itself into the sea, amidst the sands and dams thrown up on the coast, which beingat this point extremely flat, there was a constant accumulation of stagnant water in theditches, for which it was difficult to obtain any outlet. Graveiines became the tomb of allthe garrisons sent there.
At the commencement of the seventeenth century, Philip III. , King of Spain, turned hisattention to its improvement, and constructed a canal near Gravelines to carry off the waterby a shorter course ; and at about 900 toises from the counterscarp, where high waterflowed, a large sluice with a double pair of gates was formed: this was soon afterwardsdestroyed by order of Cardinal Richelieu .
In 1659 it was ceded to the French , and in 1737 Marechal Vauban commenced im-proving the drainage of the district. He formed a new lock, divided by a wall of masonryinto two unequal passages, intended to aid the discharge of the waters of the river at thetime of land floods. Its length was 105 feet and its width 100 feet. Coffer-dams weremade use of, and after the earth was taken out to a sufficient depth, piles 8 or 9 feet inlength, and 12 inches square, were driven entirely over the whole site ; others were added tothe side and middle walls, altogether amounting to nearly a thousand, all placed in parallellines, morticed and tenoned into the longitudinal and cross sleepers above.
After these and the necessary rows of sheet piling were driven in, the ground waslevelled, and the earth taken out to the depth of 30 inches ; the spaces were then filled withmasonry. In this was laid the lower grillage or timber platform, composed of twelve longi-tudinal timbers 12 inches square, so placed that they lay under each side wall, the samenumber under the middle wall, one in the narrow passage, and two in the other. Thesewere secured to the heads of the piles by pins 12 inches in length and 1 inch square. Upon