LIGHTHOUSE. 256 LIGHTHOUSE. was the Rothersaiul tower. The caisson proper wji.s made of wood, square in plan, and sur- mounted by a cylindrical cast-iron shell 35 feet in diameter, which, lillcd with concrete, formed the light-tower. The structure was completed in 1887. Cast-iron, concrete filled, cylindrical light- house towers have heen constructed in several locations upon rock foundations. The first cast- iron lighthouse ever erected was at Point Morant, .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 tilled in with masonry and concrete to the height of i" feet. It rests upon a foundation of granite and rises to a lieight of 96 feet. It is 18V-! feet in diameter at the base and 1 1 feet at the top. A modern form of lighthouse is constructed on what is called the 'screw-pile' system, an in- vention of Alexander Jlitchell. who. with his son, laid the foundation of the liglithouse on Maplin Sand, at the mouth of the Thames, Eng- land. Two similar structures followed. Chap- man Head in 18411 and (iunlleet 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. llartman Bach. I'nited States Engineer tJorps, at the mouth of Delaware Bay, eight miles from the ocean, in 1847-50. The .screw-pile lighthouse at Sand Key, Florida Reefs, is supported on 16 piles, with an auxiliary pile in the centre to sup|)ort 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 arc bored 12 feet into the I'eef. The framework of the tower consists of cast-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 cast-iron girders and joists 20 feet al)ove the foundation. The struc- ture is 120 feet above the level of the water. The foundation is 50 feet in diameter. United States Liciithoi.se Sy.stem. The theory of coast lighting is that each coast sh.all be so set with towers that the rays from their lights shall meet and pass each other, so that a vessel on the coast shall never be out of siglit of a light. The United States is proceeding upon this theory and lights are being installed U|)on this plan. The cost of the lighthouse estab- lishment for the first eenturv of its existence in the United States was .l!0.'5',250,000. The first lighthouse on this continent was built at the en- trance to Boston Harbor and was supported by light-dues of Irf. per ton on all incoming and out- going vessels except cuasters. When the I'nited States, in 1789, accepted the cession of the title to and jurisdiction over the lighthoxises on the coast, and agreed to nuiintain them thereafter, they were eight in number, and were placed in the care of the Treasury Department. Up to 1820 the number of lights increased from 8 to 55. and each seems to have been built to meet immediate want withoiit regard to any general system. Un- der the Fifth Auditor of the Treasury. Stephen Pleasanton, who was popularly known^as general superintendent of lights from 1820 to 1852, the number of lighthouses increased from 55 to 325, and numerous buoys and other aids to navi- gation were erected. In 1852 Congress passed an act constituting the Lighthouse Board as it now exists. The board, finding from the experi- ence of the keepers of the lights at Highlands of Navesink that the lenticular a|>paratus could be managed by the average light -keeper after in- struction by an expert, and that its use was more economical in oil than was the reflector a])paratus in use, pushed its substitution with vigor, and, as they had anticipated, with a diminution of the annual expenditure for oil. The construction of lighthouses in the United States varies greatly. Those built on the New England coast previous to 1840 were either of rubl)le masonry, in the shape of conical towers, or wooden frame towers built on the roofs of the keepers' dwellings. The stone towers were gen- erally three feet thick at the base, tapering to two feet in thickness at the top. and from 20 to 50 feet in height. The iron-pile structure was introduced next, and great improvement followed in the combination of the framework, in the ar- rangement of elevated a])artments for keepers, and in making disk pile and other improved foun- dations. There are a large number of screw-pile lighthouses, chiefly in Southern waters, and where dry foundations are found, iron plates have been used to form the structures in later years. A number of brick towers and iron skeleton towers are also in use. The lights in the various light- houses are classed in six groups or orders, de- pending upon their size. The first-order lights, which are the largest, have a lenticular appa- ratus which stands nearly 12 feet in height, and is feet in diameter. Such a light costs from .$4250 to $8400. The lights of the second order are 4 feet 7 inches in diameter; those of the third. .3 feet 3% inches; those of the fourth, 19% inches; those of the fifth. U'-'j inches; and those of the sixth or smallest order are 11% inches. The total number of lights in the United States in 1900, including coast, lake and river lights, lightships, and lighted buoys, was 3163. (See table at end of article.) The river lights, of which there are about twice as many as the others, are considered as separate and distinct from the lighthouses, and are maintained from a separate fund. LiGiiTiiorsE Illumination. In the catoptric or reflecting system all of those rays proceeding from the focus of a parabolic mirror which fall upon its surface are refiected parallel to the axis so as to form a solid beam of light. When a series of such reflectors are arranged close to each <ither round a cylinder in a lighthouse, they illuminate constantly, though not with equal intensity, the whole horizon. As the property of the ])arabolic reflector is to collect the rays in- cident upon its surface into one beam of parallel rays, it would be absolutely impossible, were the flame from which the rays proceeded a mathe- matical point, to produce a light which would illuminate the whole of the horizon, unless there were an infinite number of reflectors. (See Parabola.) But as the radiant, instead of be- ing a mathematical point, is a physical object, consisting of a flame of very notable size, the rays which come from the outer portion of the luminous cone proceed, after reflection, in such divergent directions as to render it practically possible to light up, though unequally, the whole
