BREAKWATER is a barrier intended for the protection of shipping in harbors or anchorages. It sometimes happens that, in front of a semicircular bay, a small island is so situated as to form a natural breakwater. This is to some extent the case with the isle of Wight, which occupies such a position as to protect Portsmouth and Southamp ton from the south. In many other places, however, bays and harbors are without such screens. A pier may be so placed and constructed as to serve also the purpose of a B., but the term B. is generally confined to a structure used solely for protection, and not for berthage or traffic, and breakwaters are frequently insulated, so as to be cut off from any communication with the shore unless by water.
Plymouth B. is the best known of these engineering works. The sound or harbor, being open to the s., was so much exposed to storms that, early in the present century, it was determined to construct a B. across its mouth, with openings between it and the shore, on either side, for the ingress and egress of shipping. The works were com menced in 1812. The operations consisted in transporting along a tram-road large blocks of limestone got from a neighboring quarry, shipping them in vessels fitted with trap doors, and by means of these depositing them in the shape of a huge mound in the required situation. As soon as the stones began to appear above water, a perceptible benefit resulted in the relative calmness of the sound during the prevalence of storms; but the structure was frequently very roughly handled by the waves, which altered and flattened its shape. A severe storm in Nov., 1824, threw a great portion of the stones over into the sound. It was not until 1841 that the works were finally completed, by the deposition of more than 3,000.000 tons of stone, and the expenditure of nearly £1,500,000. The B. is nearly a mile long, the central portion is 1000 yards; and two wings, of 350 yards each, extend from the ends of this at a slight angle. The open channels at each end, between the B. and the shore, are each about half a mile wide. and their depth is respectively 40 and 22 ft., at low water. The B. is 133 yards wide at the base, and 15 at the top—the two sides being made very sloping for the security of the stones. The slopes and top are faced with masonry. The water-space protected by this B. comprises 1120 acres, and it is generally admitted that the money has been well spent on the work.
Holyhead B. is formed of stone quarried in Holyhead mountain, drawn along a tram way on a timber structure, and cast into the sea. It more resembles a pier than the B. at Plymouth, for it is attached at one end to the shore, and is intended to convert Holy head bay or roadstead into a harbor of refuge. The works consist of a mound of loose stones up to low water, and ashlar upright walls with a parapet above that line, with a railway on the top for trains.
Portland B. is of very great value, in converting into a harbor of refuge the expanse of water between the Dorsetshire coast and the isle, or rather peninsula of Portland. An act of parliament was obtained in 1847, authorizing the works. The B., starting from the n.e. point to the isle, stretches nearly due n. for more than 2 m., with one or two intervening openings for the ingress and egress of shipping. The works were conducted more easily than those of any other great B.; for the isle contains an abund ance of stone easily quarried, and the steep shores afforded facility for transporting the stones by their own gravity to their destination. The work—which is an upright ashlar superstructure, with a parapet founded on a mound of rubble stones—was done chiefly by convict labor; the depth is about 50 ft. at low-water. From the nature of the opera tion, any part of the B. became useful as soon as constructed, increasing the safety of Portland bay as a harbor of refuge.
Dover B. progresses slowly, and has involved an enormous outlay. There is no stone near to form a mound, as in the other breakwaters spoken of, and, in consequence, the work requires to be brought up in solid ashlar from the bottom by the diving-bell, with the interior formed of blocks of concrete. It has never been clearly stated whether the government regards this B. as a protection to a great naval station and fortified harbor, or as a chief feature as a harbor of refuge for commercial fleets. In 1844, a commission of inquiry recommended that £2,500,000 should be laid out in forming a harbor of refuge at this place. In 30'years the work has not been finished, the great depth and frequent storms constituting terrible obstacles. The water is very. deep—viz., 42 ft. at low-water; the accumulations of shingle very troublesome; and several years must elapse before it can be made evident whether the Dover B. is worth the national money expended upon it.