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 inure difficult construction comprises those structures which are located on shoals at long di— tames front 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 Rothersand lighthouse, in the North Sea,Holland. and the Fourteen-Foot Bank lighthouse, in Dela ware Bay, United States. To build the founda tion for the Rother sand lighthouse a caisson, in plan resembling a sec tion of a hi-con vex lens and built of boiler iron, was used. This caisson was 30 feet S inches wide, 46 feet 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 was a transverse diaphragm forming the top of the working chamber. Above this dia phragm the cais son was divided into four stories. the lowest for mix ing concrete, the second for the ma chinery tion. the third for sleeping and store rooms, and the fourth for the re volving steam cranes. The three upper floors were suspended front the caisson by screws so that they could be raised as the work progressed. The caisson was built on the shore, towed to its site, and sunk by the pneumatic process to a depth of 73 feet belm• low water. After being sunk the caisson was surrounded by a brush mattress three feet thick for a distance of 50 feet on all sides, to prevent the currents from scouring away the sand. The tower and lantern were then erected and work was completed in 1885.
The Fourteen-Foot Bank lighthouse was con structed by means of a pneumatic caisson as was the Rothersand tower. The caisson proper was made of wood, square in plan, and sur mounted by a cylindrical cast-iron shell 35 feet in diameter, which, filled with concrete, formed the light-tower. The structure was completed in 1887. Cast-iron, concrete filled, cylindrical light
house towers have been constructed in several locations upon rock foundations. The first cast iron lighthouse ever erected was at Point :\lorant, Jamaica, in 1842. The tower was built of nine tiers of plates three-quarters of an inch thick and 10 feet high. held together by bolts and flanges on the inside, and then filled in with masonry and concrete to the height of 27 feet. it rests upon a foundation of granite and rises to a height of 96 feet. It is feet in diameter at the base and 11 feet at the top.
A modern form of lighthouse is constructed on what is called the 'serew-pile' system, an in vention of Alexander Mtchell, who, with his SOD, laid the foundation of the lighthouse on :Nlaplin Sand, at the mouth of the Thames. Eng land. Two similar structures followed, Chap man head in 1849 and Guntleet in 1850, also near the mouth of the Thames. Other screw-pile lights were afterwards erected in different parts of Great Britain. The great feature of the screw pile is that the piles upon which the structure rests are in the form of screws and are driven in the sand or soil to a sufficient depth in the manner of a corkscrew. The first screw-pile lighthouse erected in the United States was by Col. Hartman Bach, United States Engineer Corps, at the mouth of Delaware Bay, eight miles from the ocean, in 1S47-50. The screw-pile lighthouse at Sand Key, Florida Reefs, is supported on 16 piles, with an auxiliary pile in the centre to support the stair case, making in all 17. They are eight inches in diameter with a screw of two feet in diameter at the lower ends, which are bored 12 feet into the reef. The framework of the tower consists of east-iron tubular columns framed together, hav ing wrought-iron ties at each joint, and braced diagonally on the faces of each tier. The keep er's house is supported by east-iron girders and joists 20 feet above the foundation. The struc ture is 120 feet above the level of the water. The foundation is 50 feet in diameter.