An artificial wall or rampart carried nearly across the mouth of an open harbour or bay, to protect vessels moored behind it from the violence of the sea. The two most celebrated structures of this kind are those of Cherbourg and Plymouth. The breakwater at Cherbourg was commenced about the year 1783: it was to consist of a series of truncated cones of timber, approximating at their bases, and presenting to the sea, as they rose to its surface, alternate obstacles and openings. The number of cones, as originally projected to cover a front of 2,000 toises, was ninety, which were afterwards reduced to sixty-four; but this number was never completed. Each cone was to be 150 feet in diameter at the base, and 60 feet at the top, and from 60 to 70 feet in height, the depth of water at spring tides, in the line in which they were to be sunk, varying from about 56 to 70 feet. The cones were to be formed without bottoms, and being floated off" by means of casks attached to them, were to be towed to the spots whereon they were to be sunk; they were then to be filled with stones to the tops, and left to settle for a while; after which the upper part, commencing at low-water mark, was to be built with masonry laid in pozzolana, and encased with granite stone.
The first cone was sunk June 1784, and by the year 1788 eighteen cones had been sunk, occupying a line of 1,950 toises in length; but owing to the disasters which had attended the progress of the works, (the upper parts of several of the cones having been carried away by the violent winter gales,) the undertaking was suspended; in the following year three cones, then building, were sold by auction. Subsequently a plan for casing over the whole length of the old work with blocks of stone was so far acted upon, that in 1803 the centre of the dyke had been brought above high-water mark, and here were placed a battery and a small garrison, the whole 01 which were swept away by the sea in a heavy gale of wind in 1809. Small spots only of the breakwater are now visible above the surface of the sea at low-water spring tides, except near the middle, where a shapeless mass (extending about 100 yards in length) rises to the height of from 18 to 24 feet above high water. Plymouth Sound is very much exposed; and the heavy swell constantly rolling in is much increased when the wind blows fresh from any point between S.E. and S.W. In the year 1806 the improvement of the anchorage in Plymouth Sound, by the erection of a breakwater, was suggested by the late Earl St.
Vincent to Earl Grey, then first lord of the Admiralty; and in the same year Messrs. Rennie and Whidbey were directed to report on the practicability of the plan. Their report was favourable; but from various causes the work was not decided upon until 1811. The method which was recommended for constructing the work was to sink very large blocks of stone in the line of the intended breakwater, allowing them to find their own base, and to assume those positions which gravity would permit, and that blocks of stone from 11/2 to 2 tons weight would be sufficiently heavy to resist the action of a stormy sea. The immense beds of limestone east of Catwater, which were capable of affording stones of still larger dimensions, were bought by government for the sum of 10,000f., and were opened on the 7th of August, 1812, under the superintendence of Mr. Whidbey: five days after the first stone was deposited in the Sound, and on the 31st March, 1813, the breakwater was first seen above the surface of the sea at low water. The breakwater consists of a central part of 1,000 yards in length, and of two wings, each of 350 yards, forming, with the middle portion angles of 158°, the angular points being turned towards the ocean.
The transverse section is of the form of a trapezoid, whose base on an average extends to about 290 feet; its breadth at top 48 feet; and its average depth about 56 feet. The rise of the slope facing the sea is 1 in 7; that of the slope facing the harbour 1 in 5. The average depth of water is 36 feet at low water spring tides, and the breakwater is carried about 20 feet higher, which somewhat exceeds the greatest rise of the tide. The whole length is now (1833) completed, and on the surface the blocks are formed into a convenient path from end to end; but it is decided to build or encase the upper part in solid masonry, commencing at low water mark, and to erect a lighthouse near the centre. The quantity of limestone required for its construction, was estimated by Messrs. Rennie and Whidbey at about 2,000,000 tons, and the probable expense 1,171,000f.