Page:The American Cyclopædia (1879) Volume X.djvu/460

 454 LIGHTHOUSE this latter object space is also left for a current of air to flow between the iron and brick. Another kind of iron lighthouse is the wrought- iron pile lighthouse. The lower ends of the iron piles are fitted with large cast-iron screws where the foundation is soft, and the piles are screwed to a firm bearing, or where the foun- dation is rock these ends are sharpened, and the piles are driven into the rock or hard ground by an ordinary pile driver, until they come to a firm bearing upon cast-iron disks which bear upon shoulders forged on the piles. The number of piles depends upon the plan of the structure, which may be square, hexagonal, or octagonal. The foundation having been placed, the structure, which is of wood or boil- er iron, firmly braced to the piles, and con- nected with them by iron castings, is easily built upon it. This kind of lighthouse was first built in England ; the screw pile was pat- ented about 1836 by Mitchell, and is called Mitchell's screw pile. It was introduced into the United States about 1845, and has since been used in the construction of many im- portant lighthouses on the coast. Experience has shown that iron-pile lighthouses are not suitable for foundations in water in climates where much ice is formed. The ice, moving in large fields, bends and sometimes breaks the piles, and, by forming upon the piles themselves, makes the bulk of the structure so large that the effect of the waves upon it is very much increased. On this account it is not likely that iron-pile structures will be much used north of Chesapeake bay ; but on the southern coasts they have been found particularly adapted to the necessities of the service, and about 70 of this class of structures, resting upon screw piles and iron disks, now exist in the United States. Their annual cost for repairs is very small, a yearly coat of paint being all that is needed to keep the exterior in good order. They are par- ticularly suited for bays and sounds in the south- ern waters, where light vessels have been in use until the present time. As these vessels become in need of repairs, they are withdrawn, and a screw-pile lighthouse is built upon the site, at a cost not much exceeding that of the repair of the vessel, and with an annual ex- pense of maintenance less than one half of that of the vessel. Lighthouse towers are general- ly surmounted by parapet walls, which vary in height from 3 to 7 ft. according to the order of the light. Upon the parapet wall is placed the lantern in which the illuminating apparatus is contained. The lantern is a glazed framework made of brass or iron, and varies in dimensions from 6 ft. in diameter and 4 ft. in height to 12 ft. in diameter and 9 ft. in height. It is a regular polygon, and can'be made of any num- ber of sides, depending upon the various cir- cumstances to be considered. It is surmounted by a dome constructed of copper or iron, which is generally lined with some other metal, leav- ing an air space between the two metals, to prevent condensation of moisture. A ventila- tor is placed upon the top, from which the heated air escapes, and registers are inserted near the bottom of the lantern to enable the keeper to regulate the supply of fresh air at will. Upon the convenience and proper con- struction of the lantern the efficiency of the lighthouse in a great measure depends. II. METHOD OF ILLUMINATION. ' The materials which have been used for the illumination of lighthouses are : 1, wood and coal ; 2, candles; 3, oil ; 4, gas ; and 5, the electric current act- ing upon carbon points. Wood and coal were the first fuels used for lights. They were first burned as beacon fires on headlands, and after- ward, as the necessity of increased elevation was felt, the fire was placed at the top of a tower. It is not known that any other meth- od of illumination for lighthouses was used until about 1760, when Smeaton commenced the use of wax candles in the Eddystone light- house. Another lighthouse built by him on Spurn point about 1774 was arranged for illu- mination by coal, which fact shows that the use of wax candles had not become general at that date. Tallow candles were afterward used at the Eddystone. Some of the English and Scotch lights consumed coal as late as 1816, and sev- eral on the coasts of Sweden and Norway were illuminated with this material as late as 1846. The vast improvement made in lamps by the use of the Argand burner and glass chimney made a complete revolution in the lighthouse systems of the world. The parabolic reflector with this burner was introduced into lighthouses gradu- ally from 1785, when the first apparatus of this kind was erected in the Gordouan lighthouse. In the United States the first lighthouses were lighted with tallow candles and solid-wicked lamps, suspended from the domes of the lan- terns by iron chains. The lamps were in shape and in principle like the old-fashioned links. The Argand burners and reflectors were adopt- ed in 1812, and were used till 1852, when the general introduction of the lens system com- menced. Since the adoption of Argand lamps in lighthouses, oil has been used as the com- bustible. In Europe the vegetable oils (olive and rape-seed or colza) have been generally used. Great Britain however uses sperm oil as well as colza, though the latter bids fair to su- persede it. Various other oils, animal and ve- getable, have been tried with more or less suc- cess, but hitherto none but the sperm and lard among the animal oils, and the colza among the vegetable, have come up to the requirements of lighthouse illumination. In the United States lard oil is the fluid now generally used. Its cost is about one third that of sperm oil, and al- though its freezing point is some degrees high- er, this objection is not material in climates the winter temperature of which is lower than the freezing points of both oils. Its capillarity is greater than that of sperm oil, so that a high- er wick can be used with it, without danger of smoke. Attempts have been made to use gas in lighthouses. Difficulties have been found in