Lighthouse - Structure

 

Lighthouse structure, usually with a tower, built onshore or on the seabed to serve as an aid to maritime coastal navigation, warning mariners of hazards, establishing their position, and guiding them to their destinations. 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 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 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.

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 BCE). The first authenticated lighthouse was the renowned Pharos of Alexandria, which stood some 350 feet (about 110 metres) high. The Romans erected many lighthouse towers in the course of expanding their empire, and by 400 CE 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 50 CE, and lighthouses at Boulogne, France, and Dover, England. A fragment of the original Roman lighthouse at Dover still survives.

Pharos of Alexandria

The Phoenicians, 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 CE, candles or oil lamps were used in lanterns with panes of glass or horn.

Medieval lighthouses

The decline of commerce in the Dark Ages halted lighthouse construction until the revival of trade in Europe about 1100 CE. 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. 

Lanterna of Genoa in Italy

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.

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 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, in 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 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.

Rudyerd’s lighthouse

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 N. 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.

Bell Rock Lighthouse in Scotland

20th-century lighthouses

Construction

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

Modern construction methods considerably facilitated the building of lighthouses in the open sea. On soft ground, the submerged caisson method was 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 12 metres (40 feet) in diameter, was positioned on the seabed. By excavation of sand, it was sunk into the seabed to a depth of possibly 15 metres (50 feet). At the same time, extra sections were added to the top as necessary so that it remained above high water level. The caisson was finally pumped dry and filled with concrete to form a solid base on which the lighthouse proper was built.

Roter Sand Lighthouse

Where the seabed was suitable, it was possible to build a “float out” lighthouse, consisting of a cylindrical tower on a large concrete base that could be 15 metres (50 feet) in diameter. The tower was constructed in a shore berth, towed out to position, and then sunk to the seabed, where the base was finally filled with sand. Weighing 4.5 million kg (5,000 tons) or more, these towers relied on their weight for stability and required a leveled, prepared seabed. For greater stability during towing, the cylindrical tower itself often consisted 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 was more independent of seabed conditions, was the conventional steel-piled structure used for offshore gas and oil rigs. Piles could be driven as deep as 46 metres (150 feet) into the seabed, depending on the underlying strata. The United States built about 15 light towers of this type; one prominent example was Ambrose Light off New York, which was dismantled in 2008. Helicopters were widely employed in the servicing and maintenance of offshore towers, so that late 20th-century designs normally included a helipad. Even older cylindrical masonry structures of the previous era - including the Eddystone tower - had helipads fitted above their lanterns.

Illuminants

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 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 burners 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 burner 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 used kerosene vaporized under pressure, mixed with air, and burned to heat an incandescent mantle. The vaporized oil burner was six times more powerful than the 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 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 entered general use. One device was an automatic mantle exchanger that brought a fresh mantle into use when the previous one had burned out. Another, economizing on gas, was the “sun valve,” an automatic day-night switch capable of extinguishing the light during the day. The switch used 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, also found use as an illuminant. Eventually, both oil and gas lamps were largely superseded by electricity.

Electric lamps

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 South Foreland, England, in 1858, 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, became the standard illuminant. Power output ranged from about 1,500 watts for the largest structures down to about 5 watts for buoys and minor beacons. Most lamps were 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 and, in the early 21st century, light-emitting diodes (LEDs) - they were adopted for lighthouse use wherever suitable.