/luyt"hows'/, n., pl. lighthouses /-how'ziz/.
1. a tower or other structure displaying or flashing a very bright light for the guidance of ships in avoiding dangerous areas, in following certain routes, etc.
2. either of two cylindrical metal towers placed forward on the forecastle of the main deck of a sailing ship, to house the port and starboard running lights.
[1655-65; LIGHT1 + HOUSE]

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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.

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  structure, usually with a tower, built onshore or on the seabed to serve as an aid to maritime coastal navigation, warning the mariner of hazards, establishing his position, and guiding him to his destination. From the sea a lighthouse may be identified by the distinctive shape or colour of its structure, by the colour or flash pattern of its light, or by the coded pattern of its radio signal. The development of electronic navigation systems (navigation) has had a great effect on the role of lighthouses. Powerful lights are becoming superfluous, especially for landfall, but there has been a significant increase in minor lights and lighted buoys, which are still necessary to guide the navigator through busy and often tortuous coastal waters and harbour (harbours and sea works) approaches. Among mariners there is still a natural preference for the reassurance of visual navigation, and lighted marks also have the advantages of simplicity, reliability, and low cost. In addition, they can be used by vessels with no special equipment on board, providing the ultimate backup against the failure of more sophisticated systems.

History of lighthouses

Lighthouses of antiquity
      The forerunners of lighthouses proper were beacon fires kindled on hilltops, the earliest references to which are contained in the Iliad and the Odyssey (c. 8th century BC). The first authenticated man-made lighthouse was the renowned Pharos of Alexandria, which stood some 350 feet (about 110 metres) high. The Romans (ancient Rome) erected many lighthouse towers (tower) in the course of expanding their empire, and by AD 400 there were some 30 in service from the Black Sea to the Atlantic. These included a famous lighthouse at Ostia, the port of Rome, completed in AD 50, and lighthouses at Boulogne, France, and Dover, England. A fragment of the original Roman lighthouse at Dover still survives.

      The Phoenicians (Phoenicia), trading from the Mediterranean to Great Britain, marked their route with lighthouses. These early lighthouses had wood fires or torches burning in the open, sometimes protected by a roof. After the 1st century AD, candles or oil lamps were used in lanterns with panes of glass or horn.

Medieval (Middle Ages) lighthouses
      The decline of commerce in the Dark Ages halted lighthouse construction until the revival of trade in Europe about AD 1100. The lead in establishing new lighthouses was taken by Italy and France. By 1500, references to lighthouses became a regular feature of books of travel and charts. By 1600, at least 30 major beacons existed.

      These early lights were similar to those of antiquity, burning mainly wood, coal, or torches in the open, although oil lamps and candles were also used. A famous lighthouse of this period was the Lanterna of Genoa in Italy, probably established about 1139. It was rebuilt completely in 1544 as the impressive tower that remains a conspicuous seamark today. The keeper of the light in 1449 was Antonio Columbo, uncle of the Columbus who crossed the Atlantic. Another early lighthouse was built at Meloria, Italy, in 1157, which was replaced in 1304 by a lighthouse on an isolated rock at Livorno. In France the Roman tower at Boulogne was repaired by the emperor Charlemagne in 800. It lasted until 1644, when it collapsed owing to undermining of the cliff. The most famous French lighthouse of this period was one on the small island of Cordouan in the estuary of the Gironde River near Bordeaux. The original was built by Edward The Black Prince in the 14th century. In 1584 Louis de Foix, an engineer and architect, undertook the construction of a new light, which was one of the most ambitious and magnificent achievements of its day. It was 135 feet in diameter at the base and 100 feet high, with an elaborate interior of vaulted rooms, richly decorated throughout with a profusion of gilt, carved statuary, and arched doorways. It took 27 years to build, owing to subsidence of the apparently substantial island. By the time the tower was completed in 1611, the island was completely submerged at high water. Cordouan thus became the first lighthouse to be built in the open sea, the true forerunner of such rock structures as the Eddystone Lighthouse.

      The influence of the Hanseatic League helped increase the number of lighthouses along the Scandinavian and German coasts. At least 15 lights were established by 1600, making it one of the best-lighted areas of that time.

      During this period, lights exhibited from chapels and churches on the coast frequently substituted for lighthouses proper, particularly in Great Britain.

The beginning of the modern era
      The development of modern lighthouses can be said to have started about 1700, when improvements in structures and lighting equipment began to appear more rapidly. In particular, that century saw the first construction of towers fully exposed to the open sea. The first of these was Henry Winstanley's 120-foot-high wooden tower on the notorious Eddystone (Eddystone Lighthouse) Rocks off Plymouth, England. Although anchored by 12 iron stanchions laboriously grouted into exceptionally hard red rock, it lasted only from 1699 to 1703, when it was swept away without a trace in a storm of exceptional severity; its designer and builder, on the lighthouse at the time, perished with it. It was followed in 1708 by a second wooden tower, constructed by John Rudyerd, which was destroyed by fire in 1755. Rudyerd's lighthouse was followed by John Smeaton's (Smeaton, John) famous masonry tower in 1759. Smeaton, a professional engineer, embodied an important new principle in its construction whereby masonry blocks were dovetailed together in an interlocking pattern. Despite the dovetailing feature, the tower largely relied on its own weight for stability—a principle that required it to be larger at the base and tapered toward the top. Instead of a straight conical taper, though, Smeaton gave the structure a curved profile. Not only was the curve visually attractive, but it also served to dissipate some of the energy of wave impact by directing the waves to sweep up the walls.

      Owing to the undermining of the foundation rock, Smeaton's tower had to be replaced in 1882 by the present lighthouse, constructed on an adjacent part of the rocks by Sir James Douglass, engineer-in-chief of Trinity House. In order to reduce the tendency of waves to break over the lantern during severe storms (a problem often encountered with Smeaton's tower), Douglass had the new tower built on a massive cylindrical base that absorbed some of the energy of incoming seas. The upper portion of Smeaton's lighthouse was dismantled and rebuilt on Plymouth Hoe, where it still stands as a monument; the lower portion or “stump” can still be seen on the Eddystone Rocks.

      Following the Eddystone, masonry towers were erected in similar open-sea sites, which include the Smalls, off the Welsh coast; Bell Rock in Scotland; South Rock in Ireland; and Minots Ledge off Boston, Massachusetts, U.S. The first lighthouse of the North American continent, built in 1716, was on the island of Little Brewster, also off Boston. By 1820 there were an estimated 250 major lighthouses in the world.

Modern lighthouses

  While masonry and brick continue to be employed in lighthouse construction, concrete and steel are the most widely used materials. Structurally well suited and reasonably cheap, concrete especially lends itself to aesthetically pleasing designs for shore-based lighthouses.

      Modern construction methods have considerably facilitated the building of lighthouses in the open sea. On soft ground, the submerged caisson method is used, a system applied first in 1885 to the building of the Roter Sand Lighthouse in the estuary of the Weser River in Germany and then to the Fourteen Foot Bank light in the Delaware Bay, U.S. With this method, a steel caisson or open-ended cylinder, perhaps 40 feet in diameter, is positioned on the seabed. By excavation of sand, it is sunk into the seabed to a depth of possibly 50 feet. At the same time, extra sections are added to the top as necessary so that it remains above high water level. The caisson is finally pumped dry and filled with concrete to form a solid base on which the lighthouse proper is built.

      Where the seabed is suitable, it is possible to build a “float out” lighthouse, consisting of a cylindrical tower on a large concrete base that can be 50 feet in diameter. The tower is constructed in a shore berth, towed out to position, and then sunk to the seabed, where the base is finally filled with sand. Weighing 5,000 tons (4.5 million kilograms) or more, these towers rely on their weight for stability and require a leveled, prepared seabed. For greater stability during towing, the cylindrical tower itself often consists of two or more telescopic sections, raised to full height by hydraulic jacks after being founded on the seabed. This design was pioneered largely in Sweden.

      Another design, which is more independent of seabed conditions, is the conventional steel-piled structure used for offshore oil and gas rigs. Piles may be driven as deep as 150 feet into the seabed, depending on the underlying strata. The United States has built about 15 light towers of this type, one prominent example being Ambrose Light off New York.

      Helicopters (helicopter) are widely employed in the servicing and maintenance of offshore towers, so that modern designs normally include a helipad. In fact, older cylindrical masonry structures of the previous era—including the Eddystone tower—have had helipads fitted above their lanterns.

      Wood fires were not discontinued until 1800, though after about 1550 coal, a more compact and longer-burning fuel, was increasingly favoured, particularly in northwestern Europe. A lighthouse in those days could consume 300 tons or more of coal a year. In full blaze, the coal fire was far superior to other forms of lighting, preferred by mariners to oil or candles. The disadvantage of both coal fires and early oil lamps and candles was the prodigious amount of smoke produced, which resulted in rapid blackening of the lantern panes, obscuring the light.

Oil lamps
      In 1782 a Swiss scientist, Aimé Argand, invented an oil lamp (Argand burner) whose steady smokeless flame revolutionized lighthouse illumination. The basis of his invention was a circular wick with a glass chimney that ensured an adequate current of air up the centre and the outside of the wick for even and proper combustion of the oil. Eventually, Argand lamps with as many as 10 concentric wicks were designed. These lamps originally burned fish oil, later vegetable oil, and by 1860 mineral oil. The Argand lamp became the principal lighthouse illuminant for more than 100 years.

      In 1901 the Briton Arthur Kitson invented the vaporized oil burner, which was subsequently improved by David Hood of Trinity House and others. This burner utilized kerosene vaporized under pressure, mixed with air, and burned to heat an incandescent mantle. The effect of the vaporized oil burner was to increase by six times the power of former oil wick lights. (The principle is still widely used for such utensils as camp stoves and pressure lamps.)

Gas lamps
      Early proposals to use coal gas at lighthouses did not meet with great success. A gasification plant at the site was usually impracticable, and most of the lights were too remote for a piped supply. However, acetylene gas, generated in situ from calcium carbide and water, came into use around the turn of the 20th century, and its use increased following the introduction of the dissolved acetylene process, which by dissolving the acetylene in acetone made it safe to compress for storage.

      Acetylene gas as a lighthouse illuminant had a profound influence on the advancement of lighthouse technology, mainly through the work of Gustaf Dalén (Dalén, Nils) of Sweden, who pioneered its application between 1900 and 1910. Burned either as an open flame or mixed with air in an incandescent mantle, acetylene produced a light equal to that of oil. Its great advantage was that it could be readily controlled; thus, for the first time automatic unattended lights were possible. Dalén devised many ingenious mechanisms and burners, operating from the pressure of the gas itself, to exploit the use of acetylene. Most of the equipment he designed is still in general use today. One device is an automatic mantle exchanger that brings a fresh mantle into use when the previous one burns out. Another, economizing on gas, was the “sun valve,” an automatic day-night switch capable of extinguishing the light during the day. The switch utilized the difference in heat-absorbing properties between a dull black surface and a highly polished one, producing a differential expansion arranged by suitable mechanical linkage to control the main gas valve.

      The acetylene system facilitated the establishment of many automatic unattended lighthouses in remote and inaccessible locations, normally requiring only an annual visit to replenish the storage cylinders and overhaul the mechanism. liquefied petroleum gas, such as propane, has also found use as an illuminant, although both oil and gas lamps have largely been superseded by electricity.

      Electric illumination in the form of carbon arc lamps was first employed at lighthouses at an early date, even while oil lamps were still in vogue. The first of these was at Dungeness, England, in 1862, followed by a number of others. The majority of these, however, were eventually converted to oil, since the early arc lamps were difficult to control and costly to operate. In 1913 the Helgoland Lighthouse in the North Sea off Germany was equipped with arc lamps and searchlight mirrors to give a light of 38 million candlepower, the most powerful lighthouse in the world at that time.

      The electric-filament lamp, which came into general use in the 1920s, is now the standard illuminant. Power output ranges from about 1,500 watts for the largest structures down to about 5 watts for buoys and minor beacons. Most lamps are of the tungsten-halogen type for better efficiency and longer life. As new types of electric lamps become available—for example, compact source discharge tube lamps—they are adopted for lighthouse use wherever suitable.

Optical equipment
Paraboloidal mirrors
      With the advent of the Argand lamp, a reliable and steady illuminant, it became possible to develop effective optical apparatuses for increasing the intensity of the light. In the first equipment of this type, known as the catoptric system, paraboloidal reflectors concentrated the light into a beam. In 1777 William Hutchinson of Liverpool, England, produced the first practical mirrors (mirror) for lighthouses, consisting of a large number of small facets of silvered glass set in a plaster cast molded to a paraboloid form. More generally, shaped metal reflectors were used, silvered or highly polished. These were prone, however, to rapid deterioration from heat and corrosion; the glass facet reflector, although not as efficient, lasted longer. The best metallic reflectors available in 1820 were constructed of heavily silvered copper in the proportion of 6 ounces of silver to 16 ounces of copper (compared with the 0.5 ounce of silver to 16 ounces of copper commonly used for plated tableware of the period). With such heavy plating, cleaning cloths were kept for subsequent recovery of the silver. These mirrors could increase the intensity of an Argand lamp, nominally about five candlepower, almost 400 times.

      Although the mirror could effectively concentrate the light into an intense beam, it was necessary to rotate it to make it visible from any direction. This produced the now familiar revolving lighthouse beam, with the light appearing as a series of flashes. Mariners were not favourably disposed to these early flashing lights, contending that a fixed steady light was essential for a satisfactory bearing. However, the greatly increased intensity and the advantage of using a pattern of flashes to identify the light gradually overcame their objections. The first revolving-beam lighthouse was at Carlsten, near Marstrand, Sweden, in 1781.

Rectangular and drum lenses (lens)
 In 1828 Augustin Fresnel (Fresnel, Augustin-Jean) of France produced the first apparatus using the refracting properties of glass, now known as the dioptric system (diopter). On a lens panel he surrounded a central bull's-eye lens with a series of concentric glass prismatic rings. The panel collected light emitted by the lamp over a wide horizontal angle and also the light that would otherwise escape to the sky or to the sea, concentrating it into a narrow, horizontal pencil beam. With a number of lens panels rotating around the lamp, he was then able to produce several revolving beams from a single light source, an improvement over the mirror that produces only a single beam. To collect more of the light wasted vertically, he added triangular prism sections above and below the main lens, which both refracted and reflected the light. By doing this he considerably steepened the angle of incidence at which rays shining up and down could be collected and made to emerge horizontally. Thus emerged the full Fresnel catadioptric system, the basis of all lighthouse lens systems today. To meet the requirement for a fixed all-around light, Fresnel modified his principle by producing a cylindrical drum lens, which concentrated the light into an all-around fan beam. Although not as efficient as the rectangular panel, it provided a steady, all-around light. Small drum lenses, robust and compact, are widely used today for buoy and beacon work, eliminating the complication of a rotating mechanism; instead of revolving, their lights are flashed on and off by an electronic code unit.

      Prior to Fresnel's invention the best mirror systems could produce a light of about 20,000 candlepower with an Argand lamp. The Fresnel lens system increased this to 80,000 candlepower, roughly equivalent to a modern automobile headlamp; with the pressure oil burner, intensities of up to 1,000,000 candlepower could be achieved. For a light of this order, the burner mantle would measure 4 inches (100 millimetres) in diameter. The rotating lens system would have four large Fresnel glass lens panels, 12 feet high, mounted about four feet from the burner on a revolving lens carriage. The lens carriage would probably weigh five tons, about half of it being the weight of the glass alone. The rotating turntable would float in a circular cast-iron trough containing mercury. With this virtually frictionless support bearing, the entire assembly could be smoothly rotated by weight-driven clockwork. If the illuminant was acetylene gas, the lens rotation could be driven by gas pressure.

      Installations of this type are still in common use, although many have been converted to electric lamps with electric-motor drives. Modern lens equipment of the same type is much smaller, perhaps 30 inches (75 centimetres) high, mounted on ball bearings and driven by an electric motor. With a 250-watt lamp, illumination of several hundred thousand candlepower can be readily obtained. Lens panels can be molded in transparent plastic, which is lighter and cheaper. Drum lenses are also molded in plastic. In addition, with modern techniques, high-quality mirrors can be produced easily and cheaply.

Intensity, visibility, and character of lights
Geographic range and luminous range
      The luminous intensity of a light, or its candlepower, is expressed in international units called candelas (candela). Intensities of lighthouse beams can vary from thousands to millions of candelas. The range at which a light can be seen depends upon atmospheric conditions and elevation. Since the geographic horizon is limited by the curvature of the Earth, it can be readily calculated for any elevation by standard geometric methods. In lighthouse work the observer is always assumed to be at a height of 15 feet, although on large ships he may be 40 feet above the sea. Assuming a light at a height of 100 feet, the range to an observer at 15 feet above the horizon will be about 16 nautical miles. This is known as the geographic range of the light. (One nautical mile, the distance on the Earth's surface traversed by one minute of arc longitude or latitude, is equivalent to 1.15 statute miles or 1.85 kilometres.)

      The luminous range of a light is the limiting range at which the light is visible under prevailing atmospheric conditions and disregarding limitations caused by its height and the Earth's curvature. A very powerful light, low in position, can thus have a clear-weather luminous range greater than that when first seen by the mariner on the horizon. Powerful lights can usually be seen over the horizon because the light is scattered upward by particles of water vapour in the atmosphere; this phenomenon is known as the loom of the light.

      Atmospheric conditions have a marked effect on the luminous range of lights. They are defined in terms of a transmission factor, which is expressed as a percentage up to a maximum of 100 percent (representing a perfectly clear atmosphere, never attained in practice). Clear weather in the British Isles corresponds to about 80 percent transmission, but in tropical regions it can rise to 90 percent, increasing the luminous range of a 10,000-candela light from 18 to 28 nautical miles. Conversely, in mist or haze at about 60 percent transmission, a light of 1,000,000 candelas would be necessary to maintain a luminous range of 18 nautical miles. In dense fog, with visibility down to 100 yards or metres, a light of 10,000,000,000 candelas could scarcely be seen at half a nautical mile. Because average clear-weather conditions vary considerably from one region of the world to another, luminous ranges of all lighthouses by international agreement are quoted in an arbitrary standard clear-weather condition corresponding to a daytime meteorological visibility of 10 nautical miles, or 74 percent transmission. This is known as the nominal range of a light. Mariners use conversion tables to determine the actual luminous range in the prevailing visibility.

      Because lights of very great intensity yield diminishing returns in operational effectiveness, most very high-powered lights have been abandoned. A maximum of 100,000 candelas, with a clear-weather range of 20 nautical miles, is generally considered adequate. Nevertheless, there are still some very high-powered lights, which for special reasons may have to be visible at a distance in daylight.

      Most lighthouses rhythmically flash or eclipse their lights to provide an identification signal. The particular pattern of flashes or eclipses is known as the character of the light, and the interval at which it repeats itself is called the period. The number of different characters that can be used is restricted by international agreement through the International Association of Lighthouse Authorities in Paris, to which the majority of maritime nations belong. The regulations are too lengthy to quote in full, but essentially a lighthouse may display a single flash, regularly repeated at perhaps 5-, 10-, or 15-second intervals. This is known as a flashing light. Alternatively, it may exhibit groups of two, three, or four flashes, with a short eclipse between individual flashes and a long eclipse of several seconds between successive groups. The whole pattern is repeated at regular intervals of 10 or 20 seconds. These are known as group-flashing lights. In another category, “occulting” lights are normally on and momentarily extinguished, with short eclipses interrupting longer periods of light. Analogous to the flashing mode are occulting and group-occulting characters. A special class of light is the isophase, which alternates eclipses and flashes of exactly equal duration.

      Steadily burning lights are called fixed lights. For giving mariners accurate directional information in ports, harbours, and estuarial approaches, fixed directional lights display sharply defined red and green sectors. Another sensitive and very accurate method of giving directional instruction is by range lights, which are two fixed lights of different elevation located about half a nautical mile apart. The navigator steers the vessel to keep the two lights aligned one above the other. Laser lights are also employed in this role.

      Another use for fixed lights is the control of shipping at harbour entrances. A traffic signal consists of a vertical column of high-powered red, green, and yellow projector lights that are visible in daylight.

      The daymark requirement of a lighthouse is also important; lighthouse structures are painted to stand out against the prevailing background. Shore lighthouses are usually painted white for this purpose, but in the open sea or against a light background conspicuous bands of contrasting colours, usually red or black, are utilized.

sound signals
      The limitations of purely visual navigation very early led to the idea of supplementary audible warning in lighthouses. The first sound signals were explosive. At first cannon were used, and later explosive charges were attached to retractable booms above the lantern and detonated electrically. Sometimes the charges contained magnesium in order to provide an accompanying bright flare. Such signals could be heard up to four nautical miles away. Bells (bell) also were used, the striker being actuated by weight-driven clockwork or by a piston driven by compressed gas (usually carbon dioxide). Some bells were very large, weighing up to one ton.

      About the beginning of the 20th century, compressed-air fog signals (fog signal), which sounded a series of blasts, were developed. The most widely used were the siren and the diaphone. The siren consisted of a slotted rotor revolving inside a slotted stator that was located at the throat of a horn. The diaphone worked on the same principle but used a slotted piston reciprocating in a cylinder with matching ports. The largest diaphones could be heard under good conditions up to eight nautical miles away. Operating pressures were at 2 to 3 bars (200 to 300 kilopascals), and a large diaphone could consume more than 50 cubic feet (approximately 1.5 cubic metres) of air per second. This required a large and powerful compressing plant, 50 horsepower or more, with associated air-storage tanks.

      A later compressed-air signal was the tyfon. Employing a metal diaphragm vibrated by differential air pressure, it was more compact and efficient than its predecessors.

      Modern fog signals are almost invariably electric. Like the tyfon, they employ a metal diaphragm, but in the electric signal they vibrate between the poles of an electromagnet that is energized by alternating current from an electronic power unit. Powers range from 25 watts to 4 kilowatts, with ranges from half a nautical mile to five nautical miles. Note frequencies lie between 300 and 750 hertz. Emitters can be stacked vertically, half a wavelength apart, in order to enhance the sound horizontally and reduce wasteful vertical dispersion.

Effective range
      Propagation of sound in the open air is extremely haphazard, owing to the vagaries of atmospheric conditions. Wind direction, humidity, and turbulence all have an effect. Vertical wind and temperature gradients can bend the sound up or down; in the latter case it can be reflected off the sea, resulting in shadow zones of silence. The range of audibility of a sound signal is therefore extremely unpredictable. Also, it is difficult to determine with any precision the direction of a signal, especially from the bridge of a ship in fog.

Radio aids
      Sophisticated and complex radio navigation systems such as Decca and Loran, and satellite-based global positioning systems such as Navstar, are not properly within the field of lighthouses (see navigation). Radio and radar beacons, on the other hand, provide the equivalent of a visual seamark that is unaffected by visibility conditions.

Radio beacons
      Radio beacons, which first appeared in the 1920s, transmit in the frequency band of 285–315 kilohertz. In a characteristic signal lasting one minute, the station identification, in Morse code, is transmitted two or three times, followed by a period of continuous transmission during which a bearing can be taken by a ship's direction-finding receiver. Bearing accuracy averages better than 3°. The frequency of transmission varies in different parts of the world. In the busy waters of Europe, radio beacons transmit continuously on a number of different channels within the allotted frequency band.

      Since the development of satellite-based positioning systems in the 1970s and '80s, the early importance of radio beacons as an aid for marine navigators has diminished considerably—although they have acquired a second important role in broadcasting corrections for improving the accuracy of the satellite systems. The principal users of radio beacons are now small-craft operators, particularly recreational sailors.

Radar responders
      Radar-responder beacons are employed in other fields, such as aviation; in marine navigation they are called racons. A racon transmits only in response to an interrogation signal from a ship's radar, at the time when the latter's rotating scanner bears on it. During this brief period, the racon receives some 10 radar pulses, in reaction to which it transmits back a coded reply pulse that is received and displayed on the ship's radar screen. Racons operate on both marine radar bands of 9,300–9,500 megahertz and 2,900–3,100 megahertz. A racon can greatly increase the strength of the echo from a poor radar target, such as a small buoy; it is also helpful in ranging on and identifying positions on inconspicuous and featureless coastlines and in identifying offshore oil and gas rigs.

      The first racons came into use in 1966, and there are now many hundreds in service. Early racons, employing vacuum-tube technology, were large and required several hundred watts of power. Modern racons, using solid-state electronics, are compact and light, typically 16 by 24 inches in area and 20 to 35 pounds (10 to 15 kilograms) in weight. They draw an average of one watt in power from low-voltage batteries.

      Passive radar echo enhancers are also used on poor targets, such as buoys. They are made up of flat metal sheets joined into polyhedral shapes whose geometry is such as to reflect as much of the radar pulse as possible. A typical array, some 28 by 24 inches overall, can have an echoing area equivalent to that of a flat sheet with an area of some 1,600 square feet (150 square metres).

      The acetylene-gas illumination system, being fully automatic and reliable, enabled automatic lights to be operated early on. Its main use today is in buoys, which inherently have to operate unattended. Automation on a large scale, bringing considerable savings in operating costs, came after the advent of electrical equipment and technology and the demise of compressed-air fog signals. Unattended lights are now designed to be automatic and self-sustaining, with backup plant brought on-line automatically upon failure of any component of the system. The status of the station is monitored from a remote control centre via landline, radio, or satellite link. Power is provided from public electricity supplies (where practicable), with backup provided by diesel generators or storage batteries. Where solar power with storage batteries is used, the batteries must have sufficient capacity to operate the light during the hours of darkness. In tropical and subtropical regions, day and night are of approximately equal duration throughout the year, but in temperate and polar regions the days become longer and the nights shorter during the summer, and vice versa in winter. In these areas, solar power has to operate on an annual “balance sheet” basis, with excess charge being generated and stored in large batteries during the summer so that a reserve can be drawn upon in winter. Canada and Norway successfully operate solar-powered lights of this type in their Arctic regions.

Floating lights
      Floating lights (i.e., lightships and buoys) have an important function in coastal waters, guiding both passing ships and those making for or leaving harbour. They have the great advantage of mobility and can readily be redeployed to meet changed conditions. For example, submerged hazards such as sandbanks can move over the years under the influence of the sea, and, for vessels of very deep draft, safe channels must be correctly marked around these hazards at all times.

Lightships (lightship)
      Lightships originated in the early 17th century, arising from the need to establish seamarks in positions where lighthouses were at that time impracticable. The first lightship, established in 1732 at Nore Sand in the Thames estuary, was rapidly followed by others. These early vessels were small converted merchant or fishing vessels showing lanterns suspended from crossarms at the masthead. Not until 1820 were vessels built specifically as lightships.

      Modern lightships are in most cases an alternative to costly seabed structures. Used to mark the more important hazards and key positions in traffic patterns, they are capable of providing a range of powerful aids. Power is provided by diesel generators. Lightships vary in size but can be up to 150 feet in length, 25 feet in beam, and 500 tons in displacement. They are not normally self-propelled, most often being towed into position and moored by a single chain and anchor. They need to be withdrawn for overhaul every two or three years.

      Lightships are costly items to operate and to maintain and are therefore prime candidates for automation. All lightships are now unattended, and the power of their lights and fog signals has been downgraded to a more appropriate level—e.g., 10,000 candelas for the lights, giving a luminous range of approximately 15 nautical miles, and sound signals with a range of two nautical miles. New lightships are of similar construction to the older type but are smaller—60 feet or less in length. The smallest sizes, 30 feet or less, are intended for sheltered waters and are often known as light floats.

Buoys (buoy)
      Buoys are used to mark safe channels, important reference points, approaches to harbours, isolated dangers and wrecks, and areas of special significance. They also mark traffic lanes in narrow and congested waters where traffic routing is in force (i.e., where ships are being routed into designated lanes with entry and exit points).

Structure and operation
      Usually constructed of quarter-inch steel plate, buoys vary in diameter from three to six feet and can weigh as much as eight tons. They are moored to a two- or three-ton concrete or cast-iron sinker by a single length of chain, which is normally about three times as long as the depth of water. Size and type depend on the application. Buoys in deep water need to be large in order to provide buoyancy sufficient to support the extra weight of chain. Stability is provided by a circular “skirt keel,” with possibly a ballast weight. The light is mounted on a superstructure some 10 feet above the waterline. For more powerful lights on the open sea or deep water, the light may be 15 or 20 feet above the waterline in order to increase the range. Known as “high focal plane” buoys, for stability they require a cylindrical tail tube extending downward some 10 feet from the bottom of the hull.

      Buoys are also manufactured from fibreglass-reinforced (fibreglass) plastic. They have the advantage of light weight, hence lighter moorings (often of synthetic cable), ease of handling, and resistance to corrosion. Fibreglass buoys are generally confined to sheltered waters.

      In addition to the light, a buoy may be fitted with a racon, radar reflector, and low-power fog signal. In earlier times acetylene gas was the only practicable illuminant, which restricted the power of the light. Modern electric buoy lights range in power from a few hundred candelas up to the region of 1,000 candelas, giving ranges of eight nautical miles or so. The lighting equipment consists of a drum lens, usually made of plastic and between 4 and 12 inches in diameter, along with a low-voltage lamp of 5 to 60 watts. Power can be provided by expendable primary batteries, which need to be replaced every year or two. In order to increase the service interval and also to accommodate more powerful lights, rechargeable batteries with onboard generators are used. Some tail-tube buoys, which tend to oscillate vertically with the motion of the sea, generate power from the oscillating water column in the tube. The water column produces an oscillating air column, which in turn drives a small air-turbine generator. However, the vast majority of buoys are solar-powered (solar energy), and buoys constitute the major part of solar-powered lights.

Buoyage systems
      A buoy's colour and profile and the colour and flash pattern of its light convey information to navigators. The beginnings of a unified system of buoy marking emerged in 1889, but it was not until 1936 that a worldwide unified system was agreed upon at the League of Nations in Geneva. World War II began before the agreement could be fully ratified, and in the aftermath of the war little progress was made. At one time there were nine different systems in use. By 1973, however, a number of spectacular marine accidents urged the international community into action, and by 1980 a new unified system had been agreed upon by 50 maritime countries.

      Maintained by the International Association of Lighthouse Authorities, the Maritime Buoyage System applies two nearly identical standards to two regions. Region A comprises Europe, Australia, New Zealand, Africa, the Persian Gulf, and most Asian states. Region B includes the Americas, Japan, Korea, and the Philippines. In both regions, the buoyage systems divide buoys into Lateral, Cardinal, and associated classes. Lateral buoys are used to mark channels (streambed). In region A a can-profile (i.e., cylindrical) red buoy with a red light indicates the port (left) side of the channel when proceeding in the direction of buoyage, while a conical green buoy indicates the starboard (right) side. The direction of buoyage is the direction taken when approaching a harbour from seaward or when following a clockwise direction around a landmass. Where necessary, it is indicated by arrows on charts. In region B the marking is reversed—i.e., red is to starboard when returning to harbour, and green is to port.

      Cardinal buoys indicate the deepest water in an area or the safest side around a hazard. They have a plain hull with a lattice or tubular superstructure that is surmounted by a distinctive “topmark.” There are four possible topmarks, corresponding to the four cardinal points of the compass—i.e., north, south, east, and west. The safest navigable water lies to the side of the buoy indicated by its topmark. The marks consist of a pair of cones arranged one above the other in the following configurations: both points upward (north), both points downward (south), base-to-base (east), and point-to-point (west). The cones are painted black, and the buoys are coloured from top to waterline with two or three horizontal bands of black and yellow.

      Buoys indicating an isolated danger with safe water all around carry two separated spheres and are painted with alternating horizontal red and black bands. Safe-water buoys, marking an area of safe water, carry a single red sphere and vertical red and white stripes.

      Special marks are buoys that indicate other areas or features such as pipelines, prohibited zones, or recreational areas. These buoys are all yellow, with an X-shaped topmark and a yellow light. They may also have a hull shape indicating the safe side of passage.

National lighthouse systems
Lists of Lights
      All maritime countries publish Light Lists, which are comprehensive catalogs of the characteristics and location of all lightships, buoys, and beacons under their control. In the United Kingdom they are issued by the Hydrographic Office, under the Board of Admiralty, and in the United States they are issued by the U.S. Coast Guard (United States Coast Guard), under the federal Department of Transportation. Changes in status are disseminated by Notices to Mariners, which update lights lists and charts. Urgent information is broadcast at scheduled times on dedicated radio channels or satellite link. All this information is promulgated in a standard format recommended by the International Hydrographic Organization, based in Monaco.

Lighthouse administration
      In most countries lighthouse administration comes under a department of central government. It is usually financed out of general taxation, but it is sometimes funded from a levy on shipping that may on occasion be supplemented by the central government. This type of funding applies to lights intended for general navigation. Lights provided by ports and harbours specifically for ships using the port are paid for separately by port and harbour dues.

      In the United States, lighthouses and other aids are administered by the Aids to Navigation Division of the U.S. Coast Guard. The Coast Guard has at least 3,000 lighthouses and a further 6,000 minor lights, plus 4,000 lighted buoys. In 1989, expenditure, derived from general taxation, was some $450 million (£280 million).

      In England (United Kingdom) and Wales, lighthouses are administered by the Corporation of Trinity House, an autonomous nongovernmental agency. Trinity House evolved from a royal charter granted in 1514 to a medieval guild or fraternity of Thames river pilots based in the parish of Deptford Stronde. Its charter was later extended to the provision of seamarks. At that time most lights were operated by private owners, who, under concessions purchased from the crown, had the necessary authority to collect payment in the form of light dues. Because of increasing dissatisfaction with the level of charges and with poor service, by act of Parliament in 1836 all privately operated lights in England and Wales were bought and transferred to Trinity House. Seamarks off Scotland were placed under the authority of the Northern Lighthouse Board in Edinburgh, and Irish seamarks were placed under the Commissioners for Irish Lights in Dublin. These three authorities, which today operate a total of 1,100 lighthouses, lightships, buoys, and beacons, still share the pooled user charges, known as the General Lighthouse Fund, for the whole of the British Isles. In 1990 the fund was in the region of £55 million ($105 million).

Ian C. Clingan

Additional Reading
Kenneth Sutton Jones, Pharos: The Lighthouse Yesterday, Today, and Tomorrow (1985), a comprehensive and detailed treatment, includes a discussion of future trends. David Stevenson, The World's Lighthouses before 1820 (1959), is a classic work by a member of the famous family of Scottish lighthouse engineers. Patrick Beaver, A History of Lighthouses (1971), provides historical descriptions of the construction of important lighthouses in various parts of the world, as well as the people involved.douglas B. Hague and Rosemary Christie, Lighthouses: Their Architecture, History, and Archaeology (1975), offers a history of lighthouse building, construction, engineering, and architecture from earliest times throughout the world. Robert Stevenson, English Lighthouse Tours, ed. by David Stevenson (1946), is an edited version of a detailed journal kept during a tour of English lighthouses. Hilary Poland Mead, Trinity House (1947), discusses the origins and history of the lighthouse authority for England and Wales. George R. Putnam, Lighthouses and Lightships of the United States, new and rev. ed. (1933), provides a historical survey of most of the major lighthouses and lightships of the United States through the early 20th century; it may be supplemented by Francis Ross Holland, Jr., America's Lighthouses: Their Illustrated History Since 1716 (1972, reissued 1988). United States, Coast Guard, Historically Famous Lighthouses (1950), and Coast Guard History, new ed. (1982), provide brief insights into the work of the U.S. Coast Guard and some of its more famous lighthouses.Ian C. Clingan

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Universalium. 2010.

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