Page:The American Cyclopædia (1879) Volume III.djvu/241

BREAKWATER 235 make safe harbors and roadsteads. The outer mole of the harbor of Civita Vecchia was built by the emperor Trajan for this purpose ; and the piers of ancient Pirteus and of Rhodes are of the same class of structures. Josephus says that Herod, in order to form a port between Dora and Joppa, ordered mighty stones to be cast into the sea in 20 fathoms water, to pre- pare a foundation ; the greater number of them being 50 ft. in length, 9 ft. deep, and 10 ft. wide, and some even larger. In the use of such immense blocks of stone, the true princi- ples of constructing a permanent barrier to the waves appear to have been better understood than they were 17 centuries afterward. In modern times, the great breakwaters are those of Cherbourg in France, of Plymouth in Eng- land, and of Delaware bay in this country. From the experience acquired by their con- struction and history, principles before little understood have been established, upon which such works must be built to withstand the enormous forces opposed to their permanency. These were so little known in the last cen- tury, that one of the commissioners appointed by direction of Louis XVI. to report upon the best locality for establishing, opposite the Eng- lish coast, a port and naval arsenal, recommend- ed the construction of a dike of 2,000 toisesin length, in water 70 ft. deep, in front of the har- bor of Cherbourg, by sinking a vast number of ships filled with masonry as a nucleus, and cov- ering these with heavy stones to within 18 ft. of the surface. And when at last four of the ablest naval officers and engineers of France were appointed to execute the work, which was regarded as one of the most stupendous operations, certainly the greatest piece of hy- draulic architecture, ever undertaken by man, the plan they adopted was one which proved impracticable after having been prosecuted from the year 1784 to 1789, at enormous expense. This plan was the construction of huge trun- cated cones of timber, which, of the reduced size at which they were actually built, meas- ured 36 ft. in height, with a circumference of 472 ft. at base, and 339 ft. at top, the angle of the slope being 60. This was strengthened by an interior concentric cone, 5 ft. 10 in. within the outer one. The frame of each was made of 80 large upright timbers 24 ft. long and 1 ft, square. On these were erected 80 more of 14 ft. in length, making 320 of these uprights for the two exterior and two interior portions. The structure was then planked, hooped, and firmly bolted together. The first cone was built and floated at Havre, then taken to pieces, trans- ported to Cherbourg, and floated off and sunk on June 6, 1784; the second on July 7, in the presence of 10,000 spectators ; but before the cavity of this one could be filled with stones, its upper part was demolished in August by a storm of five days' continuance, and the stones it contained were spread over the bottom, in- terfering with the placing of the next cone. The original plan was to set 90 of these cones, of 150 ft. diameter at base, 60 at top, and 65 ft. in height, in succession, and fill them with loose stones or masonry, and the spaces between them with a network of iron chains, to break the force of the waves. The number was after- ward reduced to 64. After the second cone went to pieces, the government directed that the remainder should be set 192 ft. apart. This distance, by a new order, was increased to 1,280 ft., the spaces to be filled in with loose stones. At last, when 18 cones had been sunk at enor- mous expense, and with serious damage to many of them, the plan was abandoned, and the tops of those left standing were cut off down to low- water mark in 1789, and the system of construc- tion by sinking rocks was recognized as the only process sure to succeed. The filling in of stone was continued till, at the end of the year 1790, the quantity sunk was estimated at 5,300,- 000 tons; and the total expenditure, by the estimate presented to the legislative assembly in 1792, was about 31,000,000 francs, or $5,800,- 000. The commission appointed in 1792 by the departments of war, marine, and the interior, reported, after careful examination of the dike and of the partial protection it already afford- ed at different stages of the tide, that its sta- bility could not be depended upon except by the use of larger blocks of stone as a facing than had before been employed, these stones to be at least of 15 to 20 ft. cube ; and they rec- ommended that the dike be raised 31 ft. above the level of the lowest tide, which would make it about 9 ft. above that of the highest tides. But the revolution succeeding, further work was interrupted. In 1802. by advice of a new commission appointed two years previously by a new government, it was determined to raise the central portion of the breakwater to the height before recommended, for 195 metres (640 ft.) in length, and to give it a breadth at top of 19 '5 metres, in order to construct upon it a battery of 20 pieces of the heaviest artil- lery ; and it was proposed to finally complete the two extremities in the same manner. At that time the old work, which had originally been raised to low- water mark, was reduced by the action of the sea to 15 or 18 ft. below it, and the profile imparted to it was regarded as that of greatest stability with least expenditure of material. The interior slope was one of equal height and base, 12'5 metres. The slope exposed to the sea had at bottom a height of 6 - 3 metres to a base of 9, succeeded by one of 6 '2 to a base of 47'5 ; its original form was a uniform slope of 1 in height to 3 of base. The sea washing over the top tended to move the stones from the outside to the inside ; and it was essential to oppose this action by raising the top above the surface of the water. In 1803 the central portion was completed to low- water mark, and a superstructure or parapet, of blocks of 60 to 80 cubic feet each, was rais- ed along the south or inner side to the height of the highest tides, along which the smaller stones used in the construction, pressed upward