text:
Devonian

The Devonian  is a geologic period and system of the Paleozoic era during the Phanerozoic eon, spanning 60.3 million years from the end of the preceding Silurian period at  million years ago (Ma), to the beginning of the succeeding Carboniferous period at  Ma. It is named after Devon, South West England, where rocks from this period were first studied.

The first significant evolutionary radiation of life on land occurred during the Devonian, as free-sporing land plants (pteridophytes) began to spread across dry land, forming extensive coal forests which covered the continents. By the middle of the Devonian, several groups of vascular plants had evolved leaves and true roots, and by the end of the period the first seed-bearing plants (pteridospermatophytes) appeared. This rapid evolution and colonization process, which had begun during the Silurian, is known as the Silurian-Devonian Terrestrial Revolution. The earliest land animals, predominantly arthropods such as myriapods, arachnids and hexapods, also became well-established early in this period, after beginning their colonization of land at least from the Ordovician period.

Fishes, especially jawed fish, reached substantial diversity during this time, leading the Devonian to often be dubbed the Age of Fishes. The armored placoderms began dominating almost every known aquatic environment. In the oceans, cartilaginous fishes such as primitive sharks became more numerous than in the Silurian and Late Ordovician. Tetrapodomorphs, which include the ancestors of all four-limbed vertebrates (i.e. tetrapods), began diverging from freshwater lobe-finned fish as their more robust and muscled pectoral and pelvic fins gradually evolved into forelimbs and hindlimbs, though they were not fully established for life on land until the Late Carboniferous.

The first ammonites, a subclass of cephalopod molluscs, appeared. Trilobites, brachiopods and the great coral reefs were still common during the Devonian. The Late Devonian extinction, which started about 375 Ma, severely affected marine life, killing off most of the reef systems, most of the jawless fish, half of all placoderms, and nearly all trilobites save for a few species of the order Proetida. The subsequent end-Devonian extinction, which occurred at around 359 Ma, further impacted the ecosystems and completed the extinction of all calcite sponge reefs and placoderms.

Devonian palaeogeography was dominated by the supercontinent Gondwana to the south, the small continent of Siberia to the north, and the medium-sized continent of Laurussia to the east. Major tectonic events include the closure of the Rheic Ocean, the separation of South China from Gondwana, and the resulting expansion of the Paleo-Tethys Ocean. The Devonian experienced several major mountain-building events as Laurussia and Gondwana approached; these include the Acadian Orogeny in North America and the beginning of the Variscan Orogeny in Europe. These early collisions preceded the formation of the single supercontinent Pangaea in the Late Paleozoic.

History
The period is named after Devon, a county in southwestern England, where a controversial argument in the 1830s over the age and structure of the rocks found throughout the county was resolved by adding the Devonian Period to the geological timescale. The Great Devonian Controversy was a lengthy debate between Roderick Murchison, Adam Sedgwick and Henry De la Beche over the naming of the period. Murchison and Sedgwick won the debate and named it the Devonian System.

While the rock beds that define the start and end of the Devonian Period are well identified, the exact dates are uncertain. According to the International Commission on Stratigraphy, the Devonian extends from the end of the Silurian Ma, to the beginning of the Carboniferous  Ma – in North America, at the beginning of the Mississippian subperiod of the Carboniferous.

In 19th-century texts the Devonian has been called the "Old Red Age", after the red and brown terrestrial deposits known in the United Kingdom as the Old Red Sandstone in which early fossil discoveries were found. Another common term is "Age of the Fishes", referring to the evolution of several major groups of fish that took place during the period. Older literature on the Anglo-Welsh basin divides it into the Downtonian, Dittonian, Breconian, and Farlovian stages, the latter three of which are placed in the Devonian.

The Devonian has also erroneously been characterised as a "greenhouse age", due to sampling bias: most of the early Devonian-age discoveries came from the strata of western Europe and eastern North America, which at the time straddled the Equator as part of the supercontinent of Euramerica where fossil signatures of widespread reefs indicate tropical climates that were warm and moderately humid. In fact the climate in the Devonian differed greatly during its epochs and between geographic regions. For example, during the Early Devonian, arid conditions were prevalent through much of the world including Siberia, Australia, North America, and China, but Africa and South America had a warm temperate climate. In the Late Devonian, by contrast, arid conditions were less prevalent across the world and temperate climates were more common.

Subdivisions
The Devonian Period is formally broken into Early, Middle and Late subdivisions. The rocks corresponding to those epochs are referred to as belonging to the Lower, Middle and Upper parts of the Devonian System.

The Early Devonian lasted from to  Ma. It began with the Lochkovian Stage  to  Ma, which was followed by the Pragian from  to  Ma and then by the Emsian, which lasted until the Middle Devonian began,  Ma. During this time, the first ammonoids appeared, descending from bactritoid nautiloids. Ammonoids during this time period were simple and differed little from their nautiloid counterparts. These ammonoids belong to the order Agoniatitida, which in later epochs evolved to new ammonoid orders, for example Goniatitida and Clymeniida. This class of cephalopod molluscs would dominate the marine fauna until the beginning of the Mesozoic Era.
 * Early Devonian

The Middle Devonian comprised two subdivisions: first the Eifelian, which then gave way to the Givetian Ma. During this time the jawless agnathan fishes began to decline in diversity in freshwater and marine environments partly due to drastic environmental changes and partly due to the increasing competition, predation, and diversity of jawed fishes. The shallow, warm, oxygen-depleted waters of Devonian inland lakes, surrounded by primitive plants, provided the environment necessary for certain early fish to develop such essential characteristics as well developed lungs and the ability to crawl out of the water and onto the land for short periods of time.
 * Middle Devonian

Finally, the Late Devonian started with the Frasnian, to  Ma, during which the first forests took shape on land. The first tetrapods appeared in the fossil record in the ensuing Famennian subdivision, the beginning and end of which are marked with extinction events. This lasted until the end of the Devonian, Ma.
 * Late Devonian

Climate
The Devonian was a relatively warm period, although significant glaciers may have existed during the Early and Middle Devonian. The temperature gradient from the equator to the poles was not as large as it is today. The weather was also very arid, mostly along the equator where it was the driest. Reconstruction of tropical sea surface temperature from conodont apatite implies an average value of 30 C in the Early Devonian. Early Devonian mean annual surface temperatures were approximately 16 °C. levels dropped steeply throughout the Devonian Period. The newly evolved forests drew carbon out of the atmosphere, which were then buried into sediments. This may be reflected by a Mid-Devonian cooling of around 5 C-change. The Late Devonian warmed to levels equivalent to the Early Devonian; while there is no corresponding increase in concentrations, continental weathering increases (as predicted by warmer temperatures); further, a range of evidence, such as plant distribution, points to a Late Devonian warming. The climate would have affected the dominant organisms in reefs; microbes would have been the main reef-forming organisms in warm periods, with corals and stromatoporoid sponges taking the dominant role in cooler times. The warming at the end of the Devonian may even have contributed to the extinction of the stromatoporoids. At the terminus of the Devonian, Earth rapidly cooled into an icehouse, marking the beginning of the Late Paleozoic icehouse.

Paleogeography
The Devonian world involved many continents and ocean basins of various sizes. The largest continent, Gondwana, was located entirely within the Southern Hemisphere. It corresponds to modern day South America, Africa, Australia, Antarctica, and India, as well as minor components of North America and Asia. The second-largest continent, Laurussia, was northwest of Gondwana, and corresponds to much of modern-day North America and Europe. Various smaller continents, microcontinents, and terranes were present east of Laurussia and north of Gondwana, corresponding to parts of Europe and Asia. The Devonian Period was a time of great tectonic activity, as the major continents of Laurussia and Gondwana drew closer together.

Sea levels were high worldwide, and much of the land lay under shallow seas, where tropical reef organisms lived. The enormous "world ocean", Panthalassa, occupied much of the Northern Hemisphere as well as wide swathes east of Gondwana and west of Laurussia. Other minor oceans were the Paleo-Tethys Ocean and Rheic Ocean.

Laurussia
By the early Devonian, the continent Laurussia (also known as Euramerica) was fully formed through the collision of the continents Laurentia (modern day North America) and Baltica (modern day northern and eastern Europe). The tectonic effects of this collision continued into the Devonian, producing a string of mountain ranges along the southeastern coast of the continent. In present-day eastern North America, the Acadian Orogeny continued to raise the Appalachian Mountains. Further east, the collision also extended the rise of the Caledonian Mountains of Great Britain and Scandinavia. As the Caledonian Orogeny wound down in the later part of the period, orogenic collapse facilitated a cluster of granite intrusions in Scotland.

Most of Laurussia was located south of the equator, but in the Devonian it moved northwards and began to rotate counterclockwise towards its modern position. While the most northern parts of the continent (such as Greenland and Ellesmere Island) established tropical conditions, most of the continent was located within the natural dry zone along the Tropic of Capricorn, which (as nowadays) is a result of the convergence of two great air-masses, the Hadley cell and the Ferrel cell. In these near-deserts, the Old Red Sandstone sedimentary beds formed, made red by the oxidised iron (hematite) characteristic of drought conditions. The abundance of red sandstone on continental land also lends Laurussia the name "the Old Red Continent". For much of the Devonian, the majority of western Laurussia (North America) was covered by subtropical inland seas which hosted a diverse ecosystem of reefs and marine life. Devonian marine deposits are particularly prevalent in the midwestern and northeastern United States. Devonian reefs also extended along the southeast edge of Laurussia, a coastline now corresponding to southern England, Belgium, and other mid-latitude areas of Europe.

In the Early and Middle Devonian, the west coast of Laurussia was a passive margin with broad coastal waters, deep silty embayments, river deltas and estuaries, found today in Idaho and Nevada. In the Late Devonian, an approaching volcanic island arc reached the steep slope of the continental shelf and began to uplift deep water deposits. This minor collision sparked the start of a mountain-building episode called the Antler orogeny, which extended into the Carboniferous. Mountain building could also be found in the far northeastern extent of the continent, as minor tropical island arcs and detached Baltic terranes re-join the continent. Deformed remnants of these mountains can still be found on Ellesmere Island and Svalbard. Many of the Devonian collisions in Laurussia produce both mountain chains and foreland basins, which are frequently fossiliferous.

Gondwana
Gondwana was by far the largest continent on the planet. It was completely south of the equator, although the northeastern sector (now Australia) did reach tropical latitudes. The southwestern sector (now South America) was located to the far south, with Brazil situated near the South Pole. The northwestern edge of Gondwana was an active margin for much of the Devonian, and saw the accretion of many smaller land masses and island arcs. These include Chilenia, Cuyania, and Chaitenia, which now form much of Chile and Patagonia. These collisions were associated with volcanic activity and plutons, but by the Late Devonian the tectonic situation had relaxed and much of South America was covered by shallow seas. These south polar seas hosted a distinctive brachiopod fauna, the Malvinokaffric Realm, which extended eastward to marginal areas now equivalent to South Africa and Antarctica. Malvinokaffric faunas even managed to approach the South Pole via a tongue of Panthalassa which extended into the Paraná Basin.

The northern rim of Gondwana was mostly a passive margin, hosting extensive marine deposits in areas such as northwest Africa and Tibet. The eastern margin, though warmer than the west, was equally active. Numerous mountain building events and granite and kimberlite intrusions affected areas equivalent to modern day eastern Australia, Tasmania, and Antarctica.

Asian terranes
Several island microcontinents (which would later coalesce into modern day Asia) stretched over a low-latitude archipelago to the north of Gondwana. They were separated from the southern continent by an oceanic basin: the Paleo-Tethys. Although the western Paleo-Tethys Ocean had existed since the Cambrian, the eastern part only began to rift apart as late as the Silurian. This process accelerated in the Devonian. The eastern branch of the Paleo-Tethys was fully opened when South China and Annamia (a terrane equivalent to most of Indochina), together as a unified continent, detached from the northeastern sector of Gondwana. Nevertheless, they remained close enough to Gondwana that their Devonian fossils were more closely related to Australian species than to north Asian species. Other Asian terranes remained attached to Gondwana, including Sibumasu (western Indochina), Tibet, and the rest of the Cimmerian blocks. While the South China-Annamia continent was the newest addition to the Asian microcontinents, it was not the first. North China and the Tarim Block (now northwesternmost China) were located westward and continued to drift northwards, powering over older oceanic crust in the process. Further west was a small ocean (the Turkestan Ocean), followed by the larger microcontinents of Kazakhstania, Siberia, and Amuria. Kazakhstania was a volcanically active region during the Devonian, as it continued to assimilate smaller island arcs. The island arcs of the region, such as the Balkhash-West Junggar Arc, exhibited biological endemism as a consequence of their location.

Siberia was located just north of the equator as the largest landmass in the Northern Hemisphere. At the beginning of the Devonian, Siberia was inverted (upside down) relative to its modern orientation. Later in the period it moved northwards and began to twist clockwise, though it was not near its modern location. Siberia approached the eastern edge of Laurussia as the Devonian progressed, but it was still separated by a seaway, the Ural Ocean. Although Siberia's margins were generally tectonically stable and ecologically productive, rifting and deep mantle plumes impacted the continent with flood basalts during the Late Devonian. The Altai-Sayan region was shaken by volcanism in the Early and Middle Devonian, while Late Devonian magmatism was magnified further to produce the Vilyuy Traps, flood basalts which may have contributed to the Late Devonian Mass Extinction. The last major round of volcanism, the Yakutsk Large Igneous Province, continued into the Carboniferous to produce extensive kimberlite deposits.

Similar volcanic activity also affected the nearby microcontinent of Amuria (now Manchuria, Mongolia and their vicinities). Though certainly close to Siberia in the Devonian, the precise location of Amuria is uncertain due to contradictory paleomagnetic data.

Closure of the Rheic Ocean
The Rheic Ocean, which separated Laurussia from Gondwana, was wide at the start of the Devonian, having formed after the drift of Avalonia away from Gondwana. It steadily shrunk as the period continued, as the two major continents approached near the equator in the early stages of the assembly of Pangaea. The closure of the Rheic Ocean began in the Devonian and continued into the Carboniferous. As the ocean narrowed, endemic marine faunas of Gondwana and Laurussia combined into a single tropical fauna. The history of the western Rheic Ocean is a subject of debate, but there is good evidence that Rheic oceanic crust experienced intense subduction and metamorphism under Mexico and Central America.

The closure of the eastern part of the Rheic Ocean is associated with the assemblage of central and southern Europe. In the early Paleozoic, much of Europe was still attached to Gondwana, including the terranes of Iberia, Armorica (France), Palaeo-Adria (the western Mediterranean area), Bohemia, Franconia, and Saxothuringia. These continental blocks, collectively known as the Armorican Terrane Assemblage, split away from Gondwana in the Silurian and drifted towards Laurussia through the Devonian. Their collision with Laurussia leads to the beginning of the Variscan Orogeny, a major mountain-building event which would escalate further in the Late Paleozoic. Franconia and Saxothuringia collided with Laurussia near the end of the Early Devonian, pinching out the easternmost Rheic Ocean. The rest of the Armorican terranes followed, and by the end of the Devonian they were fully connected with Laurussia. This sequence of rifting and collision events led to the successive creation and destruction of several small seaways, including the Rheno-Hercynian, Saxo-Thuringian, and Galicia-Moldanubian oceans. Their sediments were eventually compressed and completely buried as Gondwana fully collided with Laurussia in the Carboniferous.

Marine biota

Sea levels in the Devonian were generally high. Marine faunas continued to be dominated by conodonts, bryozoans, diverse and abundant brachiopods, the enigmatic hederellids, microconchids, and corals. Lily-like crinoids (animals, their resemblance to flowers notwithstanding) were abundant, and trilobites were still fairly common. Bivalves became commonplace in deep water and outer shelf environments. The first ammonites also appeared during or slightly before the early Devonian Period around 400 Ma. Bactritoids make their first appearance in the Early Devonian as well; their radiation, along with that of ammonoids, has been attributed by some authors to increased environmental stress resulting from decreasing oxygen levels in the deeper parts of the water column. Among vertebrates, jawless armored fish (ostracoderms) declined in diversity, while the jawed fish (gnathostomes) simultaneously increased in both the sea and fresh water. Armored placoderms were numerous during the early ages of the Devonian Period and became extinct in the Late Devonian, perhaps because of competition for food against the other fish species. Early cartilaginous (Chondrichthyes) and bony fishes (Osteichthyes) also become diverse and played a large role within the Devonian seas. The first abundant genus of cartilaginous fish, Cladoselache, appeared in the oceans during the Devonian Period. The great diversity of fish around at the time has led to the Devonian being given the name "The Age of Fishes" in popular culture.

The Devonian saw significant expansion in the diversity of nektonic marine life driven by the abundance of planktonic microorganisms in the free water column as well as high ecological competition in benthic habitats, which were extremely saturated; this diversification has been labeled the Devonian Nekton Revolution by many researchers. However, other researchers have questioned whether this revolution existed at all; a 2018 study found that although the proportion of biodiversity constituted by nekton increased across the boundary between the Silurian and Devonian, it decreased across the span of the Devonian, particularly during the Pragian, and that the overall diversity of nektonic taxa did not increase significantly during the Devonian compared to during other geologic periods, and was in fact higher during the intervals spanning from the Wenlock to the Lochkovian and from the Carboniferous to the Permian. The study's authors instead attribute the increased overall diversity of nekton in the Devonian to a broader, gradual trend of nektonic diversification across the entire Palaeozoic.

Reefs
A now-dry barrier reef, located in present-day Kimberley Basin of northwest Australia, once extended 350 km, fringing a Devonian continent. Reefs are generally built by various carbonate-secreting organisms that can erect wave-resistant structures near sea level. Although modern reefs are constructed mainly by corals and calcareous algae, Devonian reefs were either microbial reefs built up mostly by autotrophic cyanobacteria or coral-stromatoporoid reefs built up by coral-like stromatoporoids and tabulate and rugose corals. Microbial reefs dominated under the warmer conditions of the early and late Devonian, while coral-stromatoporoid reefs dominated during the cooler middle Devonian.

Terrestrial biota
By the Devonian Period, life was well underway in its colonization of the land. The moss forests and bacterial and algal mats of the Silurian were joined early in the period by primitive rooted plants that created the first stable soils and harbored arthropods like mites, scorpions, trigonotarbids and myriapods (although arthropods appeared on land much earlier than in the Early Devonian and the existence of fossils such as Protichnites suggest that amphibious arthropods may have appeared as early as the Cambrian). By far the largest land organism at the beginning of this period was the enigmatic Prototaxites, which was possibly the fruiting body of an enormous fungus, rolled liverwort mat, or another organism of uncertain affinities that stood more than 8 metres tall, and towered over the low, carpet-like vegetation during the early part of the Devonian. Also, the first possible fossils of insects appeared around 416 Ma, in the Early Devonian. Evidence for the earliest tetrapods takes the form of trace fossils in shallow lagoon environments within a marine carbonate platform/shelf during the Middle Devonian, although these traces have been questioned and an interpretation as fish feeding traces (Piscichnus) has been advanced.

The greening of land
Many Early Devonian plants did not have true roots or leaves like extant plants, although vascular tissue is observed in many of those plants. Some of the early land plants such as Drepanophycus likely spread by vegetative growth and spores. The earliest land plants such as Cooksonia consisted of leafless, dichotomous axes with terminal sporangia and were generally very short-statured, and grew hardly more than a few centimetres tall. Fossils of Armoricaphyton chateaupannense, about 400 million years old, represent the oldest known plants with woody tissue. By the Middle Devonian, shrub-like forests of primitive plants existed: lycophytes, horsetails, ferns, and progymnosperms evolved. Most of these plants had true roots and leaves, and many were quite tall. The earliest-known trees appeared in the Middle Devonian. These included a lineage of lycopods and another arborescent, woody vascular plant, the cladoxylopsids and progymnosperm Archaeopteris. These tracheophytes were able to grow to large size on dry land because they had evolved the ability to biosynthesize lignin, which gave them physical rigidity and improved the effectiveness of their vascular system while giving them resistance to pathogens and herbivores. In Eifelian age, cladoxylopsid trees formed the first forests in Earth history. By the end of the Devonian, the first seed-forming plants had appeared. This rapid appearance of many plant groups and growth forms has been referred to as the Devonian Explosion or the Silurian-Devonian Terrestrial Revolution.

The 'greening' of the continents acted as a carbon sink, and atmospheric concentrations of carbon dioxide may have dropped. This may have cooled the climate and led to a massive extinction event. (See Late Devonian extinction).

Animals and the first soils
Primitive arthropods co-evolved with this diversified terrestrial vegetation structure. The evolving co-dependence of insects and seed plants that characterized a recognizably modern world had its genesis in the Late Devonian Epoch. The development of soils and plant root systems probably led to changes in the speed and pattern of erosion and sediment deposition. The rapid evolution of a terrestrial ecosystem that contained copious animals opened the way for the first vertebrates to seek terrestrial living. By the end of the Devonian, arthropods were solidly established on the land.

Late Devonian extinction
The Late Devonian extinction is not a single event, but rather is a series of pulsed extinctions at the Givetian-Frasnian boundary, the Frasnian-Famennian boundary, and the Devonian-Carboniferous boundary. Together, these are considered one of the "Big Five" mass extinctions in Earth's history. The Devonian extinction crisis primarily affected the marine community, and selectively affected shallow warm-water organisms rather than cool-water organisms. The most important group to be affected by this extinction event were the reef-builders of the great Devonian reef systems.

Amongst the severely affected marine groups were the brachiopods, trilobites, ammonites, and acritarchs, and the world saw the disappearance of an estimated 96% of vertebrates like conodonts and bony fishes, and all of the ostracoderms and placoderms. Land plants as well as freshwater species, such as our tetrapod ancestors, were relatively unaffected by the Late Devonian extinction event (there is a counterargument that the Devonian extinctions nearly wiped out the tetrapods ).

The reasons for the Late Devonian extinctions are still unknown, and all explanations remain speculative. Canadian paleontologist Digby McLaren suggested in 1969 that the Devonian extinction events were caused by an asteroid impact. However, while there were Late Devonian collision events (see the Alamo bolide impact), little evidence supports the existence of a large enough Devonian crater.

title: Devonian


text:
Dungeon Master (disambiguation)

A Dungeon Master is the organizer of a Dungeons & Dragons role-playing game.

Dungeon Master may also refer to:

Games

 * Dungeon Master (video game), a 1987 video game series
 * The Dungeon Master (video game), a 1983 video game for the ZX Spectrum
 * Zork III: The Dungeon Master, a 1982 interactive fiction game

Film and television

 * Dungeon Master, a character in Dungeons & Dragons (TV series)
 * The Dungeonmaster, a 1984 film
 * The Dungeon Masters, a 2008 documentary film

Other uses

 * Dungeon monitor, or dungeon master, a person charged with supervising a playspace at BDSM events
 * The Dungeon Master: The Disappearance of James Dallas Egbert III, a 1985 non-fiction book by William Dear

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/

title: Dungeon Master (disambiguation)


text:
David Thompson (explorer)

David Thompson (30 April 1770 – 10 February 1857) was an Anglo-Canadian fur trader, surveyor, and cartographer, known to some native people as "Koo-Koo-Sint" or "the Stargazer". Over Thompson's career, he travelled 90000 km across North America, mapping 4.9 e6km2 of the continent along the way. For this historic feat, Thompson has been described as the "greatest practical land geographer that the world has produced".

Early life
David Thompson was born in Westminster, Middlesex, to recent Welsh migrants David and Ann Thompson. When Thompson was two, his father died. Due to his widowed mother not having financial resources, she placed Thompson, 29 April 1777, the day before his seventh birthday, and his older brother in the Grey Coat Hospital, a school for the disadvantaged of Westminster. Thompson graduated to the Grey Coat mathematical school, well known for teaching navigation and surveying.

He received an education for the Royal Navy: including mathematics of trigonometry and geometry, practical navigation including using of nautical instruments, finding latitudes and longitudes and making navigational calculations from observing the sun, moon and tide, and drawing maps and charts, taking land measurements, and sketching landscapes. He later built on these skills to make his career. In 1784, when Thompson was 14, the Grey Coat treasurer paid the Hudson's Bay Company the sum of five pounds, upon which the youth became an apprentice employee of the company, contracted for a period of seven years to be trained as a clerk.

He set sail on a ship to North America on 28 May of that year, leaving England.

Hudson's Bay Company (HBC)
On 2 September 1784, Thompson arrived in Churchill (now in Manitoba) and was put to work as a clerk/secretary, copying the personal papers of the governor of Fort Churchill, Samuel Hearne. The next year he was transferred to nearby York Factory, and over the next few years spent time as a secretary at Cumberland House, and South Branch House of the Hudson's Bay Company before being transferred to Manchester House in 1787. During those years he learned to keep accounts and other records, calculate values of furs (it was noted that he also had several expensive beaver pelts at that time even when a secretary's job would not pay terribly well), track supplies and other duties.

On 23 December 1788, Thompson seriously fractured his tibia, forcing him to spend the next two winters at Cumberland House convalescing. It was during this time that he greatly refined and expanded his mathematical, astronomical, and surveying skills under the tutelage of Hudson's Bay Company surveyor Philip Turnor. It was also during this time that he lost sight in his right eye.

In 1790, with his apprenticeship nearing its end, Thompson requested a set of surveying tools in place of the typical parting gift of fine clothes offered by the company to those completing their indenture. He received both. He entered the employ of the Hudson's Bay Company as a fur trader. In 1792 he completed his first significant survey, mapping a route to Lake Athabasca (where today's Alberta/Saskatchewan border is located).

Between February and May 1793, Thompson made 34 observations of the longitude of Cumberland House using lunar distances. The mean of these observations was 102°12′ W, about 2' east of the modern value. The mean error of the 34 observations was about 15' of longitude. Broughton (2009) notes that the precision of the type of sextant used by Thompson was 15" of arc, corresponding to 7.5' of longitude giving an absolute limit to the precision of an individual observation. The error in Thompson's mean was several times less than this. The time he took on these observations, about 3 hours of calculation each, indicates that he understood the power of averages.

In recognition of his map-making and surveying skills, the company promoted Thompson to the surveyor in 1794. He continued working for the Hudson's Bay Company until 23 May 1797 when, frustrated by an order to cease surveying and focus on the fur trade, he left. He walked 80 mi in the snow in order to enter the employ of the competition, the North West Company. There he continued to work as a fur trader and surveyor.

North West Company
Thompson's decision to defect to the North West Company (NWC) in 1797 without providing the customary one-year notice was not well received by his former employers. But the North West Company was more supportive of Thompson pursuing his work on surveying and mapping the interior of what was to become Canada, as they judged it in the company's interest to know the exact locations of their settlements and the distances between them. In 1797, Thompson was sent south by his employers to survey part of the Canada-US boundary along the water routes from Lake Superior to Lake of the Woods to satisfy unresolved questions of territory arising from the Jay Treaty of 1794 between Great Britain and the United States after the American Revolutionary War.

By 1798 Thompson had completed a survey of 6750 km from Grand Portage, through Lake Winnipeg, to the headwaters of the Assiniboine and Mississippi rivers, as well as two sides of Lake Superior. In 1798, the company sent him to Red Deer Lake (Lac La Biche in present-day Alberta) to establish a trading post. (The English translation of Lac la Biche: Red Deer Lake, was first recorded on the Mackenzie map of 1793.) Thompson spent the next few seasons trading based in Fort George (now in Alberta), and during this time led several expeditions into the Rocky Mountains.

On 10 July 1804, at the annual meeting of the North West Company in Kaministiquia, Thompson was made a full partner of the company. He became a 'wintering partner', who was based in the field rather than Montreal, and was granted two of the 92 NWC's shares worth more than £4,000. He spent the next few seasons based there managing the fur trading operations, but still finding time to expand his surveys of the waterways around Lake Superior. At the 1806 company meeting, officers decided to send Thompson back into the interior. Concern over the United States-backed expedition of Lewis and Clark prompted the North West Company to charge Thompson with the task of finding a route to the Pacific to open up the lucrative trading territories of the Pacific Northwest.

Columbia River travels

After the general meeting in 1806, Thompson travelled to Rocky Mountain House and prepared for an expedition to follow the Columbia River to the Pacific Ocean. In June 1807 Thompson crossed the Rocky Mountains and spent the summer surveying the Columbia basin; he continued to survey the area over the next few seasons. Thompson mapped and established trading posts in Northwestern Montana, Idaho, Washington, and Western Canada. Trading posts he founded included Kootenae House, Kullyspell House and Saleesh House; the latter two were the first trading posts west of the Rockies in Idaho and Montana, respectively. These posts established by Thompson extended North West Company fur trading territory into the Columbia Basin drainage area. The maps he made of the Columbia River basin east of the Cascade Mountains were of such high quality and detail that they continued to be useful into the 20th-century.

In early 1810, Thompson was returning eastward toward Montreal but, while en route at Rainy Lake, received orders to return to the Rocky Mountains and establish a route to the mouth of the Columbia. The North West Company was responding to the plans of American entrepreneur John Jacob Astor to send a ship around the Americas to establish a fur trading post of the Pacific Fur Company on the Pacific Coast. During his return, Thompson was delayed by an angry group of Peigan natives at Howse Pass. He was ultimately forced to seek a new route across the Rocky Mountains and found one through the Athabasca Pass.

David Thompson was the first European to navigate the full length of the Columbia River. Between Kettle Falls (3 July 1811) and the Junction of the Columbia and Snake Rivers (9 July), he was travelling through country that had never been visited by Europeans, and took time to visit the villages along the way to establish good relations, helped by copious quantities of tobacco. In 1805 Lewis and Clark had descended the Snake River, and continued down the Columbia. On reaching the junction Thompson erected a pole and a notice claiming the country for Great Britain and stating the intention of the North West Company to build a trading post at the site. This notice was found later that year by Astor company workers looking to establish an inland fur post, contributing to their selection of a more northerly site at Fort Okanogan. The North West Company established its post of Fort Nez Percés near the Snake River confluence several years later. Continuing down the Columbia, Thompson passed over the Celilo Falls, almost losing the canoe on the rocks, and portaged around the rapids of The Dalles and the Cascades Rapids. On 14 July 1811, Thompson reached the partially constructed Fort Astoria at the mouth of the Columbia, arriving two months after the Pacific Fur Company's ship, the Tonquin.

Before returning upriver and across the mountains, Thompson hired Naukane, a Native Hawaiian Takane labourer brought to Fort Astoria by the Pacific Fur Company's ship Tonquin. Naukane, known as Coxe to Thompson, accompanied Thompson across the continent to Lake Superior before journeying on to England.

Thompson wintered at Saleesh House before beginning his final journey in 1812 back to Montreal, where the North West Company was based.

In his published journals, Thompson recorded seeing large footprints (“which measured fourteen inches in length by eight inches in breadth”) near what is now Jasper, Alberta, in 1811. It has been suggested that these prints were similar to what has since been called the sasquatch. However, Thompson noted that these tracks showed "a small Nail at the end of each [toe]", which led him to surmise it was a bear, but he had doubts, saying, "I held it to be the track of a large old grizzled bear; yet the shortness of the nails, the ball of the foot, and its great size was not that of a Bear".

The years 1807-1812 are the most carefully scrutinized in his career and comprise his most enduring historical legacy, due to his development of the commercial routes across the Rockies, and his mapping of the lands they traverse.

Appearance and personality
In 1820, the English geologist, John Jeremiah Bigsby, attended a dinner party given by The Hon. William McGillivray at his home, Chateau St. Antoine, one of the early estates in Montreal's Golden Square Mile. He describes the party and some of the guests in his entertaining book The Shoe and Canoe, giving an excellent description of David Thompson:

"I was well placed at table between one of the Miss McGillivray's and a singular-looking person of about fifty. He was plainly dressed, quiet, and observant. His figure was short and compact, and his black hair was worn long all round, and cut square, as if by one stroke of the shears, just above the eyebrows. His complexion was of the gardener's ruddy brown, while the expression of his deeply-furrowed features was friendly and intelligent, but his cut-short nose gave him an odd look. His speech betrayed the Welshman, although he left his native hills when very young. I might have been spared this description of Mr David Thompson by saying he greatly resembled Curran the Irish Orator..."

"I afterwards travelled much with him, and have now only to speak of him with great respect, or, I ought to say, with admiration... No living person possesses a tithe of his information respecting the Hudson's Bay countries... Never mind his Bunyan-like face and cropped hair; he has a very powerful mind, and a singular faculty of picture-making. He can create a wilderness and people it with warring savages, or climb the Rocky Mountains with you in a snow-storm, so clearly and palpably, that only shut your eyes and you hear the crack of the rifle, or feel the snow-flakes melt on your cheeks as he talks."

Marriage and children
On 10 June 1799 at Île-à-la-Crosse, Thompson married Charlotte Small, a thirteen-year-old Métis daughter of Scottish fur trader Patrick Small and a Cree mother. Their marriage was formalised thirteen years later at the Scotch Presbyterian Church in Montreal on 30 October 1812. He and Charlotte had 13 children together; five of them were born before he left the fur trade. The family did not adjust easily to life in Eastern Canada; they lived in Montreal while he was travelling. Two of the children, John (aged 5) and Emma (aged 7), died of round worms, a common parasite. By the time of Thompson's death, the couple had been married 57 years, the longest marriage known in Canada pre-Confederation.

Later life
Upon his arrival back in Montreal, Thompson retired with a generous pension from the North West Company. He settled in nearby Terrebonne and worked on completing his great map, a summary of his lifetime of exploring and surveying the interior of North America. The map covered the wide area stretching from Lake Superior to the Pacific, and was given by Thompson to the North West Company. Thompson's 1814 map, his greatest achievement, was so accurate that 100 years later it was still the basis for many of the maps issued by the Canadian government. It now resides in the Archives of Ontario.

In 1815, Thompson moved his family to Williamstown, Upper Canada, and a few years later was employed to survey the newly established borders with the United States from Lake of the Woods to the Eastern Townships of Quebec, established by Treaty of Ghent after the War of 1812. In 1843 Thompson completed his atlas of the region from Hudson Bay to the Pacific Ocean.

Afterwards, Thompson returned to a life as a land owner, but soon financial misfortune would ruin him. By 1831 he was so deeply in debt he was forced to take up a position as a surveyor for the British American Land Company to provide for his family. His luck continued to worsen and he was forced to move in with his daughter and son-in-law in 1845. He began work on a manuscript chronicling his life exploring the continent, but this project was left unfinished when his sight failed him completely in 1851.

Death and afterward

The land mass mapped by Thompson amounted to 3.9 e6km2 of wilderness (one-fifth of the continent). His contemporary, the great explorer Alexander Mackenzie, remarked that Thompson did more in ten months than he would have thought possible in two years.

Despite these significant achievements, Thompson died in Montreal in near obscurity on 10 February 1857, his accomplishments almost unrecognised. He never finished the book of his 28 years in the fur trade, based on his 77 field notebooks, before he died. In the 1890s geologist J.B. Tyrrell resurrected Thompson's notes and in 1916 published them as David Thompson's Narrative, as part of the General Series of the Champlain Society. Further editions and re-examinations of Thompson's life and works were published in 1962 by Richard Glover, in 1971 by Victor Hopwood, and in 2015 by William Moreau.

Thompson's body was interred in Montreal's Mount Royal Cemetery in an unmarked grave. It was not until 1926 that efforts by J.B. Tyrrell and the Canadian Historical Society resulted in the placing of a tombstone to mark his grave. The next year, Thompson was named a National Historic Person by the federal government, one of the earliest such designations. A federal plaque reflecting that status is at Jasper National Park, Alberta. Meantime, Thompson's achievements are central reasons for other national historic designations:
 * David Thompson on the Columbia River National Historic Event, marked at Castlegar, BC
 * Athabasca Pass National Historic Site (NHS), at Jasper National Park
 * Boat Encampment NHS, BC
 * Howse Pass NHS, Banff National Park, Alberta
 * Kootenae House NHS, BC
 * Rocky Mountain House NHS, Alberta

In 1957, one hundred years after his death, Canada's post office department honoured him with his image on a postage stamp. The David Thompson Highway in Alberta was named in his honour, along with David Thompson High School on the side of the highway near Leslieville, Alberta. There are also two David Thompson Secondary Schools, one in Vancouver, BC, and one in Invermere, BC.

His prowess as a geographer is now well-recognized. He has been called "the greatest land geographer that the world has produced."

There is a monument dedicated to David Thompson (maintained by the state of North Dakota) near the former town site of the ghost town Verendrye, North Dakota, located approximately 2 mi north and 1 mi west of Karlsruhe, North Dakota. Thompson Falls, Montana, and British Columbia's Thompson River and Thompson Falls on the Blaeberry River are also named after the explorer.

The year 2007 marked the 150th year of Thompson's death and the 200th anniversary of his first crossing of the Rocky Mountains. Commemorative events and exhibits were planned across Canada and the United States from 2007 to 2011 as a celebration of his accomplishments.

In 2007, a commemorative plaque was placed on a wall at the Grey Coat Hospital, the school for the disadvantaged of Westminster David Thompson attended as a boy, by English author and TV presenter Ray Mears.

Thompson was the subject of a 1964 National Film Board of Canada short film David Thompson: The Great Mapmaker , as well as the BBC2 programme Ray Mears' Northern Wilderness (Episode 5), broadcast in November 2009. He's also the subject of 2010 KSPS-TV film Uncharted Territory: David Thompson on the Columbia Plateau.

He is referenced in the 1981 folk song "Northwest Passage" by Stan Rogers.

The national park service, Parks Canada, announced in 2018 that it had named its new research vessel RV David Thompson, to be used for underwater archaeology, including sea floor mapping, and for marine science in the Pacific, Atlantic, Arctic Oceans, and the Great Lakes. It will be the main platform for research on the Wrecks of HMS Erebus and HMS Terror National Historic Site.

The David Thompson Astronomical Observatory at Fort William Historical Park was named to commemorate David Thompson and his discoveries.

Works

 * 1814: Map of the North-West Territory of the Province of Canada
 * 1897: New light on the early history of the greater Northwest (edited by Elliott Coues) Volume I; Volume II; Volume III
 * 1916: David Thompson's narrative of his explorations in western America, 1784–1812 (edited by J.B. Tyrrell)
 * 1950: David Thompson's journals relating to Montana and adjacent regions, 1808–1812 (edited by M. Catherine White)
 * 1962: David Thompson's narrative, 1784–1812 (edited by Richard Glover)
 * 1974: David Thompson's journal of the international boundary survey, 1817–1827: western Lake Erie, August–September 1819 (edited by Clarke E. Leverette)
 * 1993: Columbia Journals (edited by Barbara Belyea)
 * 2006: "Moccasin Miles – The Travels of Charlotte Small Thompson 1799–1812 " Contemporary and Historical Maps: Charlotte Small (S. Leanne Playter/Andreas N. Korsos|Publisher: Arcturus Consulting)
 * 2006/2007: "David Thompson in Alberta 1787–1812"; "David Thompson on the Columbia River 1807–1812"; "The Explorations and Travels of David Thompson 1784–1812"; "Posts and Forts of the North American Fur Trade 1600–1870" Contemporary and Historical Maps: David Thompson (Andreas N. Korsos|Publisher: Arcturus Consulting)
 * 2010 : Official Documentary of Thompson was released by national geographic, ca.

title: David Thompson (explorer)


text:
Dioscoreales

The Dioscoreales are an order of monocotyledonous flowering plants, organized under modern classification systems, such as the Angiosperm Phylogeny Group or the Angiosperm Phylogeny Web. Among monocot plants, Dioscoreales are grouped with the lilioid monocots, wherein they are a sister group to the Pandanales. In total, the order Dioscoreales comprises three families, 22 genera and about 850 species.

Dioscoreales contains the family Dioscoreaceae, which notably includes the yams (Dioscorea) and several other bulbous and tuberous plants, some of which are heavily cultivated as staple food sources in certain countries.

Certain species are found solely in arid climates (incl. parts of Southern Africa), and have adapted to this harsh environment as caudex-forming, perennial caudiciformes, including Dioscorea elephantipes, the "elephant's foot" or "elephant-foot yam".

Older systems tended to place all lilioid monocots with reticulate veined leaves (such as Smilacaceae and Stemonaceae together with Dioscoraceae) in Dioscoreales; as currently circumscribed by phylogenetic analysis, using combined morphology and molecular methods, Dioscreales now contains many reticulate-veined vines within the Dioscoraceae, as well as the myco-heterotrophic Burmanniaceae and the autotrophic Nartheciaceae.

Description
Dioscoreales are vines or herbaceous forest floor plants. They may be achlorophyllous or saprophytic. Synapomorphies include tuberous roots, glandular hairs, seed coat characteristics and the presence of calcium oxalate crystals. Other characteristics of the order include the presence of saponin steroids, annular vascular bundles that are found in both the stem and leaf. The leaves are often unsheathed at the base, have a distinctive petiole and reticulate veined lamina. Alternatively they may be small and scale-like with a sheathed base. The flowers are actinomorphic, and may be bisexual or dioecious, while the flowers or inflorescence bear glandular hairs. The perianth may be conspicuous or reduced and the style is short with well developed style branches. The tepals persist in the development of the fruit, which is a dry capsule or berry. In the seed, the endotegmen is tanniferous and the embryo short.

All of the species except the genera placed in Nartheciaceae express simultaneous microsporogenesis. Plants in Nartheciaceae show successive microsporogenesis which is one of the traits indicating that the family is sister to all the other members included in the order.

Pre-Darwinian
For the early history from Lindley (1853) onwards, see Caddick et al. (2000) Table 1, Caddick et al. (2002a) Table 1 and Table 2 in Bouman (1995). The taxonomic classification of Dioscoreales has been complicated by the presence of a number of morphological features reminiscent of the dicotyledons, leading some authors to place the order as intermediate between the monocotyledons and the dicotyledons.

While Lindley did not use the term "Dioscoreales", he placed the family Dioscoraceae together with four other families in what he referred to as an Alliance (the equivalent of the modern Order) called Dictyogens. He reflected the uncertainty as to the place of this Alliance by placing it as a class of its own between Endogens (monocots) and Exogens (dicots) The botanical authority is given to von Martius (1835) by APG for his description of the family Dioscoreae or Ordo, while other sources cite Hooker (Dioscoreales Hook.f.) for his use of the term "Dioscorales" in 1873 with a single family, Dioscoreae. However, in his more definitive work, the Genera plantara (1883), he simply placed Dioscoraceae in the Epigynae "Series".

Post-Darwinian
Although Charles Darwin's Origin of Species (1859) preceded Bentham and Hooker's publication, the latter project was commenced much earlier and George Bentham was initially sceptical of Darwinism. The new phyletic approach changed the way that taxonomists considered plant classification, incorporating evolutionary information into their schemata, but this did little to further define the circumscription of Dioscoreaceae. The major works in the late nineteenth and early twentieth century employing this approach were in the German literature. Authors such as Eichler, Engler and Wettstein placed this family in the Liliiflorae, a major subdivision of monocotyledons. it remained to Hutchinson (1926) to resurrect the Dioscoreales to group Dioscoreaceae and related families together. Hutchinson's circumscription of Dioscoreales included three other families in addition to Dioscoreaceae, Stenomeridaceae, Trichopodaceae and Roxburghiaceae. Of these only Trichopodaceae was included in the Angiosperm Phylogeny Group (APG) classification (see below), but was subsumed into Dioscoraceae. Stenomeridaceae, as Stenomeris was also included in Dioscoreaceae as subfamily Stenomeridoideae, the remaining genera being grouped in subfamily Dioscoreoideae. Roxburghiaceae on the other hand was segregated in the sister order Pandanales as Stemonaceae. Most taxonomists in the twentieth century (the exception was the 1981 Cronquist system which placed most such plants in order Liliales, subclass Liliidae, class Liliopsida=monocotyledons, division Magnoliophyta=angiosperms) recognised Dioscoreales as a distinct order, but demonstrated wide variations in its composition.

Dahlgren, in the second version of his taxonomic classification (1982) raised the Liliiflorae to a superorder and placed Dioscoreales as an order within it. In his system, Dioscoreales contained only three families, Dioscoreaceae, Stemonaceae (i.e. Hutchinson's Roxburghiaceae) and Trilliaceae. The latter two families had been treated as a separate order (Stemonales, or Roxburghiales) by other authors, such as Huber (1969). The APG would later assign these to Pandanales and Liliales respectively. Dahlgren's construction of Dioscoreaceae included the Stenomeridaceae and Trichopodaceae, doubting these were distinct, and Croomiaceae in Stemonaceae. Furthermore, he expressed doubts about the order's homogeneity, especially Trilliaceae. The Dioscoreales at that time were marginally distinguishable from the Asparagales. In his examination of Huber's Stemonales, he found that the two constituent families had as close an affinity to Dioscoreaceae as to each other, and hence included them. He also considered closely related families and their relationship to Dioscoreales, such as the monogeneric Taccaceae, then in its own order, Taccales. Similar considerations were discussed with respect to two Asparagales families, Smilacaceae and Petermanniaceae.

In Dahlgren's third and final version (1985) that broader circumscription of Dioscoreales was created within the superorder Lilianae, subclass Liliidae (monocotyledons), class Magnoliopsida (angiosperms) and comprised the seven families Dioscoreaceae, Petermanniaceae, Smilacaceae, Stemonaceae, Taccaceae, Trichopodaceae and Trilliaceae. Thismiaceae has either been treated as a separate family closely related to Burmanniaceae or as a tribe (Thismieae) within a more broadly defined Burmanniaceae, forming a separate order, Burmanniales, in the Dahlgren system. The related Nartheciaceae were treated as tribe Narthecieae within the Melanthiaceae in a third order, the Melanthiales, by Dahlgren. Dahlgren considered the Dioscoreales to most strongly resemble the ancestral monocotyledons, and hence sharing "dicotyledonous" characteristics, making it the most central monocotyledon order. Of these seven families, Bouman considered Dioscoreaceae, Trichopodaceae, Stemonaceae and Taccaceae to represent the "core" families of the order. However, that study also indicated both a clear delineation of the order from other orders particularly Asparagales, and a lack of homogeneity within the order.

Molecular phylogenetics and the Angiosperm Phylogeny Group
The increasing availability of molecular phylogenetics methods in addition to morphological characteristics in the 1990s led to major reconsiderations of the relationships within the monocotyledons. In that large multi-institutional examination of the seed plants using the plastid gene rbcL the authors used Dahlgren's system as their basis, but followed Thorne (1992) in altering the suffixes of the superorders from "-iflorae" to "-anae". This demonstrated that the Lilianae comprised three lineages corresponding to Dahlgren's orders Dioscoreales, Liliales, and Asparagaless.

Under the Angiosperm Phylogeny Group system of 1998, which took Dahlgren's system as a basis, the order was placed in the monocot clade and comprised the five families Burmanniaceae, Dioscoreaceae, Taccaceae, Thismiaceae and Trichopodaceae.

In APG II (2003), a number of changes were made to Dioscoreales, as a result of an extensive study by Caddick and colleagues (2002), using an analysis of three genes, rbcL, atpB and 18S rDNA, in addition to morphology. These studies resulted in a re-examination of the relationships between most of the genera within the order. Thismiaceae was shown to be a sister group to Burmanniaceae, and so was included in it. The monotypic families Taccaceae and Trichopodaceae were included in Dioscoreaceae, while Nartheciaceae could also be grouped within Dioscoreales. APG III (2009) did not change this, so the order now comprises three families Burmanniaceae, Dioscoreaceae and Nartheciaceae.

Although further research on the deeper relationships within Dioscoreales continues, the APG IV (2016) authors felt it was still premature to propose a restructuring of the order. Specifically these issues involve conflicting information as to the relationship between Thismia and Burmanniaceae, and hence whether Thismiaceae should be subsumed in the latter, or reinstated.

Phylogeny
Molecular phylogenetics in Dioscoreales poses special problems due to the absence of plastid genes in mycoheterotrophs. Dioscoreales is monophyletic and is placed as a sister order to Pandanales, as shown in Cladogram I.

Evolution
The data for the evolution of the order is collected from molecular analyses since there are no such fossils found. It is estimated that Dioscoreales and its sister clade Pandanales split up around 121 million years ago during Early Cretaceous when the stem group was formed. Then it took 3 to 6 million years for the crown group to differentiate in Mid Cretaceous.

Subdivision
The three families of Dioscreales constitutes about 22 genera and about 849 species making it one of the smaller monocot orders. Of these, the largest group is Dioscorea (yams) with about 450 species. By contrast the second largest genus is Burmannia with about 60 species, and most have only one or two.

Some authors, preferring the original APG (1998)families, continue to treat Thismiaceae separately from Burmanniaceae and Taccaceae from Dioscoreaceae. But in the 2015 study of Hertwerk and colleagues, seven genera representing all three families were examined with an eight gene dataset. Dioscoreales was monophyletic and three subclades were represented corresponding to the APG families. Dioscoreaceae and Burmanniaceae were in a sister group relationship.

Etymology
Named after the type genus Dioscorea, which in turn was named by Linnaeus in 1753 to honour the Greek physician and botanist Dioscorides.

Distribution and habitat
Species from this order are distributed across all of the continents except Antarctica. They are mainly tropical or subtropical representatives, but some members of families Dioscoreaceae and Nartheciaceae are found in cooler regions of Europe and North America. Order Dioscoreales contains plants that are able to form an underground organ for reservation of nutritions as many other monocots. An exception is the family Burmanniaceae which is entirely myco-heterotrophic and contains species that lack photosynthetic abilities.

Ecology
The three families included in order Dioscoreales also represent three different ecological groups of plants. Dioscoreaceae contains mainly vines (Dioscorea) and other crawling species (Epipetrum). Nartheciaceae on the other hand is a family composed of herbaceous plants with a rather lily-like appearance (Aletris) while Burmanniaceae is entirely myco-heterotrophic group.

Uses
Many members of Dioscoreaceae produce tuberous starchy roots (yams) which form staple foods in tropical regions. They have also been the source of steroids for the pharmaceutical industry, including the production of oral contraceptives.

Articles and chapters

 * , In
 * , In
 * Excerpts
 * , In
 * , In
 * , In
 * , In
 * , In
 * , In
 * , In
 * , In
 * , In
 * , In
 * , In
 * , In

title: Dioscoreales


text:
Talk:Disability

Impraid people
his daily lfe study playing etc 103.158.159.210 (talk) 02:38, 30 October 2023 (UTC)

Wiki Education assignment: Disability, Communication and Visual Culture
— Assignment last updated by Lyric.miller (talk) 01:09, 6 December 2023 (UTC)

Wiki Education assignment: Introduction to Community Economic and Social Development II
— Assignment last updated by Samandeepkaur002 (talk) 21:38, 8 April 2024 (UTC)

“Disability language” is written by chatgpt
See subject. 128.12.122.74 (talk) 18:34, 22 May 2024 (UTC)

 * And what's your point as well as evidence? Mason (talk) 00:08, 23 May 2024 (UTC)

title: Talk:Disability


text:
Default

Default may refer to:

Law

 * Default (law), the failure to do something required by law
 * Default (finance), failure to satisfy the terms of a loan obligation or failure to pay back a loan
 * Default judgment, a binding judgment in favor of either party based on some failure to take action by the other party
 * Default rule, a rule of law that can be overridden by a contract, trust, will, or other legally effective agreement

Science, technology

 * Default (computer science), a preset setting or value that will be used if no choice is done during program use or installation and setup
 * Default password, allows the device to be accessed during its initial setup, or after resetting to factory defaults
 * defaults (software), a command line utility for plist (preference) files for macOS and GNUstep

Music

 * "Default" (Atoms for Peace song), 2012
 * Default (band), a Canadian post-grunge and alternative rock band
 * "Default" (Django Django song), 2012
 * By Default, a 2016 album by Band of Skulls

Other uses

 * Default (2014 film), an American action thriller film
 * Default (2018 film), a South Korean drama film
 * Default (tennis), a disqualification in the game of tennis

title: Default


text:
Deposition

Deposition may refer to:
 * Deposition (law), taking testimony outside of court
 * Deposition (politics), the removal of a person of authority from political power
 * Deposition (university), a widespread initiation ritual for new students practiced from the Middle Ages until the 18th century

Art

 * Deposition from the Cross, the depiction of the removal of Jesus from the cross
 * Deposition (Bellini), a 1515–16 painting by Giovanni Bellini and his workshop
 * Deposition of Christ (Bronzino), a 1545 oil painting
 * The Deposition (Michelangelo), a 1547–55 marble sculpture
 * The Deposition from the Cross (Pontormo), a 1528 oil painting
 * Deposition from the Cross, Volterra (Rosso Fiorentino)
 * The Deposition (Raphael), a 1507 oil painting
 * The Deposition (Rubens), a 1602 painting by Peter Paul Rubens (previously attributed to van Dyck) now in the Galleria Borghese
 * Deposition (Rogier van der Weyden) or The Descent from the Cross, an oil painting, c.1435
 * Deposition (van Dyck, 1615), a 1615 painting by Anthony van Dyck, now in the Alte Pinakothek in Munich
 * Deposition (van Dyck, 1618), a 1618-20 painting, now in the Kunsthistorisches Museum in Vienna
 * Deposition (van Dyck, 1619), a c.1619 painting, now in the Ashmolean Museum
 * Deposition (van Dyck, 1629), a c.1629 painting, now in the Royal Museum of Fine Arts Antwerp
 * Deposition (van Dyck, 1629-30) or Lamentation over the Dead Christ, lost 1945
 * Deposition (van Dyck, 1634), a 1634 painting, now in the Alte Pinakothek in Munich
 * Deposition (van Dyck, 1635), a 1635 painting, now in the now in the Royal Museum of Fine Arts Antwerp
 * Deposition (van Dyck, 1640), a 1634-40 painting, now in the Valdes Izaguirre collection

Science

 * Deposition (chemistry), molecules settling out of a solution
 * Deposition (geology), material such as sediment being added to a landform
 * Deposition (phase transition), the process by which a gas is transformed into a solid
 * Deposition (aerosol physics), a process where aerosol particles set down onto surfaces

Technology

 * Cathodic arc deposition, a physical vapor deposition technique using an electric arc to vaporize material from a cathode target
 * Chemical vapor deposition, a chemical process used in the semiconductor industry to produce thin films
 * Electron beam-induced deposition, a process of decomposing gaseous molecules by an electron beam
 * Electrospark deposition, a micro-welding manufacturing process typically used to repair damage to mechanical components
 * Electrophoretic deposition
 * Ion beam-assisted deposition, a combination of ion implantation with a physical vapor deposition technique
 * Molecular vapor deposition, the gas-phase reaction between surface reactive chemicals and an appropriately receptive surface
 * Nanoparticle deposition, the process of attaching nanoparticles to solid surfaces called substrates to create coatings of nanoparticles
 * Particle deposition, the spontaneous attachment of particles to surfaces
 * Physical vapor deposition, a variety of vacuum deposition methods used to produce thin films and coatings
 * Pulsed laser deposition, a physical vapor deposition technique using a high-power pulsed laser beam in a vacuum chamber

Other uses

 * Defrocking or deposition, the opposite of ordination in many Christian churches
 * "Depositions" (Superstore), an episode of the TV series Superstore
 * "The Deposition" (The Office), an episode of the TV series The Office

title: Deposition


text: Talk:Deposition

title: Talk:Deposition


text:
Dentistry

Dentistry, also known as dental medicine and oral medicine, is the branch of medicine focused on the teeth, gums, and mouth. It consists of the study, diagnosis, prevention, management, and treatment of diseases, disorders, and conditions of the mouth, most commonly focused on dentition (the development and arrangement of teeth) as well as the oral mucosa. Dentistry may also encompass other aspects of the craniofacial complex including the temporomandibular joint. The practitioner is called a dentist.

The history of dentistry is almost as ancient as the history of humanity and civilization, with the earliest evidence dating from 7000 BC to 5500 BC. Dentistry is thought to have been the first specialization in medicine which has gone on to develop its own accredited degree with its own specializations. Dentistry is often also understood to subsume the now largely defunct medical specialty of stomatology (the study of the mouth and its disorders and diseases) for which reason the two terms are used interchangeably in certain regions. However, some specialties such as oral and maxillofacial surgery (facial reconstruction) may require both medical and dental degrees to accomplish. In European history, dentistry is considered to have stemmed from the trade of barber surgeons.

Dental treatments are carried out by a dental team, which often consists of a dentist and dental auxiliaries (such as dental assistants, dental hygienists, dental technicians, and dental therapists). Most dentists either work in private practices (primary care), dental hospitals, or (secondary care) institutions (prisons, armed forces bases, etc.).

The modern movement of evidence-based dentistry calls for the use of high-quality scientific research and evidence to guide decision-making such as in manual tooth conservation, use of fluoride water treatment and fluoride toothpaste, dealing with oral diseases such as tooth decay and periodontitis, as well as systematic diseases such as osteoporosis, diabetes, celiac disease, cancer, and HIV/AIDS which could also affect the oral cavity. Other practices relevant to evidence-based dentistry include radiology of the mouth to inspect teeth deformity or oral malaises, haematology (study of blood) to avoid bleeding complications during dental surgery, cardiology (due to various severe complications arising from dental surgery with patients with heart disease), etc.

Terminology
The term dentistry comes from dentist, which comes from French dentiste, which comes from the French and Latin words for tooth. The term for the associated scientific study of teeth is odontology (from ὀδούς) – the study of the structure, development, and abnormalities of the teeth.

Dental treatment
Dentistry usually encompasses practices related to the oral cavity. According to the World Health Organization, oral diseases are major public health problems due to their high incidence and prevalence across the globe, with the disadvantaged affected more than other socio-economic groups.

The majority of dental treatments are carried out to prevent or treat the two most common oral diseases which are dental caries (tooth decay) and periodontal disease (gum disease or pyorrhea). Common treatments involve the restoration of teeth, extraction or surgical removal of teeth, scaling and root planing, endodontic root canal treatment, and cosmetic dentistry

By nature of their general training, dentists, without specialization can carry out the majority of dental treatments such as restorative (fillings, crowns, bridges), prosthetic (dentures), endodontic (root canal) therapy, periodontal (gum) therapy, and extraction of teeth, as well as performing examinations, radiographs (x-rays), and diagnosis. Dentists can also prescribe medications used in the field such as antibiotics, sedatives, and any other drugs used in patient management. Depending on their licensing boards, general dentists may be required to complete additional training to perform sedation, dental implants, etc.

Dentists also encourage the prevention of oral diseases through proper hygiene and regular, twice or more yearly, checkups for professional cleaning and evaluation. Oral infections and inflammations may affect overall health and conditions in the oral cavity may be indicative of systemic diseases, such as osteoporosis, diabetes, celiac disease or cancer. Many studies have also shown that gum disease is associated with an increased risk of diabetes, heart disease, and preterm birth. The concept that oral health can affect systemic health and disease is referred to as "oral-systemic health".

Education and licensing

John M. Harris started the world's first dental school in Bainbridge, Ohio, and helped to establish dentistry as a health profession. It opened on 21 February 1828, and today is a dental museum. The first dental college, Baltimore College of Dental Surgery, opened in Baltimore, Maryland, US in 1840. The second in the United States was the Ohio College of Dental Surgery, established in Cincinnati, Ohio, in 1845. The Philadelphia College of Dental Surgery followed in 1852. In 1907, Temple University accepted a bid to incorporate the school.

Studies show that dentists that graduated from different countries, or even from different dental schools in one country, may make different clinical decisions for the same clinical condition. For example, dentists that graduated from Israeli dental schools may recommend the removal of asymptomatic impacted third molar (wisdom teeth) more often than dentists that graduated from Latin American or Eastern European dental schools.

In the United Kingdom, the first dental schools, the London School of Dental Surgery and the Metropolitan School of Dental Science, both in London, opened in 1859. The British Dentists Act of 1878 and the 1879 Dentists Register limited the title of "dentist" and "dental surgeon" to qualified and registered practitioners. However, others could legally describe themselves as "dental experts" or "dental consultants". The practice of dentistry in the United Kingdom became fully regulated with the 1921 Dentists Act, which required the registration of anyone practising dentistry. The British Dental Association, formed in 1880 with Sir John Tomes as president, played a major role in prosecuting dentists practising illegally. Dentists in the United Kingdom are now regulated by the General Dental Council.

In many countries, dentists usually complete between five and eight years of post-secondary education before practising. Though not mandatory, many dentists choose to complete an internship or residency focusing on specific aspects of dental care after they have received their dental degree. In a few countries, to become a qualified dentist one must usually complete at least four years of postgraduate study; Dental degrees awarded around the world include the Doctor of Dental Surgery (DDS) and Doctor of Dental Medicine (DMD) in North America (US and Canada), and the Bachelor of Dental Surgery/Baccalaureus Dentalis Chirurgiae (BDS, BDent, BChD, BDSc) in the UK and current and former British Commonwealth countries.

All dentists in the United States undergo at least three years of undergraduate studies, but nearly all complete a bachelor's degree. This schooling is followed by four years of dental school to qualify as a "Doctor of Dental Surgery" (DDS) or "Doctor of Dental Medicine" (DMD). Specialization in dentistry is available in the fields of Anesthesiology, Dental Public Health, Endodontics, Oral Radiology, Oral and Maxillofacial Surgery, Oral Medicine, Orofacial Pain, Pathology, Orthodontics, Pediatric Dentistry (Pedodontics), Periodontics, and Prosthodontics.

Specialties
Some dentists undertake further training after their initial degree in order to specialize. Exactly which subjects are recognized by dental registration bodies varies according to location. Examples include:
 * Anesthesiology – The specialty of dentistry that deals with the advanced use of general anesthesia, sedation and pain management to facilitate dental procedures.
 * Cosmetic dentistry – Focuses on improving the appearance of the mouth, teeth and smile.
 * Dental public health – The study of epidemiology and social health policies relevant to oral health.
 * Endodontics (also called endodontology) – Root canal therapy and study of diseases of the dental pulp and periapical tissues.
 * Forensic odontology – The gathering and use of dental evidence in law. This may be performed by any dentist with experience or training in this field. The function of the forensic dentist is primarily documentation and verification of identity.
 * Geriatric dentistry or geriodontics – The delivery of dental care to older adults involving the diagnosis, prevention, and treatment of problems associated with normal aging and age-related diseases as part of an interdisciplinary team with other health care professionals.
 * Oral and maxillofacial pathology – The study, diagnosis, and sometimes the treatment of oral and maxillofacial related diseases.
 * Oral and maxillofacial radiology – The study and radiologic interpretation of oral and maxillofacial diseases.
 * Oral and maxillofacial surgery (also called oral surgery) – Extractions, implants, and surgery of the jaws, mouth and face.
 * Oral biology – Research in dental and craniofacial biology
 * Oral Implantology – The art and science of replacing extracted teeth with dental implants.
 * Oral medicine – The clinical evaluation and diagnosis of oral mucosal diseases
 * Orthodontics and dentofacial orthopedics – The straightening of teeth and modification of midface and mandibular growth.
 * Pediatric dentistry (also called pedodontics) – Dentistry for children
 * Periodontology (also called periodontics) – The study and treatment of diseases of the periodontium (non-surgical and surgical) as well as placement and maintenance of dental implants
 * Prosthodontics (also called prosthetic dentistry) – Dentures, bridges and the restoration of implants.
 * Some prosthodontists super-specialize in maxillofacial prosthetics, which is the discipline originally concerned with the rehabilitation of patients with congenital facial and oral defects such as cleft lip and palate or patients born with an underdeveloped ear (microtia). Today, most maxillofacial prosthodontists return function and esthetics to patients with acquired defects secondary to surgical removal of head and neck tumors, or secondary to trauma from war or motor vehicle accidents.
 * Special needs dentistry (also called special care dentistry) – Dentistry for those with developmental and acquired disabilities.
 * Sports dentistry – the branch of sports medicine dealing with prevention and treatment of dental injuries and oral diseases associated with sports and exercise. The sports dentist works as an individual consultant or as a member of the Sports Medicine Team.
 * Veterinary dentistry – The field of dentistry applied to the care of animals. It is a specialty of veterinary medicine.

History
Tooth decay was low in pre-agricultural societies, but the advent of farming society about 10,000 years ago correlated with an increase in tooth decay (cavities). An infected tooth from Italy partially cleaned with flint tools, between 13,820 and 14,160 years old, represents the oldest known dentistry, although a 2017 study suggests that 130,000 years ago the Neanderthals already used rudimentary dentistry tools. The Indus valley has yielded evidence of dentistry being practised as far back as 7000 BC, during the Stone Age. The Neolithic site of Mehrgarh (now in Pakistan's south western province of Balochistan) indicates that this form of dentistry involved curing tooth related disorders with bow drills operated, perhaps, by skilled bead-crafters. The reconstruction of this ancient form of dentistry showed that the methods used were reliable and effective. The earliest dental filling, made of beeswax, was discovered in Slovenia and dates from 6500 years ago. Dentistry was practised in prehistoric Malta, as evidenced by a skull which had a dental abscess lanced from the root of a tooth dating back to around 2500 BC.

An ancient Sumerian text describes a "tooth worm" as the cause of dental caries. Evidence of this belief has also been found in ancient India, Egypt, Japan, and China. The legend of the worm is also found in the Homeric Hymns, and as late as the 14th century AD the surgeon Guy de Chauliac still promoted the belief that worms cause tooth decay.

Recipes for the treatment of toothache, infections and loose teeth are spread throughout the Ebers Papyrus, Kahun Papyri, Brugsch Papyrus, and Hearst papyrus of Ancient Egypt. The Edwin Smith Papyrus, written in the 17th century BC but which may reflect previous manuscripts from as early as 3000 BC, discusses the treatment of dislocated or fractured jaws. In the 18th century BC, the Code of Hammurabi referenced dental extraction twice as it related to punishment. Examination of the remains of some ancient Egyptians and Greco-Romans reveals early attempts at dental prosthetics. However, it is possible the prosthetics were prepared after death for aesthetic reasons.

Ancient Greek scholars Hippocrates and Aristotle wrote about dentistry, including the eruption pattern of teeth, treating decayed teeth and gum disease, extracting teeth with forceps, and using wires to stabilize loose teeth and fractured jaws. Some say the first use of dental appliances or bridges comes from the Etruscans from as early as 700 BC. The Phoenicians crafted the oldest documented dentures during the 6th–4th century BC, fashioning them from gold wire and incorporating two ivory teeth. In ancient Egypt, Hesy-Ra is the first named "dentist" (greatest of the teeth). The Egyptians bound replacement teeth together with gold wire. Roman medical writer Cornelius Celsus wrote extensively of oral diseases as well as dental treatments such as narcotic-containing emollients and astringents. The earliest dental amalgams were first documented in a Tang dynasty medical text written by the Chinese physician Su Kung in 659, and appeared in Germany in 1528.

During the Islamic Golden Age Dentistry was discussed in several famous books of medicine such as The Canon in medicine written by Avicenna and Al-Tasreef by Al-Zahrawi who is considered the greatest surgeon of the Middle Ages, Avicenna said that jaw fracture should be reduced according to the occlusal guidance of the teeth; this principle is still valid in modern times. Al-Zahrawi invented over 200 surgical tools that resemble the modern kind.

Historically, dental extractions have been used to treat a variety of illnesses. During the Middle Ages and throughout the 19th century, dentistry was not a profession in itself, and often dental procedures were performed by barbers or general physicians. Barbers usually limited their practice to extracting teeth which alleviated pain and associated chronic tooth infection. Instruments used for dental extractions date back several centuries. In the 14th century, Guy de Chauliac most probably invented the dental pelican (resembling a pelican's beak) which was used to perform dental extractions up until the late 18th century. The pelican was replaced by the dental key which, in turn, was replaced by modern forceps in the 19th century.

The first book focused solely on dentistry was the "Artzney Buchlein" in 1530, and the first dental textbook written in English was called "Operator for the Teeth" by Charles Allen in 1685.

In the United Kingdom, there was no formal qualification for the providers of dental treatment until 1859 and it was only in 1921 that the practice of dentistry was limited to those who were professionally qualified. The Royal Commission on the National Health Service in 1979 reported that there were then more than twice as many registered dentists per 10,000 population in the UK than there were in 1921.

Modern dentistry

It was between 1650 and 1800 that the science of modern dentistry developed. The English physician Thomas Browne in his A Letter to a Friend (c. 1656 pub. 1690) made an early dental observation with characteristic humour:

""

The French surgeon Pierre Fauchard became known as the "father of modern dentistry". Despite the limitations of the primitive surgical instruments during the late 17th and early 18th century, Fauchard was a highly skilled surgeon who made remarkable improvisations of dental instruments, often adapting tools from watchmakers, jewelers and even barbers, that he thought could be used in dentistry. He introduced dental fillings as treatment for dental cavities. He asserted that sugar-derived acids like tartaric acid were responsible for dental decay, and also suggested that tumors surrounding the teeth and in the gums could appear in the later stages of tooth decay.

Fauchard was the pioneer of dental prosthesis, and he invented many methods to replace lost teeth. He suggested that substitutes could be made from carved blocks of ivory or bone. He also introduced dental braces, although they were initially made of gold, he discovered that the teeth position could be corrected as the teeth would follow the pattern of the wires. Waxed linen or silk threads were usually employed to fasten the braces. His contributions to the world of dental science consist primarily of his 1728 publication Le chirurgien dentiste or The Surgeon Dentist. The French text included "basic oral anatomy and function, dental construction, and various operative and restorative techniques, and effectively separated dentistry from the wider category of surgery".

After Fauchard, the study of dentistry rapidly expanded. Two important books, Natural History of Human Teeth (1771) and Practical Treatise on the Diseases of the Teeth (1778), were published by British surgeon John Hunter. In 1763, he entered into a period of collaboration with the London-based dentist James Spence. He began to theorise about the possibility of tooth transplants from one person to another. He realised that the chances of a successful tooth transplant (initially, at least) would be improved if the donor tooth was as fresh as possible and was matched for size with the recipient. These principles are still used in the transplantation of internal organs. Hunter conducted a series of pioneering operations, in which he attempted a tooth transplant. Although the donated teeth never properly bonded with the recipients' gums, one of Hunter's patients stated that he had three which lasted for six years, a remarkable achievement for the period.

Major advances in science were made in the 19th century, and dentistry evolved from a trade to a profession. The profession came under government regulation by the end of the 19th century. In the UK, the Dentist Act was passed in 1878 and the British Dental Association formed in 1879. In the same year, Francis Brodie Imlach was the first ever dentist to be elected President of the Royal College of Surgeons (Edinburgh), raising dentistry onto a par with clinical surgery for the first time.

Hazards in modern dentistry
Long term occupational noise exposure can contribute to permanent hearing loss, which is referred to as noise-induced hearing loss (NIHL) and tinnitus. Noise exposure can cause excessive stimulation of the hearing mechanism, which damages the delicate structures of the inner ear. NIHL can occur when an individual is exposed to sound levels above 90 dBA according to the Occupational Safety and Health Administration (OSHA). Regulations state that the permissible noise exposure levels for individuals is 90 dBA. For the National Institute for Occupational Safety and Health (NIOSH), exposure limits are set to 85 dBA. Exposures below 85 dBA are not considered to be hazardous. Time limits are placed on how long an individual can stay in an environment above 85 dBA before it causes hearing loss. OSHA places that limitation at 8 hours for 85 dBA. The exposure time becomes shorter as the dBA level increases.

Within the field of dentistry, a variety of cleaning tools are used including piezoelectric and sonic scalers, and ultrasonic scalers and cleaners. While a majority of the tools do not exceed 75 dBA, prolonged exposure over many years can lead to hearing loss or complaints of tinnitus. Few dentists have reported using personal hearing protective devices, which could offset any potential hearing loss or tinnitus.

Evidence-based dentistry
There is a movement in modern dentistry to place a greater emphasis on high-quality scientific evidence in decision-making. Evidence-based dentistry (EBD) uses current scientific evidence to guide decisions. It is an approach to oral health that requires the application and examination of relevant scientific data related to the patient's oral and medical health. Along with the dentist's professional skill and expertise, EBD allows dentists to stay up to date on the latest procedures and patients to receive improved treatment. A new paradigm for medical education designed to incorporate current research into education and practice was developed to help practitioners provide the best care for their patients. It was first introduced by Gordon Guyatt and the Evidence-Based Medicine Working Group at McMaster University in Ontario, Canada in the 1990s. It is part of the larger movement toward evidence-based medicine and other evidence-based practices, especially since a major part of dentistry involves dealing with oral and systemic diseases. Other issues relevant to the dental field in terms of evidence-based research and evidence-based practice include population oral health, dental clinical practice, tooth morphology etc.

Ethical and medicolegal issues
Dentistry is unique in that it requires dental students to have competence-based clinical skills that can only be acquired through supervised specialized laboratory training and direct patient care. This necessitates the need for a scientific and professional basis of care with a foundation of extensive research-based education. According to some experts, the accreditation of dental schools can enhance the quality and professionalism of dental education.

title: Dentistry


text:
Diameter

[[File:Circle-withsegments.svg|thumb|right|Circle with

]]

In geometry, a diameter of a circle is any straight line segment that passes through the centre of the circle and whose endpoints lie on the circle. It can also be defined as the longest chord of the circle. Both definitions are also valid for the diameter of a sphere.

In more modern usage, the length $$d$$ of a diameter is also called the diameter. In this sense one speaks of diameter rather than  diameter (which refers to the line segment itself), because all diameters of a circle or sphere have the same length, this being twice the radius $$r.$$

 * $$d = 2r \qquad\text{or equivalently}\qquad r = \frac{d}{2}.$$

For a convex shape in the plane, the diameter is defined to be the largest distance that can be formed between two opposite parallel lines tangent to its boundary, and the is often defined to be the smallest such distance. Both quantities can be calculated efficiently using rotating calipers. For a curve of constant width such as the Reuleaux triangle, the width and diameter are the same because all such pairs of parallel tangent lines have the same distance.

For an ellipse, the standard terminology is different. A diameter of an ellipse is any chord passing through the centre of the ellipse. For example, conjugate diameters have the property that a tangent line to the ellipse at the endpoint of one diameter is parallel to the conjugate diameter. The longest diameter is called the major axis.

The word "diameter" is derived from διάμετρος (diametros), "diameter of a circle", from διά (dia), "across, through" and μέτρον (metron), "measure". It is often abbreviated $$\text{DIA}, \text{dia}, d,$$ or $$\varnothing.$$

Generalizations
The definitions given above are only valid for circles, spheres and convex shapes. However, they are special cases of a more general definition that is valid for any kind of $$n$$-dimensional (convex or non-convex) object, such as a hypercube or a set of scattered points. The or  of a subset of a metric space is the least upper bound of the set of all distances between pairs of points in the subset. Explicitly, if $$S$$ is the subset and if $$\rho$$ is the metric, the diameter is $$\operatorname{diam}(S) = \sup_{x, y \in S} \rho(x, y).$$

If the metric $$\rho$$ is viewed here as having codomain $$\R$$ (the set of all real numbers), this implies that the diameter of the empty set (the case $$S = \varnothing$$) equals $$- \infty$$ (negative infinity). Some authors prefer to treat the empty set as a special case, assigning it a diameter of $$0,$$ which corresponds to taking the codomain of $$\rho$$ to be the set of nonnegative reals.

For any solid object or set of scattered points in $$n$$-dimensional Euclidean space, the diameter of the object or set is the same as the diameter of its convex hull. In medical terminology concerning a lesion or in geology concerning a rock, the diameter of an object is the least upper bound of the set of all distances between pairs of points in the object.

In differential geometry, the diameter is an important global Riemannian invariant.

In planar geometry, a diameter of a conic section is typically defined as any chord which passes through the conic's centre; such diameters are not necessarily of uniform length, except in the case of the circle, which has eccentricity $$e = 0.$$

Symbol
The symbol or variable for diameter, ⌀, is sometimes used in technical drawings or specifications as a prefix or suffix for a number (e.g. "⌀ 55 mm"), indicating that it represents diameter. Photographic filter thread sizes are often denoted in this way.

The symbol has a Unicode code point at, in the Miscellaneous Technical set, and should not be confused with several other Unicode characters that resemble it but have unrelated meanings. It has the compose sequence Compose.

Diameter vs. radius
The diameter of a circle is exactly twice its radius. However, this is true only for a circle, and only in the Euclidean metric. Jung's theorem provides more general inequalities relating the diameter to the radius.

title: Diameter


text:
Direct examination

The  direct examination or examination-in-chief is one stage in the process of adducing evidence from witnesses in a court of law. Direct examination is the questioning of a witness by the lawyer/side/party that called such witness in a trial. Direct examination is usually performed to elicit evidence in support of facts which will satisfy a required element of a party's claim or defense.

In direct examination, one is generally prohibited from asking leading questions. This prevents a lawyer from feeding answers to a favorable witness. An exception to this rule occurs if one side has called a witness, but it is either understood or becomes clear, that the witness is hostile to the calling lawyer's side of the controversy, the lawyer who called the witness may then ask the court to declare the person on the stand a hostile witness. If the court does so, the lawyer may thereafter ask witness leading questions during direct examination.

The techniques of direct examination are taught in courses on trial advocacy. Each direct examination is integrated with the overall case strategy through either a theme and theory or, with more advanced strategies, a line of effort.

title: Direct examination