LIGHTHOUSE, a lofty tower or other structure, erected at the entrance of a harbor, or at some important or danger ous part of a coast, and having a strong light at the top, to guide vessels, and warn them of danger. Originally they were lighted up with fires, but now oil, gas, and electricity are used, the power of the light being increased by the adop tion of glass reflectors, lenses, and prisms.
In the United States these matters are under the supervision of the Lighthouse Board. The most important lighthouse in the LTnited States is the great tower at Barnegat, erected in 1903, provided with lights of 30,000,000 candle power. The illumination can be seen 100 miles at sea.
None of the early lighthouse buildings now exist. The Pharos of Alexandria (331 B. C.) gave its name to its suc cessors. The Romans built lighthouses at Ostia, Ravenna, Puteoli, and other ports. The Phcenician Pharos at Coruna was repaired during the reign of the Emperor Trajan, was re-established as a lighthouse about 1634, and in 1847 had a dioptric apparatus placed in it. Till the end of the 18th century the lighthouses of Great Britain and Amer ica were few in number, and of an in ferior description in the great essential of a lighthouse—viz., sending the great
est number of rays of light toward the horizon. As an example of a modern lighthouse tower we may take Skerry yore, which is 139 feet in height and 42 feet in diameter at the base, containing a mass of 58,580 cubic feet of granite. (See EDDYSTONE.) To Augustin Fresnel belongs the honor of inventing and first employing, in 1822, the dioptric system for lighthouse pur poses in combination with a central lamp having four concentric wicks. He de vised the lighthouse lens, which is plano convex, 3 feet 3 inches in height by 2 feet 6 inches in breadth, composed of a central disk, surrounded by annular rings gradually decreasing in breadth as they recede from the center. If these lenses be assembled on a frame with eight or more sides, having a lamp in their common focus, and be made to revolve, a dioptric revolving light is duced. The electric light was first shown to the mariner in 1858 from the Fore. land lighthouse.