LIGHTHOUSE

a buil erected on some Conspicuous part of the coast to warn and guide ships. The earliest form -Was.a simple tower with a beacon fire. Lighthouses may be situated on the land, or on rocks or shoals swept by the sea. Those on land are ordinary architectural buildings, and their construction does not call for special notice. There are various methods of constru lighthouses on rocks or shoals. If the rock affords a good foundation, a tower of masonry is usually the most convenient, such towers being constructed on the lines of Smeaton’s Eddy stone lighthouse, with certain modifications. The circular form is adopted as off least resistance to wind and water, an the center of gravity is kept low. The lowest parts of the structure, which have to withstand the full shock of the waves, are built with a vertical face. The height of the tower depends upon the work it has to do, but it must always be sufficient to keep the light unobscured by spray. 4 Lights may be divided into two cla es, fixed and flashing. Fixed or continuous lights are used for harbor lights, but seldom for other purposes. Flashing lights may show a single flash, or a group of two or more flashes, but in every case the periods of darkne are greater than those of the light. Fixed lights are often arranged so as to be entirely eclipsed at reguiar intervals, and they are then called oot (0) banks or other on the fairway, and colored light on the dangerous places. The apparatus employed in flashing and occulting lights is usually rotated by weight-driven clockwork mechanism, which ate indicates when rewinding is req 3 Spring-driven clockwork is also used for met Mmeaton’s onone Hdd ystone tone lig light consisted | of twenty-four candles. Oil, coal or acetylene gas, and electricity are now used. The use of gas or electricity generally nece itates the construction of special generating plant, and on this account oil is muc instruments used in Llig hthouses may be divided into three _ cla es—catoptric, dioptric, and catadioptric. In catopiric instruments the rays of light are reflected by plane, parabolic, spherical, or other mirrors. lights and beacons. The source of light _ for these is often ofl gas, stored at a ' pre ure of several atmospheres. Acetylene gas is also used to some extent. frghtships are largely employed in places where it is not po ible to con-