Alderney 13. is a great work, consisting of ashlar wallo and parapet, built on a stone mound up to low-water from a depth of 72 feet. Small breakwaters have been con structed at Cette near Marseilles, at the mouth of the Delaware in the Fuited States, and at Buffalo iu lake Erie; but they do not call for description.
Cherbourg B. is the greatest and the most costly ever constructed. Nearly 100 years ago, 31. de Cessart proposed to the French government the formation of a 13. at Cher bourg, to be commenced by the construction of a number of hollow cones formed of timber-framing, sunk in a line as close as they could be placed to each other, and then filled with stones. These cones, of which there were to be 64, each about 70 ft. high, 150 ft. in diameter at the base, and 60 ft. at the top, were intended to form a nucleus to the stone breakwater, to prevent the stones, during its formation, being knocked about and too much spread out by the action of the waves. In 1784 to 1788, 16 cones were con structed, and 13 of them sunk; but so great was the destruction which they underwent during stormy weather, that the government at length abandoned the plan, and carried on the stone breakwater without the aid of the cones. It was completed under Napoleon III, at a' cost exceeding £2,500,000. The B. itself was finished in 1853, but since that
year large fortifications have been built upon the upper works. The length is nearly 2} in.; the B. is 300 ft. wide at the bottom, and 31 at the top. The chief mass consists of rubble or unshaped stones, thrown down from ships; but there is a larger ratio of wrought and finished masonry than in the Plymouth B., consisting of granite blocks imbedded in cement. The depth of water is about 60 ft. at low-water spring-tides; and the B. rises to 12 ft. above high-water level. The water-space included within and protected by the B., is about 2000 acres, but two thirds of this has scarcely depth enough for the largest-sized ships. The relation which this B. bears to the vast military and naval arrangements of the place will be noticed under CHERBOURG.
Many substitutes have been proposed for solid breakwaters, such as floating break waters constructed of timber framework, open iron screens, Me., but noue of them have been shown to be suitable for actual practice. Close timber-work, filled in with stones, is found to be quite efficacious; but on most of our coasts the timber is liable to be eaten by the marine worm, which is an almost insuperable objection to its being used under water.