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70-449: Buzzards Bay Entrance Light is a lighthouse located in open water at the entrance to Buzzards Bay, about four nautical miles west southwest of Cuttyhunk Island, Massachusetts . The light has a racon showing the letter "B". In 1996 the present structure replaced a Texas Tower built in 1961, which in turn had replaced the lightships Hens & Chickens (LV-5) and Vineyard Sound (LV-10). Since it sits in water 50 ft (15m) deep,

140-402: 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

210-640: 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 Many Fresnel lens installations have been replaced by rotating aerobeacons , which require less maintenance.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

280-444: A conventional lighthouse would have been difficult, forcing the choice of structure. This article relating to a United States lighthouse is a stub . You can help Misplaced Pages by expanding it . Lighthouse Lighthouses mark dangerous coastlines, hazardous shoals , reefs , rocks, and safe entries to harbors; they also assist in aerial navigation . Once widely used, the number of operational lighthouses has declined due to

350-436: 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

420-457: 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

490-534: A fund of anecdotes about the dangers he tended to place himself in and his lucky narrow escapes. For example, in 1794, he was aboard the sloop Elizabeth of Stromness , returning from the Orkney Islands , when it became becalmed off Kinnaird Head . Unlike others aboard the ship, he had the good fortune to be taken off it and rowed ashore. After he left it, a gale arose and drove the ship back to Orkney, where it foundered: All aboard were drowned. Another time, he

560-414: 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

630-427: 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

700-602: 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

770-521: 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

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840-402: 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

910-411: 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

980-415: 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

1050-477: 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

1120-505: 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

1190-419: 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

1260-587: 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. Robert Stevenson (civil engineer) Robert Stevenson , FRSE , FGS , FRAS , FSA Scot , MWS (8 June 1772 – 12 July 1850)

1330-510: 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

1400-643: The Andersonian Institute in Glasgow. In the winter, when it was too chilly for construction work, he attended lectures at the University of Edinburgh in philosophy, mathematics, chemistry, natural history, moral philosophy, logic, and agriculture. He was not granted a degree because he did not have the proficiency in Latin or Greek that was a requirement for a degree in those days. In 1797, he

1470-655: The Encyclopædia Britannica and the Edinburgh Encyclopædia, and published papers in a number of scientific journals. He was inducted into the Scottish Engineering Hall of Fame in 2016. In 1799, Robert married his stepsister, Jean Smith, who was his stepfather Thomas Smith's eldest daughter by his first wife. A number of their children died young, including three who died of childhood diseases early in 1808, during

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1540-622: 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

1610-513: 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

1680-466: 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

1750-405: 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

1820-495: 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

1890-491: 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

1960-406: 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

2030-533: 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

2100-597: The condition of the eastern coastline of the United Kingdom , entitled Account of the Bell Rock Lighthouse . In it, he presented convincing evidence that the North Sea was eroding that coastline, and in particular that the great sandbanks were disappearing — the spoils taken by the sea. He hypothesized that freshwater and saltwater areas at river mouths exist as separate and distinct streams, and carried out tests of this hypothesis. He contributed articles to

2170-432: The construction of Bell Rock. Three of Stevenson's sons became engineers: Alan , David and Thomas . Robert's other surviving child was Jane (1801–1864). Jane became her father's secretary and helped him write and illustrate his account of the Bell Rock Lighthouse construction. He had two grandchildren who became well-known: His son Alan was the father of the author and journalist Katharine de Mattos , and his son Thomas

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2240-488: 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

2310-627: The construction of roads, bridges, harbours, canals, railways, and aids to river navigation. He designed and oversaw the construction in Glasgow of the Hutcheson Bridge , and in Edinburgh of the Regent Bridge and approaches to it from the east. He also produced a number of designs for canals and railways which were not built, and new and improved designs for bridges, some of which were later implemented by his successors. He invented

2380-473: 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

2450-475: 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

2520-464: 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 the development of the lighthouse. In antiquity, the lighthouse functioned more as an entrance marker to ports than as

2590-571: The family moved to 1 Blair Street, off the Royal Mile in Edinburgh . Robert's new stepfather was Thomas Smith , a tinsmith , lamp maker, ingenious mechanic , and civil engineer , who had been appointed to the newly formed Northern Lighthouse Board in 1786. In 1798 or 1799, when Robert was about 26, the family moved to a newly built home, 2 Baxters Place, at the head of Leith Walk . In 1815, Robert's stepfather died, and Robert inherited

2660-625: The house, where he continued to live until about 1820. Stevenson served as an apprentice civil engineer to his stepfather, Thomas Smith. He was so successful at it that, at age 19, he was given responsibility for supervising the erection of a lighthouse on Little Cumbrae island in the River Clyde . His next project was overseeing the building of lighthouses on Orkney . While working on these projects, he continued his civil engineering studies: He diligently practised surveying and architectural drawing , and attended maths and physics lectures at

2730-455: 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

2800-535: 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

2870-509: 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

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2940-469: 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

3010-470: 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

3080-421: 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

3150-496: 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

3220-464: The movable jib and the balance crane as necessary aids to lighthouse construction, and, as George Stephenson noted, he led the trend toward using malleable rather than cast-iron rails in the construction of railways. In 1815, he was made a Fellow of the Royal Society of Edinburgh . His proposers were John Barclay , John Playfair and David Brewster . In 1824, he published a paper about

3290-405: The same disease around the same time. Since this left Alan's widow, Jean Lillie Stevenson, in much-reduced financial circumstances, Robert was educated, as a young child, at a charity school . Robert's mother intended him to join the ministry , so when he was a bit older she enrolled him in the school of a locally famous Glasgow linguist , a Mr Macintyre. But when Robert was 15, she remarried and

3360-465: 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

3430-451: 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

3500-554: 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

3570-569: The situation. Stevenson served as the engineer to the Northern Lighthouse Board until 1842 - nearly fifty years. During that time he designed numerous lighthouses and oversaw their construction and the addition of later improvements to them. His many innovations included his choice of light sources and mountings, his reflector design, his use of Fresnel lenses , and his use of rotation and shuttering systems that provided lighthouses with individual signatures ( characteristics ) — allowing them to be identified by seafarers. For this latter innovation, he

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3640-528: 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

3710-599: 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

3780-486: 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

3850-480: 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

3920-472: The two men's sons when they were older - Robert's son Alan Stevenson and John Rennie's son, Sir John Rennie . Samuel Smiles , a popular engineering author of the time, published an account taken from Rennie, which gave prominence to Rennie's claim. However, the Northern Lighthouse Board gave full credit to Stevenson, as have historians since then. Stevenson's work on the Bell Rock and elsewhere provided

3990-535: 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

4060-643: 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

4130-437: 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

4200-543: Was a Scottish civil engineer , and designer and builder of lighthouses . His works include the Bell Rock Lighthouse . Robert Stevenson was born in Glasgow . His father was Alan Stevenson, a partner in a West Indies sugar trading house in the city. Alan died of an epidemic fever on the island of St. Christopher in the West Indies on 26 May 1774, a few days before Robert's second birthday. Robert's uncle died of

4270-555: Was a scheme long in the gestation, and then long (and extremely hazardous) in the construction. Its structure was based upon the design of the Eddystone Lighthouse , which had been built by John Smeaton — but Stevenson made several improvements to that design. John Rennie was a consulting engineer on the project. After the project was complete, there was some contention as to who should get more credit—Rennie or Stevenson. That contention grew particularly strong as between

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4340-474: Was appointed engineer to the Lighthouse Board, succeeding to his stepfather's place there. In 1799, he married Smith's eldest daughter Jean, who was also his stepsister, and, in 1800, Smith made him his business partner. The most important work of Stevenson's life was the Bell Rock Lighthouse , built between 1807 and 1810 when he was in his mid-30s. The lighthouse still stands today. Its construction

4410-709: Was awarded a gold medal by King William I of the Netherlands . Engineering skills were in high demand after the Battle of Waterloo , which marked the end of the continental wars, as the focus turned toward improving the country's infrastructure. So Stevenson was kept busy. In addition to his work for the Northern Lighthouse Board, he served as a consulting engineer on many projects, collaborating with other engineers such as John Rennie , Alexander Nimmo , Thomas Telford , William Walker , Archibald Elliot , and William Cubitt . These projects included

4480-459: 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 ",

4550-683: 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

4620-538: Was removed. Stevenson died on 12 July 1850, at 1 Baxters Place in Edinburgh . He is buried in the Stevenson family plot in the New Calton Burial Ground . Stevenson College, Edinburgh , named after Robert Stevenson, was founded in 1970. For a good portion of his life, Stevenson lived at 1 Baxter's Place, Edinburgh. In 1985, the building was named “Robert Stevenson House” in his memory. (The name

4690-456: Was the father of the author Robert Louis Stevenson . Stevenson lived in Baxter's Place at the head of Leith Walk for most of his later years, moving from No. 2 to the far larger No. 1 around 1820. In 1985, the building was named "Robert Stevenson House" in his honour. It was used as an office from 1985 to 2015. The building was converted into a Marriott Hotel in 2017, at which point the name

4760-506: 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

4830-598: 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

4900-490: Was with a crew of men on the Bell Rock, which was only above the surface of the water at the lowest tide, when one of the crew boats drifted away. The remaining boats did not have enough room to carry everyone back to the mainland. Once the tide rose, the rock would have been submerged, and anyone not in a boat would have been stranded in the water. Luckily, before the tide rose, the Bell Rock pilot boat happened to arrive on an errand to deliver some mail to Stevenson, and thus saved

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