silting of a concrete superstructure resting on a rubble-stone base. Several of the most important of these structures are briefly described in the accompanying table, rearranged from a table published by Mr. Louis Y. Schermerhorn, civil engineer, in a report to the United States Gov ernment.
The first five of these structures are of peculiar construction, in the respect that the concrete superstructure is built of blocks resting on the rubble base at an inclination from the vertical, and having their tops bound together by a super imposed mass of concrete. This construction is supposed to permit the blocks to settle indepen dently without destroying the integrity of the structure as a whole.
The Presque Isle Point Breakwater, near Mar quette, on Lake Superibr, is a timber crib structure filled with stone. The cribs were each 24 feet wide and 50 feet long, and varied in depth to suit the varying depth of water. These cribs were framed with solid 12 X 12-inch timber side walls, solidly bound together with 12 X 12 Before passing to a description of a few typ ieal breakwaters at the Great Lake ports in the United States, some mention should be made of a number of important ocean breakwaters con inch cross-tics 24 feet long, running front wall to wall and dovetailed at the ends. Timbers 6 X 12 inehes and 24 feet long tie the walls of the cribs lengthwise. These cribs were sunk close to gether, end to end, fastened to each other and tilled with stone. After filling, the tops of the cribs were covered svith a continuous plank deck ing. The cost of this breakwater was $32 per linear foot. At Buffalo, N. Y.. in Lake Erie, a new breakwater now under construction is a tim ber crib structure filled with stone. At this
point the depth of water is 30 feet. and the lake bottom is covered with mud about 38 feet thick. The method of construction is to dredge a chan nel through this mud to hard bottom and fill it with gravel, and to found the timber cribs on this tilling. NVhile the timber crib structure is the standard breakwater construction for the Great Lake ports, the practice has become familiar in recent years. when repairing such structures, to remove the timber work to a safe point below tow water and replace it by a superstructure of eon erete. A notable example of this class of work is found in the new breakwater at Cleveland, Ohio.
Floating Breakiratcr.--The use of floating structures of iron or wood has often been sug gested for breakwaters. but they have seldom been used. The idea generally has been to anchor the structures at intervals, so that the waves striking against them would become disinte grated or deflected.
Further information on breakwaters may be obtained by reference to the following works: Harbors and Docks (London, 1885) ; Proreedings of the Institution of Ciril Engineers (London, 1S3S, current) ; Reports of the Chief of Engineers, United Plates of America (Washington. 1860. current) ; Cresy, Eneyclo mrdia of Ciril Engineering (London. I847) ; Engineering .Veirs (New York, 1577, current) ; Spon, Dictionary of Engineering (London, 1880 SI) ; Stevenson. The ronstruetion of Harbors (New York. 1886) ; De Cordemoy, Lcs ports (Paris, 1900). See also the article lImumEs in this Eneyeloptudia, in which are included maps showing the harbors formed by a number of the above-mentioned breakwaters.