Chap. VI.
FRANCE.
239
through rings attached to piles driven conveniently for the purpose. When the masonsentered, they commenced their work, by filling up the voids between the bottom timbers withpuzzolana and lime, mixed with gravel andchippings of stone; upon this, properly levelled,was built a stone wall 8 feet in thickness, the external faces of which were carried up withblocks from 12 to 14 inches square, dovetailed into each other; the space between beingfilled with moellon.
As the caissoons sunk as they were loaded, it became necessary to keep them from swervingor getting out of their line, and greater attention was required when they had reachedwithin a short distance of the bottom. Proper examination being made of their position,when found correct and perpendicular, they were loaded with the rest of the materialsto be employed in the construction, which, when raised within 2 feet below the level of highwater, was allowed to remain for a considerable time to acquire the requisite hardness,after which the several partitions which separated the caissoons were removed, and thespaces of 2 feet between them filled up, and the wall completed throughout its wholelength. This portion of the work required that sheet planks shod with iron should bedriven in a manner to surround the several intervals, after which beton was let down intothem, and the facing brought up on both sides to unite with the others, when it was madegood throughout.
In the ports of the Mediterranean slips were made at very early times for the con-struction and launching of vessels of considerable draught, and also for the purpose ofhauling vessels that needed repair on shore.
When they required careening, a frame made of timber, and called a cradle, wasplaced under the vessel and supported it in a state of equilibrium, until it was brought tothe stocks and shored up. This cradle, of a very early invention, was formed of threelongitudinal timbers, 20 inches square, which extended the whole length of the ship, onebeing under the keel, and the others at the side a little above. 'Die vessel was girdedwith strong cables to these timbers, which were mounted on eight rollers that were turnedby means of handspikes. The form given to the slips was that of a w’edge, the inclinedplane of which was in length about 220 feet ; the bottom projected into the sea, xvherea timber platform terminated it. Its inclination, like that of the stone bottom, was equalto a fourteenth, the rise being about 5 inches to every 6 feet, which permitted the vesselwhen launched to glide easily into the water, without subjecting it to any sudden shockor interruption. As the vessel had neither its cargo nor ballast, 16 feet water was reckonedsufficient for it to float.
Another slip at Toulon was commenced by taking out the ground to a solid foundation,for 220 feet in length, and 60 in width, using for the latter part of the operation the dredgingmachines employed for cleansing the harbour. The bottom had a sufficient fall or in-clination towards the sea, at the point where the slip was commenced. The stocks wereestablished 18 feet below high water at one end, and 22 feet at the other ; the constructionwas carried on by the means of caissoons, 60 feet in length, and of a depth sufficient tomaintain 4 feet above the level of the water at the highest state of the tide. They werefixed in their places and inclosed in a space 230 feet in length, and about 60 in width, andin this case the caissoons were not loaded, but the water was suffered gradually to enter andsink them ; afterwards they were loaded with the stone intended for the constructions,and the openings which admitted the water being closed, the water they contained waspumped out. Another precaution, taken after the caissoons were grounded, was toplank and pile around them at the points of their junction, and form a clay dam, so thatwhen the ends or divisions which separated the caissoons were taken out, the work mightbe made good and the wall continued in its construction without any interruption. Thisarrangement had very considerable advantages over the preceding, and was another steptowards improvement.
Quay at Rouen , built by De Cessart. — The great road from Paris to Havre and Dieppe ,passing along the ancient quay of Rouen , it was found inconveniently narrow, and in 1779,a new quay was completed, 120 feet in advance of the original wall.
The total length of the new quay was 110 toises ; this was divided into seven equaldistances, by caissoons 66 feet in length, 16 feet wide, and 14 feet high, their base containing1056 square feet. Ninety-two piles were driven, 3 feet 6 inches apart, in the thickness ofthe wall, and 3 feet in the length of the caissoon ; each pile bore the weight of 18,633 pounds,for the wall alone, and adding one half more for the weight of merchandize placed on it,would then have 27,000 pounds.
All the piles were from 12 to 15 inches thick, driven with a ram weighing 1200 pounds,falling 20 feet, each pile receiving a percussion equal to 300,000 pounds. The heads of thepiles were cut off 6 feet below low water, at 12 feet behind the wall; piles were driven atevery 6 feet, to attach land ties, which supported the masonry ; sand and gravel were thenthrown from the inside of the wall, to form a slope on the river side of sixty degrees, whichextended 60 feet into the river, so that vessels of 400 tons could approach the quay at lowwater.