Plan of the coast of Cherbourg / Wm. Faden (1787)

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Plan of the coast of Cherbourg / Wm. Faden (1787)

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Title:: Plan of the coast of Cherbourg / Wm. Faden (1787)

Alternative Title:: The atlas factice of Sir William Beauchamp Proctor

Date of Original:: 1787

Identifier:: RB_GEN_G1793_C6-1800_009.tif

Permalink:: https://n2t.net/ark:/81984/d39z90p13

Notes:: This map was originally bound together with other maps in the book "The atlas factice of Sir William Beauchamp Proctor". Framed.

Caption:: From map: "PLAN OF THE COAST OF CHERBOURG, with A SKETCH OF THE WORKS which the French are now executing TO IN CLOSE AND DEFEND THE ROAD &c"

From map: "AAA Conical Caissons filled up with Stones and higher than High water, Seven of which are already immersed, and several ready to Launch."

From map: "bbb Heaps of loose Granite Stones, with which they are filling the Intervals between each Caisson, and which they are to raise above High water."

From map: "C A Bason (which the Troops are ordered to Dig) to be filled up at High Tide, and the water let out at Low Tide, to cleanse the West Entrance."

From map: "DESCRIPTION OF THE TRUNCATED CONES, OR CONICAL CAISSONS, now constructing to cover the ROAD of CHERBOURG Invented by Monsieru DE CESSART, INSPECTEUR GÉNÉRAL DES PONTS ET CHAUSSÉES."

From map: "THE Conical Caissons are of various sizes, from 147 feet diameter at the base, to 170½ and 179 feet. The first Caisson was 158 feet diameter; consequently the whole surface of its base was 19596 square feet, and, including the casks, covered about half an acre of ground. It was judged proper not to close the bottom of the Caisson, nor let it touch the ground, but only to rest it on the upright beams which compose the frame; wherby likewise it migth be floated with more facility. The perpendicular height, as well as the diameter of the upper circle, is 65 feet, though some of the Caissons may be 80 feet high, according to the depth of the sea.

The Caissons are composed of 80 or 90 upright beams (according to their respective diameters), which are lined from end to end, and are made of pieces of timber from 24 to 30 feet in length: these are sustained by 20 circular ribs on the inside, and by 8 ribs on the outside, which, as well as the beams, are about 13 inches square. They are fastened together by iron bolts, weighing from 21 to 32 pounds, which are tinned over with a new composition, to prevent the marine acid from rusting them. There are 6000 of these bolts in each frame.

The talus, or slope of the Caissons, is 7½ inches per foot under an angle of 60 degrees. Its immersion is 60 feet at the highest spring tides, and 38 feet at the lowest neap tides. 25243½ cubic yards of stone (weighing 52465 tons, at about 2½ tons to each cubic yard), will fill one of these Caissons: the whole extent of the base is besides loaded with about 93½ tons of stones; and this last is done before they set it afloat, in order that they Caissons, on its immersion, may fix itself at the bottom of the sea, so as to refill the force of the tides of flood; the perpendicular rise whereof, being 19 feet, would be able to raise the Caisson one seventh part of its whole height.

The weight of a Caisson so loaded is 770½. They set it afloat by means of 64 large casks, about 12 feet 9 inches in length, and about 6½ feet in diameter, and of 30 or 40 others of smaller dimensions. These are made fast around the base of the Caisson with strong cables , 7½ inches thick. From an experiment made in the port of Cherbourg, each large casks is able to raise a mass of cannon balls weighing nearly 14½ tons, and the whole of the casks will raise a weight equal to 915 tons; so that there remains an overplus weight equal to about 144½ tons, in case of accidents.

The cable-netting at the base is designed to counteract the force of the surrounding casks, which, from their great buoyance, would otherwise tear the frame to pieces as soon as floated. The 4 large casks placed in the center are to keep the netting perfectly tight, their power being equal to 46 tons. When the Caisson is immersed, the netting remains at bottom, but the casks are preserved.

The Caissons are towed to the place of their immersion by means of a capstan, placed on a ponton, and worked by 40 men ; four large flat-bottomed boats, with 72 oars each, tow the ponton. In calm weather they advance 12 or 13 yards in a minute, or near half a mile in one hour. There is likewise a frigate moored beyond the place of immersion, with a capstan, to which a cable (fastened also to the Caisson) is secured, to direct the towing, that it may not deviate from its proper course.

The immersion is completed in one hour at most, by the successive removal of the casks; which is effected by means of knives, with long handles, which have a communication from the gallery with the ropes that fasten the casks. These knives weigh about 120 pounds each, and being numbered four by four, strike together upon the braces of four casks, diametrically opposite to each other, in the whole circumference of the base. This is done in order to keep an equal balance, according to the progress of the immersion ; and with view it is only after the whole is sunk, that they cut the four casks fastened at right angles in the center of the base.

If a Caisson with its ballast, weighing 770½ tons, to be added 25243½ cubic yards (weighing 52465 tons) of stones, which the Caisson contains, the whole mass will then weight 53235½ tons. Now from that calucation, supppsoing the Caisson entirely covered with water, by the most violent storm the action of the sea will operate against the conical mass, at most, only with the force of 25243½ cubic yards of water, weighing 19732 tons ; which, being deducted from the entire mass (53235½ tons), there will remain to the Caisson, if taken in the most unfaourable view, when entirely covered with water, a preponderating weight of 33503½ tons, to keep it firmly fixed against all the power of the waves.

The stability of the Caissons will be secured hereafter by the great quanity of live muscles which have been strewed on the north south sides of the work, and will form, with the sea-weed, a rocky crust which will every day increase in bulk : besides, the sand that will be thrown into each cone to fill up all the intertices, and the dyke, or causeway, to be formed of loose stones between the Caissons, and to be raised to their height, will render them immovable.

About 150 small vessels, called chaff-marées, are employed in transporting stones for filling the Caissons and the intervals."