lighthouse

lighthouse, towerlike structure erected to give guidance and warning to ships and aircraft by either visible or radioelectrical means. Lighthouses were long built to conform in structure to their geographical location. Until the beginning of the 19th cent. tallow candles, coal fires, and oil lamps were used as illuminating agents; coal gas followed, to be succeeded by acetylene. Electricity was used for the first time at South Foreland Light, England, in 1858. Other 19th-century innovations were rapidly revolving lights, the incandescent oil-vapor light, fog bells, whistles, sirens, diaphones (fog signals similar to sirens), and the Fresnel lens (used to focus the beam).

In modern lighthouses there are three kinds of lighting systems: the catoptric system, in which rays of light are reflected from silvered mirrors to form a parallel beam visible at a distance; the dioptric, or refractive, system, in which the rays pass through optical glass and are refracted as they enter and emerge from it; and the catadioptric system, in which rays are both refracted and reflected. Increased use of radio beams and radar has made the conventional lighthouse obsolete.

History

Lighthouses date back to ancient Egypt, where priests maintained the beacon fires. For about 1,500 years the lighthouse of Pharos, built in the 3d cent. B.C., guided ships into the Nile; it was lighted by a wood fire and showed smoke by day and a glow by night. The Romans built famous lighthouses in Ostia, Ravenna, and Messina and on both sides of the English Channel.

In the United States the tower for the Boston Light on Little Brewster Island was built in 1716; the first structure of the Brant Point Light, Nantucket, was built in 1746; and Beavertail Light on Conanicut Island, Narragansett Bay, was erected in 1749. In 1789 the U.S. government took over the care of lighthouses from their former private owners. The government set up (1852) the Lighthouse Board, which was eventually superseded by the Lighthouse Service, established (1910) to supervise lighthouses and lightships (see lightship). In 1939 this service was transferred from the Dept. of Commerce to the U.S. Coast Guard.

Bibliography

See H. C. Adamson, Keepers of the Lights (1955); D. A. Stevenson, The World's Lighthouses before 1820 (1960); F. R. Holland, America's Lighthouses: Their Illustrated History Since 1716 (1972).

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lighthouse

Structure, usually with a tower, built onshore or on the seabed to signal danger or provide aid to seafarers. The first known lighthouse was the Pharos of Alexandria. The modern lighthouse dates only from the early 18th century. Initially made of wood, these towers were often washed away in severe storms. The first lighthouse made of interlocking masonry blocks was built on the treacherous Eddystone Rocks reef, off Plymouth, England (1759). Interlocking masonry blocks remained the principal material of lighthouse construction until they were replaced by concrete and steel in the 20th century. Modern construction methods have facilitated the building of offshore lighthouses. The most common illuminant is the electric-filament lamp. Refinements in lenses (e.g., the Fresnel lens) and reflectors made it possible to substantially increase the light's intensity. Radio and satellite-based navigation systems have greatly reduced the need for large lighthouses in sighting land.