HISTORY OF ENGINEERING.
Book I.
220
when brought to a level, a bed of mortar was spread over the whole; and on this, in thespace comprised between the chamber walls, was a floor of three-inch oak plank, secureddown upon the cross timbers, for the purpose of preventing the springs from workingupwards. Under the walls it was omitted, that the masonry might unite more firmlywith the foundations.
'Flic floor was again crossed by other timbers placed directly over those below, andhalved on the five longitudinal pieces forming a third layer, or grille, which extended onlyover the chamber; these were pinned down with iron; when these timbers were laid,others, which formed the sills to the chief parts of the lock, were placed.
The intervals of this timber platform were then filled up with masonry, and levelled asbefore. A floor of three-inch oak plank was pinned down to the chief timbers; overthis, in a contrary direction, was another floor two inches thick, a fall being preserved ofone in forty-eight in the direction of its length, to allow the water to run ofF when repairswere required.
After six lines of sheet piles had been driven in different directions, the Marechal Vaubanadded a double row at the pointing sills, which, with the other parts of the construction,were then commenced.
'I'lie side walls and their counterforts were set out, and the various courses of facing wereall cramped and worked with care; a backing of clay, 5 or 6 feet in thickness, was rammeddown as low as the first course of masonry, and brought up to the level of high water.
Sluice of the Canal of Bcrgucs , which was used for the purpose of deepening the port ofDunkerque. It consisted of two gates, one contrived to hang within the other; the lesseropened seawards, and the whole turned so as to admit boats into the canal. The outergate, being scarcely anything more than a margin to the first, carried a triangular pieceof framing, which was requisite in order to render the inner gate secure; this is calleda port hustjuevs, and is more curious than useful. At low water the triangular framewas unhooked from the great gate, and the lesser one being relieved, opened suddenlyand allowed the water to pass through with violence, pushing any obstructions before it.At high water the small gate was again closed, and the triangular piece of framing called avalet was hooked to the outer frame, and effectually prevented its opening.
A turning gate in the canal of Bergues , its designed and executed by M. Clement, in theyear 1705, is an excellent example. The width of the lock was 27 feet 6 inches, so thateach gate was in width half that dimension. The ports were 15 by 17 inches, as were thehorizontal rails: these together formed the skeleton framing, in which revolved the turninggate, 13 feet 6 inches in width, and 10 feet in height. Where the rails entered the uprights,they were secured on both sides by strong irons. The upper horizontal rail was 15 by 13.and the braces 11 by 9.
The gate was supported when closed in a reveal on each side, and turned upon its centre;when this was required, it was only necessary to raise one of the paddles, and to keep theother closed, for as the water was allowed to pass through one opening freely, the closed onereceiving the entire weight, was on that side more pressed, and consequently impelledforward by the weight of water against it. Both sides were then closed, and the force,being equal over each half, prevented it from turning cither way.
Sluice at the Mouth of the Quad of the Moe'rc , about 15 feet in width, was not con-trived for the purposes of navigation. It had one pair of gates and a sluice gate, raised bya wheel and capstan, or a wheel termed herisson, formed by a number of handspikes. Thegates were opened at low tide, and after the water had left the canal, the sluice gate wasraised to a height sufficient to allow the proper quantity of water to pass, and the force withwhich it ran out effectually scoured the mouth. The sea again entered the canal, where itwas confined by shutting the gates, when the sluice was afterwards lowered ; at the time oflow water it was let out, and the operation continued as long as it was deemed necessaryfor scouring purposes.
The vanne of the canal of Moore was about 16 feet in width, and 13 or Id feet of waterusually pressed against it; its thickness was about 8 inches, and it was firmly secured byeight bands of solid iron. To raise it, a species of tread-wheel, upwards of 29 feet indiameter, was placed on each side, worked by men ; to the axle that turned round, ropes wereattached, and by the aid of blocks and pulleys, the vanne or gate was lifted to the heightrequired.
The channel of the port of Dunkerque was at first bounded by jetties constructed withfascines, which were laid in the year 1679, at the time Louis XIV . became possessed of theport.
It being found necessary that the sluices should operate more powerfully, and deepen thechannel, the jetties were carried out to a greater distance. Jetties formed with fascines arcadmirably adapted to receive the surf, but require a constant outlay to maintain them, andshould therefore only be used as a temporary expedient, or to facilitate the arrangements ofmore solid constructions, for which purpose, when properly formed, they are exceedinglyuseful. The breadth given to the base was three times that of the height, which here was