' Directions were accordingly given to M. de Caux, Commanding Officer_ of Engineers at Cherbourg, to commence, as a preparatory measure, with the construction of a fort on the Island of Pelee, and another on Du Hornet, according to plans given in by Vauban in 1679 ; by these works the road stead would be flanked on the right and left. The interval, however, being found too great to afford sufficient protection to all the ships that might re quire to be anchored in the roadstead, M. de Caux presented a plan to the Minister at War for con structing an intermediate fort in the sea, which should be casemated, and sufficiently large to con tain all the buildings necessary for a garrison. The surrounding walls were proposed to be sunk in "The sons of 6000 feet square at the base, and 52 feet high. The top of the platform was to be 80 feet high from the bottom of the sea, and the area of its surface 1000 square tosses. This plan, however, was hot considered to give sufficient shelter to a fleet from the winds and waves, and new projects were called for by the Government.
In 1777, M. de la Bretonniere, Capitaine ddVais seau, one of the commissioners Who had been named to report on the comparative merits of the two road steads of Cherbourg and La Hogue, had addressed a memorial to the Minister of Marine, in which he expatiated, at great length, on the numerous advan tages held out loy the former, and particularly with regard to the security of the anchorage. He pro posed to construct, at the distance of a league in the sea, a stone dike of 2000 toises in length, leaving three open passages into the roadstead it was in tended to cover ; one in the middle, and one at each extremity. This dike, like that which was sunk be fore Rochelle, was proposed to have as its nucleus a number of ships filled with masonry, floated off and sunk in proper situations, and afterwards to be cased with large sunken stones, to the height of 50 feet above the bottom of the sea. The reason as signed for sinking the stone vessels was the supposi tion that an under current might cause so much motion at the bottom of the sea as would derange the level, and work away the loose stones ; so little appears at that time to have been known of the in creasing tranquillity of the waves of the sea, in pro portion to the increasing depth of water.
On this plan the commissioners observed, 1. That in order to construct a dike of 2000 toises in length, with sloping sides proportioned to its height, there would be required so great a number of old ships as could hardly be collected in all France in less than ten years ; and, if purchased from foreigners, the expence would be enormous. 2. That the assem bling and employing the necessary number of sea men would be next to impossible, but, if possible, highly impolitic, when, just at the close of a mari time war, commerce felt a pressing want of their ser vices; whereas it might be practicable, and would be advautageoes, to employ the military for some time before disbanding them. 3. That no camped.= would bold good between the roadstead of Cer bourg /pith an opening to the sea of 3600 tam, and a depth of 40 to 42 feet of water at the lowest ebb, and the closing up the entrance of the port of Ro chelle, which is only 740 toises in length, and the depth of water only five or six toises. 4. That the
upper part of the projected -dike, being exposed to the violent action of the sea, the stability of that part could not be depended on ; and besides, a dike covered at high spring-tides with 18 feet water would not fulfil the two indispensable conditions—smooth water, and protection against an enemy. These ar guments were deemed conclusive, the plan of .M. de la Bretonniere was abandoned.
In 1781, M. de Cessart, Insppctor-General of Bridges and Embankments,. received directions to prepare a plan that should cover a fleet of 80 to 100 ships of war in the roadstead of Cherbourg, from the attack of an enemy, and protect them against • the elements. M. de Cessart was fully aware that, to raise Xbarrier in front of this roadstead, and in the middle of the sea, capable of resisting the impetuosi ty of the waves, and repelling the enterprises of. the enemy, was no easy task. " Nothing," says he, " that I had ever performed, or that I had ever read of, in an cient or modem history, appeared to me to be worthy of being placed in comparison with the grandeur of this project." He suggested, as the preferable and only mode of answering the purpose of producing smooth water in the roadstead, that, in the place of one con tinued dike or mole, a number of large masses, se parated from each other, of a circular form, with an. elevation greatly inclined, should be substituted ; in short, a series of truncated cones, which, touching each other at their bases, might present to the sea at the surface, alternate obstacles and openings, and thus interrupt and break down the waves previous to their entering the harbour. He also considered that, as these openings at the surface would not ex ceed 72 feet, a sufficient barrier would be formed against the passage of an enemy's vessel ; and that, if necessary, in time of war, it might be rendered still more secure by placing strong chains of iron across the intervals. It was proposed to ceastruct these conical caissons, of wood the number of which to cover a front of 2000 toises would amount to go, which, at 360,000 Byres for each cone, would cause a total expence of 82,400,000 for the whole. The number, however, was afterwards reduced to 64, anchthe time estimated for completing the work 13 years. Each cone was to be 150 feet in diameter at the base, and 60 feet in diameter at the top, and from 60 to 70 feet in heighth, the depth of water at spring-tides, in the line in which they were intend ed to be sunk, varying from about 56 to 70 feet. They were proposed to be sunk ;rithout any bottoms in them, by which the upward resistance of the water acting on a base whose surface was equal to 17,678 square feet, would be avoided. The caissons floated of by casks, attached to their inner and outer circumference, being towed to the spot where they were destined to be sunk, were then to be filled with stones- to the tops, and left for a while to settle ; after which the upper part, commencing with the line of low water, was to he built with masonry laid in poonslira, and encased bath stones of granite.