Page:The New International Encyclopædia 1st ed. v. 12.djvu/281

* LIGHTHOUSE. 255 LIGHTHOUSE. which, known as ykenyvoic rock, carries the lighthouse. The engineer was Allan Stevenson, who adopted a design tor the tower resembling more nearly the frustum of a cone than did the Eddystone and Bell Kock towers. The tower is circular, 42 feet in diameter at the base, 16 feet in diameter at the top, and 138 feet high to the base of the lantern. Work was begun in 1838 and finished in 1842. Wolf Rock lighthouse, situ- ated on Wolf Rock between Seilly and Lizard Point, ofl' the English coast, has a tower in the form of a concave elliptical frustum 110% feet high, 41% feet in diameter at the top. It was begun in 1862 and finished in 1869. The Heaux de Brehat lighthouse is situated on the rock of the same name about three miles from the most northerly point of the peninsula of Brit- tan.y. It consists of a tower with an interior cylindrical opening 14 feet in diameter, 158 feet high from its base to the lantern floor. The tower consists of two parts, the lower part be- ing a concave frustum and the upper part be- ing nearly a true cylinder. The lighthouse of Ar-men, which is situated on one of a series of rocks off Cape Finisterre, France, is another ma- sonry lighthouse of French design which is no- table for its exposed situation and the consequent difficulties of construction. The height of the focal plane of the light is 96 feet above sea-level. The lighthouse of Triagoz, in the Department of Nord. is another French example of exposed sea- rock lighthouse construction. The masonry tower is square and 92 feet high, with the focal plane of the light 6 feet above this height. In America, masonry lighthouse towers of notable character have been built at Minot's Ledge. Spectacle Reef, and Tillamook Rock. Minot's Ledge, off the town of Cohasset, in !Massa- ehusetts Bay, about 20 miles east-southeast of Boston, is one of the most dangerous places in the ■world withovit a signal. The difficulties in the construction of a lighthouse tipon this rock were immense. The first structure, which was of iron, was completed in 1849. but stood only imtil April. 18.51, when it was demolished by a terrific, storm. In 1852 money was approi)riated by Con- gress for a new lighthouse, and work was com- menced in 1855. but it was not till the latter part of 1857 that the first stone was laid. The struc- ture was completed in 1860. It is a granite tower in the form of a frustum of a cone, having a base 30 feet in diameter, and a height of stone- work of 88 feet, the lower 40 feet being solid. The courses are dovetailed, and are fastened with wrought-iron dowels. The lighthouse at Spectacle Reef, in the north- ern part of Lake Huron, was built not only to resist waves, but ice-fields, often covering thou- sands of acres and moving at the rate of two or three miles per hour. That the structure should be able to withstand this force, it was so designed as to cause the ice to be broken and piled into a protecting barrier. The tower is the frustum of a cone, 32 feet in diameter at the base, and 18 feet jnst beneath the cornice at a height of 80 feet. The whole height of the stone- work is 93 feet above the base, which is 11 feet below the surface of the water. The tower is solid as high as 34 feet, above which it contains five stories, each 14 feet in diameter. The work was commenced Jlav 1. 1870, and the light was first used June 1, 1874. The cost was $375,000. Tillamook Rock is a bold isolated basaltic rock in the Pacific Ocean alioiil a mile oil Tillamook Head and 20 miles south of the Cohimljia River. The lighthouse on it, built in 1880-81, consists of a square masonry tower of no great height rising out of the centre of a masonry lunise 45 X 48 feet in plan, the tower proper being 16 feet square. The lighthouses so far described were built upon solid rock, and the foundation problem was a simple one except for the work of combating the wind and waves. Another class of lighthouses of even mure dillicult construction comprises those structures which are located on shoals at long dis- tances from the shore. Here the sinking of the foundations is a serious problem by itself, even were it not complicated by the dangers of wind and water. Perhaps the two most noted masonry towers erected on such foundations are the Rothcrsand lighthouse, in the North Sea, Holland, and the Fourteen-Foot Bank lighthouse, in Dela- ware Bay, I'nited States. To build the founda- tion for the Rothcr- sand liglithouse a caisson, in plan resembling a sec- tion of a bi-con- vex lens and built of boiler iron, was used. This caisson was 36 feet 8 inches wide, 46 feet 8 inches long, and 61 feet 8 inches high. At a height of 8 feet 4 inches above the bottom edge of this len- ticular shell there Avas a transverse diaphragm forming the top of the working chamber. Above this dia- pliragm the cais- son "vas divided into four stories, the lowest for mix- ing concrete, the second for the ma- chinery installa- tion, the third for sleeping and store rooms, and the fourth for the re- V o 1 V i n g steam cranes. The three upper floors were suspended from the caisson by screws so that they could he raised as the work ])rogressed. The caisson was built on the shore, towed to its site, and sunk by the pneumatic process to a depth of 73 feet below low water. .'Xfter being sunk the caisson was surroimded by a brush mattress three feet thick for a distance (if 50 feet on all sides, to ])revent the currents from scouring away the sand. The tower and lantern were then erected and work was completed in 188.5. The Fourteen-Foot Bank lighthouse was con- structed by means of a pneumatic caisson as FlO. 2. VERTICAL SECTIOX SHOmNO KOTUEKSAND LIOHTHOISE.