Metallurgy is a domain of materials science and engineering that studies the physical and chemical behavior of metallic elements, their inter-metallic compounds, their mixtures, which are called alloys. A special type of alloy was invented in 1995, when Taiwanese scientists invented the world's first high-entropy alloys of metals that can withstand the highest temperatures and pressures for use in industrial and technological applications such as state of the art race cars, submarines, nuclear reactors, jet airplanes, nuclear weapons, long range hypersonic missiles and many other areas of technology. Metallurgy is used to separate metals from their ore. Metallurgy is the technology of metals: the way in which science is applied to the production of metals, the engineering of metal components for usage in products for consumers and manufacturers; the production of metals involves the processing of ores to extract the metal they contain, the mixture of metals, sometimes with other elements, to produce alloys.
Metallurgy is distinguished from the craft of metalworking, although metalworking relies on metallurgy, as medicine relies on medical science, for technical advancement. The science of metallurgy is subdivided into physical metallurgy. Metallurgy is subdivided into ferrous metallurgy and non-ferrous metallurgy. Ferrous metallurgy involves processes and alloys based on iron while non-ferrous metallurgy involves processes and alloys based on other metals; the production of ferrous metals accounts for 95 percent of world metal production. The roots of metallurgy derive from Ancient Greek: μεταλλουργός, metallourgós, "worker in metal", from μέταλλον, métallon, "metal" + ἔργον, érgon, "work"; the word was an alchemist's term for the extraction of metals from minerals, the ending -urgy signifying a process manufacturing: it was discussed in this sense in the 1797 Encyclopædia Britannica. In the late 19th century it was extended to the more general scientific study of metals and related processes. In English, the pronunciation is the more common one in the Commonwealth.
The pronunciation is the more common one in the USA, is the first-listed variant in various American dictionaries. The earliest recorded metal employed by humans appears to be gold, which can be found free or "native". Small amounts of natural gold have been found in Spanish caves used during the late Paleolithic period, c. 40,000 BC. Silver, copper and meteoric iron can be found in native form, allowing a limited amount of metalworking in early cultures. Egyptian weapons made from meteoric iron in about 3000 BC were prized as "daggers from heaven". Certain metals, notably tin and copper, can be recovered from their ores by heating the rocks in a fire or blast furnace, a process known as smelting; the first evidence of this extractive metallurgy, dating from the 5th and 6th millennia BC, has been found at archaeological sites in Majdanpek and Plocnik, in present-day Serbia. To date, the earliest evidence of copper smelting is found at the Belovode site near Plocnik; this site produced a copper axe from 5500 BC.
The earliest use of lead is documented from the late neolithic settlement of Yarim Tepe in Iraq, "The earliest lead finds in the ancient Near East are a 6th millennium BC bangle from Yarim Tepe in northern Iraq and a later conical lead piece from Halaf period Arpachiyah, near Mosul. As native lead is rare, such artifacts raise the possibility that lead smelting may have begun before copper smelting." Copper smelting is documented at this site at about the same time period, although the use of lead seems to precede copper smelting. Early metallurgy is documented at the nearby site of Tell Maghzaliyah, which seems to be dated earlier, lacks pottery. Other signs of early metals are found from the third millennium BC in places like Palmela, Los Millares, Stonehenge. However, the ultimate beginnings cannot be ascertained and new discoveries are both continuous and ongoing. In the Near East, about 3500 BC, it was discovered that by combining copper and tin, a superior metal could be made, an alloy called bronze.
This represented a major technological shift known as the Bronze Age. The extraction of iron from its ore into a workable metal is much more difficult than for copper or tin; the process appears to have been invented by the Hittites in about 1200 BC. The secret of extracting and working iron was a key factor in the success of the Philistines. Historical developments in ferrous metallurgy can be found in a wide variety of past cultures and civilizations; this includes the ancient and medieval kingdoms and empires of the Middle East and Near East, ancient Iran, ancient Egypt, ancient Nubia, Anatolia, Ancient Nok, the Greeks and Romans of ancient Europe, medieval Europe and medieval China and medieval India and medieval Japan, amongst others. Many applications and devices associated or involved in metallurgy were established in ancient China, such as the innovation of the blast furnace, cast iron, hydraulic-powered trip hammers, double acting piston bellows. A 16th century book by Georg Agricola called De re metallica describes the developed and complex processes of mining metal ores, metal extraction and metallurgy of the time.
Agricola has been described as the "father of metallurgy". Extractive metallurgy is the practice of removing valuable metals from an ore and refining the extracted
The British Museum, in the Bloomsbury area of London, United Kingdom, is a public institution dedicated to human history and culture. Its permanent collection of some eight million works is among the largest and most comprehensive in existence, having been sourced during the era of the British Empire, it documents the story of human culture from its beginnings to the present. It was the first public national museum in the world; the British Museum was established in 1753 based on the collections of the Irish physician and scientist Sir Hans Sloane. It first opened in Montagu House, on the site of the current building, its expansion over the following 250 years was a result of expanding British colonisation and has resulted in the creation of several branch institutions, the first being the Natural History Museum in 1881. In 1973, the British Library Act 1972 detached the library department from the British Museum, but it continued to host the now separated British Library in the same Reading Room and building as the museum until 1997.
The museum is a non-departmental public body sponsored by the Department for Digital, Culture and Sport, as with all national museums in the UK it charges no admission fee, except for loan exhibitions. Its ownership of some of its most famous objects originating in other countries is disputed and remains the subject of international controversy, most notably in the case of the Parthenon Marbles. Although today principally a museum of cultural art objects and antiquities, the British Museum was founded as a "universal museum", its foundations lie in the will of the Irish physician and naturalist Sir Hans Sloane, a London-based doctor and scientist from Ulster. During the course of his lifetime, after he married the widow of a wealthy Jamaican planter, Sloane gathered a large collection of curiosities and, not wishing to see his collection broken up after death, he bequeathed it to King George II, for the nation, for a sum of £20,000. At that time, Sloane's collection consisted of around 71,000 objects of all kinds including some 40,000 printed books, 7,000 manuscripts, extensive natural history specimens including 337 volumes of dried plants and drawings including those by Albrecht Dürer and antiquities from Sudan, Greece, the Ancient Near and Far East and the Americas.
On 7 June 1753, King George II gave his Royal Assent to the Act of Parliament which established the British Museum. The British Museum Act 1753 added two other libraries to the Sloane collection, namely the Cottonian Library, assembled by Sir Robert Cotton, dating back to Elizabethan times, the Harleian Library, the collection of the Earls of Oxford, they were joined in 1757 by the "Old Royal Library", now the Royal manuscripts, assembled by various British monarchs. Together these four "foundation collections" included many of the most treasured books now in the British Library including the Lindisfarne Gospels and the sole surviving manuscript of Beowulf; the British Museum was the first of a new kind of museum – national, belonging to neither church nor king open to the public and aiming to collect everything. Sloane's collection, while including a vast miscellany of objects, tended to reflect his scientific interests; the addition of the Cotton and Harley manuscripts introduced a literary and antiquarian element and meant that the British Museum now became both National Museum and library.
The body of trustees decided on a converted 17th-century mansion, Montagu House, as a location for the museum, which it bought from the Montagu family for £20,000. The trustees rejected Buckingham House, on the site now occupied by Buckingham Palace, on the grounds of cost and the unsuitability of its location. With the acquisition of Montagu House, the first exhibition galleries and reading room for scholars opened on 15 January 1759. At this time, the largest parts of collection were the library, which took up the majority of the rooms on the ground floor of Montagu House and the natural history objects, which took up an entire wing on the second state storey of the building. In 1763, the trustees of the British Museum, under the influence of Peter Collinson and William Watson, employed the former student of Carl Linnaeus, Daniel Solander to reclassify the natural history collection according to the Linnaean system, thereby making the Museum a public centre of learning accessible to the full range of European natural historians.
In 1823, King George IV gave the King's Library assembled by George III, Parliament gave the right to a copy of every book published in the country, thereby ensuring that the museum's library would expand indefinitely. During the few years after its foundation the British Museum received several further gifts, including the Thomason Collection of Civil War Tracts and David Garrick's library of 1,000 printed plays; the predominance of natural history and manuscripts began to lessen when in 1772 the museum acquired for £8,410 its first significant antiquities in Sir William Hamilton's "first" collection of Greek vases. From 1778, a display of objects from the South Seas brought back from the round-the-world voyages of Captain James Cook and the travels of other explorers fascinated visitors with a glimpse of unknown lands; the bequest of a collection of books, engraved gems, coins and drawings by Clayton Mordaunt Cracherode in 1800 did much to raise the museum's reputation. The museum's first notable addition towards its collection of antiquities, since its foundation, was by Sir William Hamilton, British Ambassador to Naples, who sold his collection of Greek and Roman artefacts to
Natural resources are resources that exist without actions of humankind. This includes all valued characteristics such as magnetic, electrical properties and forces etc. On earth it includes: sunlight, water, land along with all vegetation and animal life that subsists upon or within the heretofore identified characteristics and substances. Particular areas such as the rainforest in Fatu-Hiva are characterized by the biodiversity and geodiversity existent in their ecosystems. Natural resources may be further classified in different ways. Natural resources are components that can be found within the environment; every man-made product is composed of natural resources. A natural resource may exist as a separate entity such as fresh water, as well as a living organism such as a fish, or it may exist in an alternate form that must be processed to obtain the resource such as metal ores, rare earth metals and most forms of energy. There is much debate worldwide over natural resource allocations, this is true during periods of increasing scarcity and shortages.
There are various methods of categorizing natural resources, these include source of origin, stage of development, by their renewability. On the basis of origin, natural resources may be divided into two types: Biotic — Biotic resources are obtained from the biosphere, such as forests and animals, the materials that can be obtained from them. Fossil fuels such as coal and petroleum are included in this category because they are formed from decayed organic matter. Abiotic – Abiotic resources are those that come from non-living, non-organic material. Examples of abiotic resources include land, fresh water, rare earth metals and heavy metals including ores such as gold, copper, etc. Considering their stage of development, natural resources may be referred to in the following ways: Potential resources — Potential resources are those that may be used in the future—for example, petroleum in sedimentary rocks that, until drilled out and put to use remains a potential resource Actual resources — Those resources that have been surveyed and qualified and, are used—development, such as wood processing, depends on technology and cost Reserve resources — The part of an actual resource that can be developed profitably in the future Stock resources — Those that have been surveyed, but cannot be used due to lack of technology—for example, hydrogenMany natural resources can be categorized as either renewable or non-renewable: Renewable resources — Renewable resources can be replenished naturally.
Some of these resources, like sunlight, wind, etc, are continuously available and their quantity is not noticeably affected by human consumption. Though many renewable resources do not have such a rapid recovery rate, these resources are susceptible to depletion by over-use. Resources from a human use perspective are classified as renewable so long as the rate of replenishment/recovery exceeds that of the rate of consumption, they replenish compared to Non-renewable resources. Non-renewable resources – Non-renewable resources either form or do not form in the environment. Minerals are the most common resource included in this category. By the human perspective, resources are non-renewable when their rate of consumption exceeds the rate of replenishment/recovery; some resources naturally deplete in amount without human interference, the most notable of these being radio-active elements such as uranium, which decay into heavy metals. Of these, the metallic minerals can be re-used by recycling them, but coal and petroleum cannot be recycled.
Once they are used they take millions of years to replenish. Resource extraction involves any activity; this can range in scale to global industry. Extractive industries are, along with agriculture, the basis of the primary sector of the economy. Extraction produces raw material, processed to add value. Examples of extractive industries are hunting, mining and gas drilling, forestry. Natural resources can add substantial amounts to a country's wealth, however, a sudden inflow of money caused by a resource boom can create social problems including inflation harming other industries and corruption, leading to inequality and underdevelopment, this is known as the "resource curse". Extractive industries represent a large growing activity in many less-developed countries but the wealth generated does not always lead to sustainable and inclusive growth. People accuse extractive industry businesses as acting only to maximize short-term value, implying that less-developed countries are vulnerable to powerful corporations.
Alternatively, host governments are assumed to be only maximizing immediate revenue. Researchers argue; these present opportunities for international governmental agencies to engage with the private sector and host governments through revenue management and expenditure accountability, infrastructure development, employment creation and enterprise development and impacts on children girls and women. A strong civil society can play an important role in ensuring effective management of natural resources. Norway can ser
Agriculture in Chile
Agriculture in Chile encompasses a wide range of different activities due to its particular geography, climate and human factors. Agriculture is one of the bases of Chile's economy, now agriculture and allied sectors—like forestry and fishing—account only for 4.9% of the GDP as of 2007 and employed 13.6% of the country's labor force. Some major agricultural products of Chile include grapes, onions, corn, peaches, asparagus, beef, wool and timber. Due to its geographical isolation and strict customs policies, Chile is free from diseases such as Mad Cow, fruit fly and Phylloxera, this plus being located in the southern hemisphere and its wide range of agriculture conditions are considered Chile’s main comparative advantages. However, the mountainous landscape of Chile limits the extent and intensity of agriculture so that arable land corresponds only to 2.62% of the total territory. Chile’s principal growing region and agricultural heartland is the Central Valley bounde by the Chilean Coast Range to the west, the Andes to the east Aconcagua River to the north and Bío-Bío River to the south.
In the northern half of Chile, cultivation is dependent on irrigation. South of the Central Valley, cultivation is replaced by aquaculture, silviculture and cattle farming; the Norte Grande region covers most of Atacama Desert and its crop agriculture is contracted around small oases, such as Pica and Pozo Almonte. The use of water from Loa River and other streams is limited due to contamination with heavy metals from the mining industry; some minor subsistence agriculture is practised in Loa River. These waters are being used for growing non-edible plants, such as flowers. In Norte Grande's Puna plateau, indigenous Aymaras practise traditional alpaca herding. Norte Chico has a semi-arid climate. Most agriculture here is dependent on irrigation. Norte Chico papayas. Other crops cultivated include avocados. Zona Central is Chile's cultural heartland. With its mediterranean climate, Zona Central hosts the largest part of the country's wine regions. Most of Chile's premium wine regions depend on irrigation to sustain vineyards, the necessary water formed by melting snow caps in the Andes.
In the developing wine regions along the Coastal Ranges and in the far south, there is not a lack of needed rainfall but vineyards owners have to deal with other factors, such as the Humboldt Current from the Pacific which can bathe a vineyard with a blanket of cool air. For the rest of Chile's wine regions, the Coastal Ranges serve as a buffer from the current and act as a rain shadow; the vineyards in these regions are planted on the valley plains of the Andes foothills along the major rivers such as the Maipo and Maule Rivers. Wheat cultivation, cattle farming and salmon aquaculture are the main agricultural activities of Zona Sur. In northern Zona Sur Araucania Region was known until as "Chile’s granary". Wheat in Araucanía continues to be the main crop. While the area devoted to traditional crops and natural pasture has tended to shrink in Araucanía, there has been an increase in artificial and improved pasturelands; this supports a substantial mass of livestock, including beef cattle, swine and horses, abundant production of milk and by-products.
Salmon aquaculture is Los Lagos Region largest source of main export industry. Nearly all of Chile’s, much of the southern hemisphere and cranberry production is concentrated in Zona Sur. In Zona Austral, sheepherding has been a major industry since the 19th century. In the Magallanes Region alone, there is an estimate of 1.5 and 2 million sheep, distributed among 300 farmers as of 2006. Exotic species, like reindeer, have been introduced in recent years. Agriculture in the north-western parts of Zona Austral focuses on aquaculture and silviculture and is similar to that of Zona Sur. Evidence ranging from historical records, local agriculturalists, DNA analyses supports the hypothesis that the most cultivated variety of potato worldwide, Solanum tuberosum tuberosum, is indigenous to Chiloé Island and has been cultivated by the local indigenous people since before the coming of the Spanish. Agriculture is practised in Chile since pre-Hispanic times when indigenous Mapuches and Atacameños raised llama and alpaca livestock and cultivated beans, maize and quinoa.
With the arrival of the Spaniards around 1550 European plants and animals such as grapes, wheat and cattle were introduced to Chile. Due to the geographical isolation of Chile and the commerce limitations imposed by the Spanish crown few new species were introduced to colonial Chile after its conquest. Nearly all of Chile's exports and imports went by Lima in Peru, a rich colony with little need of commerce with Chile; the 17th century came to be known as the century of the fat, because Chile’s agricultural exports to Peru consisted of animal fat and charqui. Since Peru's croplands were affected by an earthquake and a plague in 1687, Chile became a major exporter of wheat, first to Peru but in the 19th century to Australia and California; the wheat cultivation increased further in the 1880s, fueled by the new arable land available after the occupation of Araucanía. The wheat period ended in poor conditions coupled with decades
California is a state in the Pacific Region of the United States. With 39.6 million residents, California is the most populous U. S. the third-largest by area. The state capital is Sacramento; the Greater Los Angeles Area and the San Francisco Bay Area are the nation's second and fifth most populous urban regions, with 18.7 million and 9.7 million residents respectively. Los Angeles is California's most populous city, the country's second most populous, after New York City. California has the nation's most populous county, Los Angeles County, its largest county by area, San Bernardino County; the City and County of San Francisco is both the country's second-most densely populated major city after New York City and the fifth-most densely populated county, behind only four of the five New York City boroughs. California's $3.0 trillion economy is larger than that of any other state, larger than those of Texas and Florida combined, the largest sub-national economy in the world. If it were a country, California would be the 5th largest economy in the world, the 36th most populous as of 2017.
The Greater Los Angeles Area and the San Francisco Bay Area are the nation's second- and third-largest urban economies, after the New York metropolitan area. The San Francisco Bay Area PSA had the nation's highest GDP per capita in 2017 among large PSAs, is home to three of the world's ten largest companies by market capitalization and four of the world's ten richest people. California is considered a global trendsetter in popular culture, innovation and politics, it is considered the origin of the American film industry, the hippie counterculture, fast food, the Internet, the personal computer, among others. The San Francisco Bay Area and the Greater Los Angeles Area are seen as global centers of the technology and entertainment industries, respectively. California has a diverse economy: 58% of the state's economy is centered on finance, real estate services and professional, scientific and technical business services. Although it accounts for only 1.5% of the state's economy, California's agriculture industry has the highest output of any U.
S. state. California is bordered by Oregon to the north and Arizona to the east, the Mexican state of Baja California to the south; the state's diverse geography ranges from the Pacific Coast in the west to the Sierra Nevada mountain range in the east, from the redwood–Douglas fir forests in the northwest to the Mojave Desert in the southeast. The Central Valley, a major agricultural area, dominates the state's center. Although California is well-known for its warm Mediterranean climate, the large size of the state results in climates that vary from moist temperate rainforest in the north to arid desert in the interior, as well as snowy alpine in the mountains. Over time and wildfires have become more pervasive features. What is now California was first settled by various Native Californian tribes before being explored by a number of European expeditions during the 16th and 17th centuries; the Spanish Empire claimed it as part of Alta California in their New Spain colony. The area became a part of Mexico in 1821 following its successful war for independence but was ceded to the United States in 1848 after the Mexican–American War.
The western portion of Alta California was organized and admitted as the 31st state on September 9, 1850. The California Gold Rush starting in 1848 led to dramatic social and demographic changes, with large-scale emigration from the east and abroad with an accompanying economic boom; the word California referred to the Baja California Peninsula of Mexico. The name derived from the mythical island California in the fictional story of Queen Calafia, as recorded in a 1510 work The Adventures of Esplandián by Garci Rodríguez de Montalvo; this work was the fifth in a popular Spanish chivalric romance series that began with Amadis de Gaula. Queen Calafia's kingdom was said to be a remote land rich in gold and pearls, inhabited by beautiful black women who wore gold armor and lived like Amazons, as well as griffins and other strange beasts. In the fictional paradise, the ruler Queen Calafia fought alongside Muslims and her name may have been chosen to echo the title of a Muslim leader, the Caliph. It's possible.
Know ye that at the right hand of the Indies there is an island called California close to that part of the Terrestrial Paradise, inhabited by black women without a single man among them, they lived in the manner of Amazons. They were robust of body with great virtue; the island itself is one of the wildest in the world on account of the craggy rocks. Shortened forms of the state's name include CA, Cal. Calif. and US-CA. Settled by successive waves of arrivals during the last 10,000 years, California was one of the most culturally and linguistically diverse areas in pre-Columbian North America. Various estimates of the native population range from 100,000 to 300,000; the Indigenous peoples of California included more than 70 distinct groups of Native Americans, ranging from large, settled populations living on the coast to groups in the interior. California groups were diverse in their political organization with bands, villages, on the resource-rich coasts, large chiefdoms, such as the Chumash and Salinan.
Trade, intermarriage a
Central Europe is the region comprising the central part of Europe. It is said to occupy continuous territory that are otherwise conventionally Western Europe, Southern Europe, Eastern Europe; the concept of Central Europe is based on a common historical and cultural identity. Central Europe is going through a phase of "strategic awakening", with initiatives such as the CEI, Centrope and the Visegrád Four. While the region's economy shows high disparities with regard to income, all Central European countries are listed by the Human Development Index as highly developed. Elements of unity for Western and Central Europe were Latin; however Eastern Europe, which remained Eastern Orthodox, was the area of Graeco-Byzantine cultural influence. According to Hungarian historian Jenő Szűcs, foundations of Central European history at the first millennium were in close connection with Western European development, he explained that between the 11th and 15th centuries not only Christianization and its cultural consequences were implemented, but well-defined social features emerged in Central Europe based on Western characteristics.
The keyword of Western social development after millennium was the spread of liberties and autonomies in Western Europe. These phenomena appeared in the middle of the 13th century in Central European countries. There were self-governments of towns and parliaments. In 1335, under the rule of the King Charles I of Hungary, the castle of Visegrád, the seat of the Hungarian monarchs was the scene of the royal summit of the Kings of Poland and Hungary, they agreed to cooperate in the field of politics and commerce, inspiring their post-Cold War successors to launch a successful Central European initiative. In the Middle Ages, countries in Central Europe adopted Magdeburg rights. Before 1870, the industrialization that had developed in Western and Central Europe and the United States did not extend in any significant way to the rest of the world. In Eastern Europe, industrialization lagged far behind. Russia, for example, remained rural and agricultural, its autocratic rulers kept the peasants in serfdom.
The concept of Central Europe was known at the beginning of the 19th century, but its real life began in the 20th century and became an object of intensive interest. However, the first concept mixed science and economy – it was connected with intensively growing German economy and its aspirations to dominate a part of European continent called Mitteleuropa; the German term denoting Central Europe was so fashionable that other languages started referring to it when indicating territories from Rhine to Vistula, or Dnieper, from the Baltic Sea to the Balkans. An example of that-time vision of Central Europe may be seen in J. Partsch's book of 1903. On 21 January 1904, Mitteleuropäischer Wirtschaftsverein was established in Berlin with economic integration of Germany and Austria–Hungary as its main aim. Another time, the term Central Europe became connected to the German plans of political and cultural domination; the "bible" of the concept was Friedrich Naumann's book Mitteleuropa in which he called for an economic federation to be established after the war.
Naumann's idea was that the federation would have at its centre Germany and the Austro-Hungarian Empire but would include all European nations outside the Anglo-French alliance, on one side, Russia, on the other. The concept failed after the German defeat in the dissolution of Austria -- Hungary; the revival of the idea may be observed during the Hitler era. According to Emmanuel de Martonne, in 1927 the Central European countries included: Austria, Germany, Poland and Switzerland; the author use both Human and Physical Geographical features to define Central Europe, but he doesn't care about the legal development, the social, economic, infrastructural developments in these countries. The interwar period brought new geopolitical system and economic and political problems, the concept of Central Europe took a different character; the centre of interest was moved to its eastern part – the countries that have appeared on the map of Europe: Czechoslovakia and Poland. Central Europe ceased to be the area of German aspiration to lead or dominate and became a territory of various integration movements aiming at resolving political and national problems of "new" states, being a way to face German and Soviet pressures.
However, the conflict of interests was too big and neither Little Entente nor Intermarium ideas succeeded. The interwar period brought new elements to the concept of Central Europe. Before World War I, it embraced German states, non-German territories being an area of intended German penetration and domination – German leadership position was to be the natural result of economic dominance. After the war, the Eastern part of Central Europe was placed at the centre of the concept. At that time the scientists took an interest in the idea: the International Historical Congress in Brussels in 1923 was committed to Central Europe, the 1933 Congress continued the discussions. Hungarian scholar Magda Adam wrote in her study Versailles System and Central Europe: "Today we know that the bane of Central Europe was the Little Entente, military alliance of Czechoslovakia and Kingdom of Serbs and Slovenes (later Yu
The Americas comprise the totality of the continents of North and South America. Together, they comprise the New World. Along with their associated islands, they cover 8% of Earth's total surface area and 28.4% of its land area. The topography is dominated by the American Cordillera, a long chain of mountains that runs the length of the west coast; the flatter eastern side of the Americas is dominated by large river basins, such as the Amazon, St. Lawrence River / Great Lakes basin, La Plata. Since the Americas extend 14,000 km from north to south, the climate and ecology vary from the arctic tundra of Northern Canada and Alaska, to the tropical rain forests in Central America and South America. Humans first settled the Americas from Asia between 17,000 years ago. A second migration of Na-Dene speakers followed from Asia; the subsequent migration of the Inuit into the neoarctic around 3500 BCE completed what is regarded as the settlement by the indigenous peoples of the Americas. The first known European settlement in the Americas was by the Norse explorer Leif Erikson.
However, the colonization never became permanent and was abandoned. The Spanish voyages of Christopher Columbus from 1492 to 1502 resulted in permanent contact with European powers, which led to the Columbian exchange and inaugurated a period of exploration and colonization whose effects and consequences persist to the present. Diseases introduced from Europe and West Africa devastated the indigenous peoples, the European powers colonized the Americas. Mass emigration from Europe, including large numbers of indentured servants, importation of African slaves replaced the indigenous peoples. Decolonization of the Americas began with the American Revolution in the 1770s and ended with the Spanish–American War in the late 1890s. All of the population of the Americas resides in independent countries; the Americas are home to over a billion inhabitants, two-thirds of which reside in the United States, Brazil, or Mexico. It is home to eight megacities: New York City, Mexico City, São Paulo, Los Angeles, Buenos Aires, Rio de Janeiro, Bogotá, Lima.
The name America was first recorded in 1507. Christie's auction house says a two-dimensional globe created by Martin Waldseemüller was the earliest recorded use of the term; the name was used in the Cosmographiae Introductio written by Matthias Ringmann, in reference to South America. It was applied to both North and South America by Gerardus Mercator in 1538. America derives from the Latin version of Italian explorer Amerigo Vespucci's first name; the feminine form America accorded with the feminine names of Asia and Europa. In modern English and South America are considered separate continents, taken together are called America or the Americas in the plural; when conceived as a unitary continent, the form is the continent of America in the singular. However, without a clarifying context, singular America in English refers to the United States of America. In the English-speaking world, the term America used to refer to a single continent until the 1950s: According to historians Kären Wigen and Martin W. Lewis, While it might seem surprising to find North and South America still joined into a single continent in a book published in the United States in 1937, such a notion remained common until World War II.
By the 1950s, however all American geographers had come to insist that the visually distinct landmasses of North and South America deserved separate designations. This shift did not seem to happen in Romance-speaking countries, where America is still considered a continent encompassing the North America and South America subcontinents, as well as Central America; the first inhabitants migrated into the Americas from Asia. Habitation sites are known in Alaska and the Yukon from at least 20,000 years ago, with suggested ages of up to 40,000 years. Beyond that, the specifics of the Paleo-Indian migration to and throughout the Americas, including the dates and routes traveled, are subject to ongoing research and discussion. Widespread habitation of the Americas occurred during the late glacial maximum, from 16,000 to 13,000 years ago; the traditional theory has been that these early migrants moved into the Beringia land bridge between eastern Siberia and present-day Alaska around 40,000–17,000 years ago, when sea levels were lowered during the Quaternary glaciation.
These people are believed to have followed herds of now-extinct pleistocene megafauna along ice-free corridors that stretched between the Laurentide and Cordilleran ice sheets. Another route proposed is that, either on foot or using primitive boats, they migrated down the Pacific coast to South America. Evidence of the latter would since have been covered by a sea level rise of hundreds of meters following the last ice age. Both routes may have