Misplaced Pages

#485514

72-458: Trevose Head Lighthouse is a lighthouse on Trevose Head on the north Cornish coast at grid reference SW850766 lying to the WSW of Padstow and was sited here as there was previously no light from Land's End to Lundy and it would be visible from Cape Cornwall to Hartland Point . The tower is 89 feet (27 m) tall, and has a range of 20 nautical miles (37 km; 23 mi), but, on

144-594: A coordinate system with parallels of latitude and meridians of longitude . The earliest reference of 60 miles to a degree is a map by Nicolaus Germanus in a 1482 edition of Ptolemy 's Geography indicating one degree of longitude at the Equator contains " milaria 60 ". An earlier manuscript map by Nicolaus Germanus in a previous edition of Geography states " unul gradul log. et latitud sub equinortiali formet stadia 500 que fanut miliaria 62 ⁠ 1 / 2 ⁠ " ("one degree longitude and latitude under

216-568: A quarter meridian . So ⁠ 10,000,000 m / 90 × 60 ⁠ = 1,851.85 m ≈ 1,852 m became the metric length for a nautical mile. France made it legal for the French Navy in 1906, and many metric countries voted to sanction it for international use at the 1929 International Hydrographic Conference. Both the United States and the United Kingdom used an average arcminute—specifically,

288-402: A rescue service , if necessary. Improvements in maritime navigation and safety, such Global Positioning System (GPS), led to the phasing out of non-automated lighthouses across the world. Although several closed due to safety concerns, Canada still maintains 49 staffed lighthouses, split roughly evenly across east and west coasts. The remaining modern lighthouses are usually illuminated by

360-640: A Diesel generator for backup. Many Fresnel lens installations have been replaced by rotating aerobeacons , which require less maintenance. In modern automated lighthouses, the system of rotating lenses is often replaced by a high intensity light that emits brief omnidirectional flashes, concentrating the light in time rather than direction. These lights are similar to obstruction lights used to warn aircraft of tall structures. Later innovations were "Vega Lights", and experiments with light-emitting diode (LED) panels. LED lights, which use less energy and are easier to maintain, had come into widespread use by 2020. In

432-520: A clear night, you can just spot the light from Pendeen Lighthouse , over 35 miles (56 km) away. The site was surveyed by order of the Trinity Board in July 1844 with a design submitted that November and approved February 1845. Building began in that May with the laying out of the road and contract entered into with the builders the next month. During gales on 20–21 November 1846 scaffolding attached to

504-406: A degree (5866 ⁠ 2 / 3 ⁠ feet per arcminute ). In 1633, William Oughtred suggested 349,800 feet to a degree (5830 feet per arcminute). Both Gunter and Oughtred put forward the notion of dividing a degree into 100 parts, but their proposal was generally ignored by navigators. The ratio of 60 miles, or 20 leagues, to a degree of latitude remained fixed while the length of the mile

576-436: A filament source. Experimental installations of laser lights, either at high power to provide a "line of light" in the sky or, utilising low power, aimed towards mariners have identified problems of increased complexity in installation and maintenance, and high power requirements. The first practical installation, in 1971 at Point Danger lighthouse , Queensland , was replaced by a conventional light after four years, because

648-453: A form of concrete that will set under water used by the Romans, and developed a technique of securing the granite blocks together using dovetail joints and marble dowels . The dovetailing feature served to improve the structural stability , although Smeaton also had to taper the thickness of the tower towards the top, for which he curved the tower inwards on a gentle gradient. This profile had

720-414: A horizontal plane, and horizontally the light is focused into one or a few directions at a time, with the light beam swept around. As a result, in addition to seeing the side of the light beam, the light is directly visible from greater distances, and with an identifying light characteristic . This concentration of light is accomplished with a rotating lens assembly. In early lighthouses, the light source

792-401: A metal turret on top of the engine house. New diesel engines and Reavell compressors were provided. The light was electrified in 1974. In 1995 the lighthouse was automated and became unmanned. The red colour was removed from the light at this time and the rotation speed of the optic was slowed. By this stage the engine house was suffering cracking due to erosion; with automation a new fog signal

SECTION 10

#1733092556486

864-571: A minute of arc of a great circle of a sphere having the same surface area as the Clarke 1866 ellipsoid . The authalic (equal area) radius of the Clarke 1866 ellipsoid is 6,370,997.2 metres (20,902,222 ft). The resulting arcminute is 1,853.2480 metres (6,080.210 ft). The United States chose five significant digits for its nautical mile, 6,080.2 feet , whereas the United Kingdom chose four significant digits for its Admiralty mile, 6,080 feet. In 1929

936-427: A more powerful hyperradiant Fresnel lens manufactured by the firm of Chance Brothers . While lighthouse buildings differ depending on the location and purpose, they tend to have common components. A light station comprises the lighthouse tower and all outbuildings, such as the keeper's living quarters, fuel house, boathouse, and fog-signaling building. The Lighthouse itself consists of a tower structure supporting

1008-526: A navigator with a line of position called a range in North America and a transit in Britain. Ranges can be used to precisely align a vessel within a narrow channel such as a river. With landmarks of a range illuminated with a set of fixed lighthouses, nighttime navigation is possible. Such paired lighthouses are called range lights in North America and leading lights in the United Kingdom. The closer light

1080-512: A number of screw-pile lighthouses. Englishman James Douglass was knighted for his work on the fourth Eddystone Lighthouse. United States Army Corps of Engineers Lieutenant George Meade built numerous lighthouses along the Atlantic and Gulf coasts before gaining wider fame as the winning general at the Battle of Gettysburg . Colonel Orlando M. Poe , engineer to General William Tecumseh Sherman in

1152-402: A particular color (usually formed by colored panes in the lantern) to distinguish safe water areas from dangerous shoals. Modern lighthouses often have unique reflectors or racon transponders so the radar signature of the light is also unique. Before modern strobe lights , lenses were used to concentrate the light from a continuous source. Vertical light rays of the lamp are redirected into

1224-411: A reservoir mounted above the burner. The lamp was first produced by Matthew Boulton , in partnership with Argand, in 1784, and became the standard for lighthouses for over a century. South Foreland Lighthouse was the first tower to successfully use an electric light in 1875. The lighthouse's carbon arc lamps were powered by a steam-driven magneto . John Richardson Wigham was the first to develop

1296-415: A single stationary flashing light powered by solar-charged batteries and mounted on a steel skeleton tower. Where the power requirement is too great for solar power alone, cycle charging of the battery by a Diesel generator is provided. The generator only comes into use when the battery needs charging, saving fuel and increasing periods between maintenance. John Smeaton is noteworthy for having designed

1368-469: A system for gas illumination of lighthouses. His improved gas 'crocus' burner at the Baily Lighthouse near Dublin was 13 times more powerful than the most brilliant light then known. The vaporized oil burner was invented in 1901 by Arthur Kitson , and improved by David Hood at Trinity House . The fuel was vaporized at high pressure and burned to heat the mantle, giving an output of over six times

1440-428: Is a unit of length used in air, marine, and space navigation , and for the definition of territorial waters . Historically, it was defined as the meridian arc length corresponding to one minute ( ⁠ 1 / 60 ⁠ of a degree) of latitude at the equator, so that Earth's polar circumference is very near to 21,600 nautical miles (that is 60 minutes × 360 degrees). Today the international nautical mile

1512-478: Is defined as 1,852 metres (about 6,076 ft; 1.151 mi). The derived unit of speed is the knot , one nautical mile per hour. There is no single internationally agreed symbol, with several symbols in use. The word mile is from the Latin phrase for a thousand paces: mille passus . Navigation at sea was done by eye until around 1500 when navigational instruments were developed and cartographers began using

SECTION 20

#1733092556486

1584-419: Is one example. Race Rocks Light in western Canada is painted in horizontal black and white bands to stand out against the horizon. For effectiveness, the lamp must be high enough to be seen before the danger is reached by a mariner. The minimum height is calculated by trigonometry (see Distance to the horizon ) as D = 1.22 H {\displaystyle D=1.22{\sqrt {H}}} , where H

1656-492: Is referred to as the beacon or front range; the further light is called the rear range. The rear range light is almost always taller than the front. When a vessel is on the correct course, the two lights align vertically, but when the observer is out of position, the difference in alignment indicates the direction of travel to correct the course. There are two types of lighthouses: ones that are located on land, and ones that are offshore. Nautical miles A nautical mile

1728-510: Is the height above water in feet, and D is the distance from the lighthouse to the horizon in nautical miles, the lighthouse range . Where dangerous shoals are located far off a flat sandy beach, the prototypical tall masonry coastal lighthouse is constructed to assist the navigator making a landfall after an ocean crossing. Often these are cylindrical to reduce the effect of wind on a tall structure, such as Cape May Light . Smaller versions of this design are often used as harbor lights to mark

1800-601: The Florida Reef along the Florida Keys, beginning with the Carysfort Reef Light in 1852. In waters too deep for a conventional structure, a lightship might be used instead of a lighthouse, such as the former lightship Columbia . Most of these have now been replaced by fixed light platforms (such as Ambrose Light ) similar to those used for offshore oil exploration . Aligning two fixed points on land provides

1872-433: The 'high' light was changed to an occulting light , now with a six-wick lamp, and the 'low' light was put out of use. Under the new arrangement the high light was eclipsed (for three seconds) three times in quick succession every minute. From 1911 a series of further improvements were made. First, the keepers' dwellings were upgraded. Then, in 1912 the light was again updated and the current rotating optic (weighing 3.6 tons)

1944-510: The 20th century. These often have a narrow cylindrical core surrounded by an open lattice work bracing, such as Finns Point Range Light . Sometimes a lighthouse needs to be constructed in the water itself. Wave-washed lighthouses are masonry structures constructed to withstand water impact, such as Eddystone Lighthouse in Britain and the St. George Reef Light of California. In shallower bays, Screw-pile lighthouse ironwork structures are screwed into

2016-462: The Swiss scientist Aimé Argand revolutionized lighthouse illumination with its steady smokeless flame. Early models used ground glass which was sometimes tinted around the wick. Later models used a mantle of thorium dioxide suspended over the flame, creating a bright, steady light. The Argand lamp used whale oil , colza , olive oil or other vegetable oil as fuel, supplied by a gravity feed from

2088-405: The United Kingdom and Ireland about a third of lighthouses had been converted from filament light sources to use LEDs, and conversion continued with about three per year. The light sources are designed to replicate the colour and character of the traditional light as closely as possible. The change is often not noticed by people in the region, but sometimes a proposed change leads to calls to preserve

2160-486: The added advantage of allowing some of the energy of the waves to dissipate on impact with the walls. His lighthouse was the prototype for the modern lighthouse and influenced all subsequent engineers. One such influence was Robert Stevenson , himself a seminal figure in the development of lighthouse design and construction. His greatest achievement was the construction of the Bell Rock Lighthouse in 1810, one of

2232-481: The beam was too narrow to be seen easily. In any of these designs an observer, rather than seeing a continuous weak light, sees a brighter light during short time intervals. These instants of bright light are arranged to create a light characteristic or pattern specific to a lighthouse. For example, the Scheveningen Lighthouse flashes are alternately 2.5 and 7.5 seconds. Some lights have sectors of

Trevose Head Lighthouse - Misplaced Pages Continue

2304-406: The choice of light sources, mountings, reflector design, the use of Fresnel lenses , and in rotation and shuttering systems providing lighthouses with individual signatures allowing them to be identified by seafarers. He also invented the movable jib and the balance-crane as a necessary part for lighthouse construction. Alexander Mitchell designed the first screw-pile lighthouse – his lighthouse

2376-525: The collapse of the Soviet government in 1990s, most of the official records on the locations, and condition, of these lighthouses were reportedly lost. Over time, the condition of RTGs in Russia degraded; many of them fell victim to vandalism and scrap metal thieves, who may not have been aware of the dangerous radioactive contents. Energy-efficient LED lights can be powered by solar panels , with batteries instead of

2448-488: The construction of lenses of large aperture and short focal length , without the mass and volume of material that would be required by a lens of conventional design. A Fresnel lens can be made much thinner than a comparable conventional lens, in some cases taking the form of a flat sheet. A Fresnel lens can also capture more oblique light from a light source, thus allowing the light from a lighthouse equipped with one to be visible over greater distances. The first Fresnel lens

2520-621: The development of the lighthouse. In antiquity, the lighthouse functioned more as an entrance marker to ports than as a warning signal for reefs and promontories , unlike many modern lighthouses. The most famous lighthouse structure from antiquity was the Pharos of Alexandria , Egypt , which collapsed following a series of earthquakes between 956 and 1323. The intact Tower of Hercules at A Coruña , Spain gives insight into ancient lighthouse construction; other evidence about lighthouses exists in depictions on coins and mosaics, of which many represent

2592-500: The distance along a great circle was 60 miles per degree. However, these referred to the old English mile of 5000 feet and league of 15,000 feet, relying upon Ptolemy's underestimate of the Earth's circumference . In the early seventeenth century, English geographers started to acknowledge the discrepancy between the angular measurement of a degree of latitude and the linear measurement of miles. In 1624 Edmund Gunter suggested 352,000 feet to

2664-473: The emitted light into a concentrated beam, thereby greatly increasing the light's visibility. The ability to focus the light led to the first revolving lighthouse beams, where the light would appear to the mariners as a series of intermittent flashes. It also became possible to transmit complex signals using the light flashes. French physicist and engineer Augustin-Jean Fresnel developed the multi-part Fresnel lens for use in lighthouses. His design allowed for

2736-412: The entrance into a harbor, such as New London Harbor Light . Where a tall cliff exists, a smaller structure may be placed on top such as at Horton Point Light . Sometimes, such a location can be too high, for example along the west coast of the United States, where frequent low clouds can obscure the light. In these cases, lighthouses are placed below the clifftop to ensure that they can still be seen at

2808-475: The equator forms 500 stadia , which make 62 ⁠ 1 / 2 ⁠ miles"). Whether a correction or convenience, the reason for the change from 62 ⁠ 1 / 2 ⁠ to 60 miles to a degree is not explained. Eventually, the ratio of 60 miles to a degree appeared in English in a 1555 translation of Pietro Martire d'Anghiera 's Decades: "[Ptolemy] assigned likewise to every degree three score miles." By

2880-547: The international nautical mile was defined by the First International Extraordinary Hydrographic Conference in Monaco as exactly 1,852 metres (which is 6,076.12 ft). The United States did not adopt the international nautical mile until 1954. Britain adopted it in 1970, but legal references to the obsolete unit are now converted to 1,853 metres (which is 6,079.40 ft). The metre

2952-455: The lantern room where the light operates. The lantern room is the glassed-in housing at the top of a lighthouse tower containing the lamp and lens. Its glass storm panes are supported by metal muntins (glazing bars) running vertically or diagonally. At the top of the lantern room is a stormproof ventilator designed to remove the smoke of the lamps and the heat that builds in the glass enclosure. A lightning rod and grounding system connected to

Trevose Head Lighthouse - Misplaced Pages Continue

3024-671: The late 16th century English geographers and navigators knew that the ratio of distances at sea to degrees was constant along any great circle (such as the equator , or any meridian), assuming that Earth was a sphere. In 1574, William Bourne stated in A Regiment for the Sea the "rule to raise a degree" practised by navigators: "But as I take it, we in England should allowe 60 myles to one degrée: that is, after 3 miles to one of our Englishe leagues, wherefore 20 of oure English leagues shoulde answere to one degrée." Likewise, Robert Hues wrote in 1594 that

3096-531: The light is concentrated, if needed, by the "lens" or "optic". Power sources for lighthouses in the 20th–21st centuries vary. Originally lit by open fires and later candles, the Argand hollow wick lamp and parabolic reflector were introduced in the late 18th century. Whale oil was also used with wicks as the source of light. Kerosene became popular in the 1870s and electricity and acetylene gas derived on-site from calcium carbide began replacing kerosene around

3168-505: The lighthouse at Ostia . Coins from Alexandria, Ostia, and Laodicea in Syria also exist. The modern era of lighthouses began at the turn of the 18th century, as the number of lighthouses being constructed increased significantly due to much higher levels of transatlantic commerce. Advances in structural engineering and new and efficient lighting equipment allowed for the creation of larger and more powerful lighthouses, including ones exposed to

3240-469: The lighthouse keepers. Efficiently concentrating the light from a large omnidirectional light source requires a very large diameter lens. This would require a very thick and heavy lens if a conventional lens were used. The Fresnel lens (pronounced / f r eɪ ˈ n ɛ l / ) focused 85% of a lamp's light versus the 20% focused with the parabolic reflectors of the time. Its design enabled construction of lenses of large size and short focal length without

3312-466: The luminosity of traditional oil lights. The use of gas as illuminant became widely available with the invention of the Dalén light by Swedish engineer Gustaf Dalén . He used Agamassan (Aga), a substrate , to absorb the gas, allowing the gas to be stored, and hence used, safely. Dalén also invented the ' sun valve ', which automatically regulated the light and turned it off during the daytime. The technology

3384-430: The measurement based on this ( ⁠ 40,075.017 km / 360 × 60 ⁠ = 1,855.3 metres) is known as the geographical mile . Using the definition ⁠ 1 / 60 ⁠ of a degree of latitude on Mars , a Martian nautical mile equals to 983 m (1,075 yd). This is potentially useful for celestial navigation on a human mission to the planet , both as a shorthand and a quick way to roughly determine

3456-421: The metal cupola roof provides a safe conduit for any lightning strikes. Immediately beneath the lantern room is usually a Watch Room or Service Room where fuel and other supplies were kept and where the keeper prepared the lanterns for the night and often stood watch. The clockworks (for rotating the lenses) were also located there. On a lighthouse tower, an open platform called the gallery is often located outside

3528-485: The most impressive feats of engineering of the age. This structure was based upon Smeaton's design, but with several improved features, such as the incorporation of rotating lights, alternating between red and white. Stevenson worked for the Northern Lighthouse Board for nearly fifty years during which time he designed and oversaw the construction and later improvement of numerous lighthouses. He innovated in

3600-402: The number of operational lighthouses has declined due to the expense of maintenance and the advent of much cheaper, more sophisticated, and more effective electronic navigational systems. Before the development of clearly defined ports , mariners were guided by fires built on hilltops. Since elevating the fire would improve visibility, placing the fire on a platform became a practice that led to

3672-403: The poles and 1,843 metres at the Equator. France and other metric countries state that in principle a nautical mile is an arcminute of a meridian at a latitude of 45°, but that is a modern justification for a more mundane calculation that was developed a century earlier. By the mid-19th century, France had defined a nautical mile via the original 1791 definition of the metre , one ten-millionth of

SECTION 50

#1733092556486

3744-401: The roof of a new engine house, containing a pair of Hornsby oil engines , the air compressor , reservoirs and other equipment. In 1920 a paraffin vapour burner replaced the oil lamp; it continued to show one red flash every 5 seconds. The fog signal equipment all remained in service until 1963, when the trumpet and siren were replaced by a set of eight 'supertyfon' air horns mounted in

3816-458: The sea. The function of lighthouses was gradually changed from indicating ports to the providing of a visible warning against shipping hazards, such as rocks or reefs. The Eddystone Rocks were a major shipwreck hazard for mariners sailing through the English Channel . The first lighthouse built there was an octagonal wooden structure, anchored by 12 iron stanchions secured in the rock, and

3888-451: The seabed and a low wooden structure is placed above the open framework, such as Thomas Point Shoal Lighthouse . As screw piles can be disrupted by ice, steel caisson lighthouses such as Orient Point Light are used in cold climates. Orient Long Beach Bar Light (Bug Light) is a blend of a screw pile light that was converted to a caisson light because of the threat of ice damage. Skeletal iron towers with screw-pile foundations were built on

3960-541: The siege of Atlanta, designed and built some of the most exotic lighthouses in the most difficult locations on the U.S. Great Lakes . French merchant navy officer Marius Michel Pasha built almost a hundred lighthouses along the coasts of the Ottoman Empire in a period of twenty years after the Crimean War (1853–1856). In a lighthouse, the source of light is called the "lamp" (whether electric or fuelled by oil) and

4032-516: The surface during periods of fog or low clouds, as at Point Reyes Lighthouse . Another example is in San Diego , California : the Old Point Loma lighthouse was too high up and often obscured by fog, so it was replaced in 1891 with a lower lighthouse, New Point Loma lighthouse . As technology advanced, prefabricated skeletal iron or steel structures tended to be used for lighthouses constructed in

4104-587: The third and most famous Eddystone Lighthouse , but some builders are well known for their work in building multiple lighthouses. The Stevenson family ( Robert , Alan , David , Thomas , David Alan , and Charles ) made lighthouse building a three-generation profession in Scotland. Richard Henry Brunton designed and built 26 Japanese lighthouses in Meiji Era Japan, which became known as Brunton's "children". Blind Irishman Alexander Mitchell invented and built

4176-522: The tower was blown away. After completion of the first tower, it was determined that the light was under certain circumstances liable to be mistaken by mariners . A second lower light was therefore proposed and (the decision having been taken in June 1847) it was constructed, 50 feet in front of the first light, with a covered passage between them for use by the lighthouse keepers . Only the first built 'high' light now remains. Designed by engineer James Walker

4248-483: The traditional light, including in some cases a rotating beam. A typical LED system designed to fit into the traditional 19th century Fresnel lens enclosure was developed by Trinity House and two other lighthouse authorities and costs about € 20,000, depending on configuration, according to a supplier; it has large fins to dissipate heat. Lifetime of the LED light source is 50,000 to 100,000 hours, compared to about 1,000 hours for

4320-476: The turn of the 20th century. Carbide was promoted by the Dalén light , which automatically lit the lamp at nightfall and extinguished it at dawn. In the second half of the 20th century, many remote lighthouses in Russia (then Soviet Union ) were powered by radioisotope thermoelectric generators (RTGs). These had the advantage of providing power day or night and did not need refuelling or maintenance. However, after

4392-450: The two original lights, 'high' and 'low', were constructed under the supervision of Henry Norris by builders Jacob & Thomas Olver of Falmouth . They were provided with a pair of first-order fixed optics by Henry Lépaute of Paris and each had an oil lamp with 4 concentric wicks manufactured by Messrs. Wilkins & Co. of Long Acre . The light was first lit on 1 December 1847. In 1882, under Engineer-in-Chief James Douglass ,

SECTION 60

#1733092556486

4464-523: The watch room (called the Main Gallery) or Lantern Room (Lantern Gallery). This was mainly used for cleaning the outside of the windows of the Lantern Room. Lighthouses near to each other that are similar in shape are often painted in a unique pattern so they can easily be recognized during daylight, a marking known as a daymark . The black and white barber pole spiral pattern of Cape Hatteras Lighthouse

4536-643: The weight and volume of material in conventional lens designs. Fresnel lighthouse lenses are ranked by order , a measure of refracting power, with a first order lens being the largest, most powerful and expensive; and a sixth order lens being the smallest. The order is based on the focal length of the lens. A first order lens has the longest focal length, with the sixth being the shortest. Coastal lighthouses generally use first, second, or third order lenses, while harbor lights and beacons use fourth, fifth, or sixth order lenses. Some lighthouses, such as those at Cape Race , Newfoundland, and Makapuu Point , Hawaii, used

4608-437: Was a kerosene lamp or, earlier, an animal or vegetable oil Argand lamp, and the lenses rotated by a weight driven clockwork assembly wound by lighthouse keepers, sometimes as often as every two hours. The lens assembly sometimes floated in liquid mercury to reduce friction. In more modern lighthouses, electric lights and motor drives were used, generally powered by diesel electric generators. These also supplied electricity for

4680-459: Was built by Henry Winstanley from 1696 to 1698. His lighthouse was the first tower in the world to have been fully exposed to the open sea. The civil engineer John Smeaton rebuilt the lighthouse from 1756 to 1759; his tower marked a major step forward in the design of lighthouses and remained in use until 1877. He modeled the shape of his lighthouse on that of an oak tree , using granite blocks. He rediscovered and used " hydraulic lime ",

4752-662: Was built on piles that were screwed into the sandy or muddy seabed. Construction of his design began in 1838 at the mouth of the Thames and was known as the Maplin Sands lighthouse, and first lit in 1841. Although its construction began later, the Wyre Light in Fleetwood, Lancashire, was the first to be lit (in 1840). Until 1782 the source of illumination had generally been wood pyres or burning coal. The Argand lamp , invented in 1782 by

4824-736: Was installed (a stack of electric emitters placed at the foot of the lighthouse) whereupon the old engine house was demolished. The fog signal was decommissioned in 2012. In 2023 the revolving optic was removed, having been in service for over 110 years. It was replaced by a fixed LED lantern, which now produces the required flash. At the same time nominal range of the light was reduced from 21 to 18 nautical miles . The former keepers' cottages (arranged in two semi-detached pairs) are nowadays available to rent as holiday accommodation. Lighthouse Lighthouses mark dangerous coastlines, hazardous shoals , reefs , rocks, and safe entries to harbors; they also assist in aerial navigation . Once widely used,

4896-460: Was installed. At the same time, the addition of a red filter to the lamp meant that (as from 1 August 1912) the lighthouse displayed one short red flash every five seconds. Work also began on installing a fog signal : a 5-inch siren attached to a 36-foot-long acoustic horn , which came into service in 1913; it was nicknamed ' Lord Rayleigh 's trumpet' after its designer, the eminent physicist and acoustician . The trumpet and siren were built on to

4968-487: Was originally defined as 1 ⁄ 10,000,000 of the length of the meridian arc from the North pole to the equator (1% of a centesimal degree of latitude), thus one kilometre of distance corresponds to one centigrad (also known as centesimal arc minute) of latitude. The Earth's circumference is therefore approximately 40,000 km. The equatorial circumference is slightly longer than the polar circumference –

5040-449: Was revised with better estimates of the earth’s circumference. In 1637, Robert Norwood proposed a new measurement of 6120 feet for an arcminute of latitude, which was within 44 feet of the currently accepted value for a nautical mile. Since the Earth is not a perfect sphere but is an oblate spheroid with slightly flattened poles, a minute of latitude is not constant, but about 1,862 metres at

5112-502: Was the predominant light source in lighthouses from the 1900s to the 1960s, when electric lighting had become dominant. With the development of the steady illumination of the Argand lamp, the application of optical lenses to increase and focus the light intensity became a practical possibility. William Hutchinson developed the first practical optical system in 1777, known as a catoptric system. This rudimentary system effectively collimated

5184-588: Was used in 1823 in the Cordouan lighthouse at the mouth of the Gironde estuary ; its light could be seen from more than 20 miles (32 km) out. Fresnel's invention increased the luminosity of the lighthouse lamp by a factor of four and his system is still in common use. The introduction of electrification and automatic lamp changers began to make lighthouse keepers obsolete. For many years, lighthouses still had keepers, partly because lighthouse keepers could serve as

#485514