satellite, natural, celestial body orbiting a planet, dwarf planet, asteroid, or star of a larger size. The most familiar natural satellite is the earth's moon; thus, satellites of other planets are often referred to as moons. Within the solar system the earth's moon is the largest satellite in relation to its planet and Charon is even larger relative to the dwarf planet Pluto, although neither is the largest in actual size. The largest natural satellite, Jupiter's Ganymede, is 3,268 mi (5,262 km) in diameter, and it and Saturn's Titan are both larger than the planet Mercury. In comparison, some satellites are quite small, e.g., Deimos, the outer satellite of Mars, is c.4 mi (6 km) in diameter. Neither of the inferior planets, Mercury or Venus, has a known satellite; all of the superior planets (those whose orbits lie beyond the orbit of the earth) have at least two known satellites (Mars, 2; Jupiter, 63; Saturn, 48; Uranus, 27; Neptune, 13). A number of satellites, e.g., Phoebe of Saturn, Triton of Neptune, and most of the small outer moons of Jupiter and Uranus, have retrograde motion and may be asteroids that were captured by the planet's gravitation. The asteroid Ida has a tiny moon, Dactyl, that is about a mile (1.6 km) in diameter and orbits about 60 mi (97 km) above Ida's surface.

natural selection: see selection.

natural rights, political theory that maintains that an individual enters into society with certain basic rights and that no government can deny these rights. The modern idea of natural rights grew out of the ancient and medieval doctrines of natural law, i.e., the belief that people, as creatures of nature and God, should live their lives and organize their society on the basis of rules and precepts laid down by nature or God. With the growth of the idea of individualism, especially in the 17th cent., natural law doctrines were modified to stress the fact that individuals, because they are natural beings, have rights that cannot be violated by anyone or by any society. Perhaps the most famous formulation of this doctrine is found in the writings of John Locke. Locke assumed that humans were by nature rational and good, and that they carried into political society the same rights they had enjoyed in earlier stages of society, foremost among them being freedom of worship, the right to a voice in their own government, and the right of property. Jean Jacques Rousseau attempted to reconcile the natural rights of the individual with the need for social unity and cooperation through the idea of the social contract. The most important elaboration of the idea of natural rights came in the North American colonies, however, where the writings of Thomas Jefferson, Samuel Adams, and Thomas Paine made of the natural rights theory a powerful justification for revolution. The classic expressions of natural rights are the English Bill of Rights (1689), the American Declaration of Independence (1776), the French Declaration of the Rights of Man and the Citizen (1789), the first 10 amendments to the Constitution of the United States (known as the Bill of Rights, 1791), and the Universal Declaration of Human Rights of the United Nations (1948).

natural resources, conservation of: see conservation of natural resources.

natural law, theory that some laws are basic and fundamental to human nature and are discoverable by human reason without reference to specific legislative enactments or judicial decisions. Natural law is opposed to positive law, which is human-made, conditioned by history, and subject to continuous change. The concept of natural law originated with the Greeks and received its most important formulation in Stoicism. The Stoics believed that the fundamental moral principles that underlie all the legal systems of different nations were reducible to the dictates of natural law. This idea became particularly important in Roman legal theory, which eventually came to recognize a common code regulating the conduct of all peoples and existing alongside the individual codes of specific places and times (see natural rights). Christian philosophers such as St. Thomas Aquinas perpetuated this idea, asserting that natural law was common to all peoples—Christian and non-Christian alike—while adding that revealed law gave Christians an additional guide for their actions. In modern times, the theory of natural law became the chief basis for the development by Hugo Grotius of the theory of international law. In the 17th cent., such philosophers as Spinoza and G. W. von Leibniz interpreted natural law as the basis of ethics and morality; in the 18th cent. the teachings of Jean Jacques Rousseau, especially as interpreted during the French Revolution, made natural law a basis for democratic and egalitarian principles. The influence of natural law theory declined greatly in the 19th cent. under the impact of positivism, empiricism, and materialism. In the 20th cent., such thinkers as Jacques Maritain saw in natural law a necessary intellectual opposition to totalitarian theories.

natural gas, natural mixture of gaseous hydrocarbons found issuing from the ground or obtained from specially driven wells. The composition of natural gas varies in different localities. Its chief component, methane, usually makes up from 80% to 95%, and the balance is composed of varying amounts of ethane, propane, butane, and other hydrocarbon compounds. Some of the hydrocarbons found in gasoline also occur as vapors in natural gas; by liquefying these hydrocarbons, gasoline can be obtained.

Although commonly associated with petroleum deposits it also occurs separately in sand, sandstone, and limestone deposits. Some geologists theorize that natural gas is a byproduct of decaying vegetable matter in underground strata, while others think it may be primordial gases that rise up from the mantle. Because of its flammability and high calorific value, natural gas is used extensively as an illuminant and a fuel.

Natural gas was known to the ancients but was considered by them to be a supernatural phenomenon because, noticed only when ignited, it appeared as a mysterious fire bursting from the ground. One of the earliest attempts to harness it for economic use occurred in the early 19th cent. in Fredonia, N.Y. Toward the latter part of the 19th cent., large industrial cities began to make use of natural gas, and extensive pipeline systems have been constructed to transport gas.

Liquefied natural gas, or LNG, is natural gas that has been pressurized and cooled so as to liquefy it for convenience in shipping and storage. The boiling point of natural gas is extremely low, and only in the 1970s did cryogenic technology (see low-temperature physics) advance enough to make the production and transport of LNG commerically feasible. Some of the natural gas moved to and from the United States is carried as LNG in special tankers.

natural childbirth: see birth.

liquefied natural gas: see under natural gas.

conservation of natural resources, the wise use of the earth's resources by humanity. The term conservation came into use in the late 19th cent. and referred to the management, mainly for economic reasons, of such valuable natural resources as timber, fish, game, topsoil, pastureland, and minerals, and also to the preservation of forests (see forestry), wildlife (see wildlife refuge), parkland, wilderness, and watershed areas. In recent years the science of ecology has clarified the workings of the biosphere; i.e., the complex interrelationships among humans, other animals, plants, and the physical environment. At the same time burgeoning population and industry and the ensuing pollution have demonstrated how easily delicately balanced ecological relationships can be disrupted (see air pollution; water pollution; solid waste).

Conservation of natural resources is now usually embraced in the broader conception of conserving the earth itself by protecting its capacity for self-renewal. Particularly complex are the problems of nonrenewable resources such as oil and coal (see energy, sources of) and other minerals in great demand. Current thinking also favors the protection of entire ecological regions by the creation of "biosphere reserves." Examples of such conservation areas include the Great Barrier Reef off Australia and Adirondack State Park in the United States. The importance of reconciling human use and conservation beyond the boundaries of parks has become another important issue.

Conservation in the United States

Conservation became part of U.S. government policy with the creation (1871) of a U.S. commissioner of fish and fisheries. The Forestry Bureau of the Dept. of Agriculture created the first national forest reserve in 1891. The Irrigation Division in the U.S. Geological Survey developed into the Bureau of Reclamation. The Geological Survey has cataloged and classified the resources of the public domain. In 1906 an act protected the Alaskan fisheries. Conservation as part of a total approach to the use of natural resources was first introduced by President Theodore Roosevelt and his chief forester, Gifford Pinchot. In 1907 President Roosevelt appointed the Inland Waterways Commission, which emphasized the connection between forests, water supply, and stream flow. In 1909 he appointed the National Conservation Commission, which published the first inventory of the country's natural resources. Roosevelt in 1907 began withdrawing large areas of western public land from sale and settlement so that their resources might be investigated, and setting apart forest reserves, following the example of President Cleveland. Approximately one fourth of all timberland is held by the government. The National Park Service was created in 1916 to preserve landscapes of important aesthetic value. In the 1930s the erosion of much arable land in the Midwest underscored the need for land reclamation and for conservation in general. The National Industrial Recovery Act of 1933 provided for conservation. The Civilian Conservation Corps, founded in 1933 to relieve unemployment, furnished the personnel for many conservation projects. The Tennessee Valley Authority, set up in 1933, was an outstanding attempt to apply principles of conservation, soil reclamation, and electrification to an entire area, although some critics claim that the extensive river damming and similar New Deal legislation did not, on the whole, have a positive effect on the environment. By 1960 the Soil Conservation Service, established in 1935, covered 95% of all farms and ranches in the United States. By the same year, under the Conservation Reserve Program, some 28 million acres of cropland had been returned to grass and forest cover. Throughout the 1950s attention was focused on the problem of conservation of water resources, particularly in the Southwest. In the 1960s pollution problems came to the fore in all industrialized countries. In the United States numerous laws were passed to protect the environment and its resources (see environmentalism).

Conservation Worldwide

The commitment of nations to conservation policies varies. Some nations, such as Iraq, Cambodia, and the republics of the former Soviet Union, have no protected areas, while 38% of Ecuador's land is protected and 44% of Luxembourg's is. (In the United States 7% of the land is protected.) Plants and animals have been protected through curtailment of whaling and the taking of porpoises in tuna seines and restrictions on logging. Endangered species have been protected by the Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES, 1979). In addition to CITES, United Nations Conference on Environment and Development (the "Earth Summit," 1992) produced an agreement to protect the world's biological diversity. The World Wildlife Fund, Greenpeace, and other organizations also have been active in promoting conservation internationally.

Natural History, American Museum of: see American Museum of Natural History.

Natural Bridges National Monument, 7,636 acres (3,093 hectares), SE Utah; est. 1908. Located in an area of colored cliffs and box canyons, the monument contains three huge natural sandstone bridges: Owachomo (also called Rock Mound), 106 ft (32 m) high with a span of 180 ft (55 m); Kachina, 210 ft (64 m) high with a span of 206 ft (63 m); and Sipapu, 220 ft (67 m) high with a span of 268 ft (82 m). See National Parks and Monuments, table.

Natural Bridge, small village, Rockbridge co., W Va., in the Shenandoah valley; founded 1774. Nearby is the famous Natural Bridge over the gorge of Cedar Creek. It is a limestone arch 215 ft (66 m) high with a span of 90 ft (27 m) and was once owned by Thomas Jefferson, who built a cabin there for visitors and kept a guest book. A public highway now crosses the bridge.

International Union for the Conservation of Nature and Natural Resources (IUCN) or World Conservation Union, international organization founded in 1948 to encourage the preservation of wildlife, natural environments, and living resources. Its members include private individuals, nongovernmental organizations and governments. The IUCN promotes research in the preservation of threatened species, ecology, sustainable development, and environmental law, education, and training.

Field Museum of Natural History, at Chicago, Ill. Founded in 1893 through the gifts of Marshall Field and others, it was first known as the Columbian Museum of Chicago and later (1943-66) as the Chicago Natural History Museum. It is especially noted for its lifelike exhibits of animals in their natural settings, displays of plant life, and anthropological and geological collections. The many expeditions sponsored by the museum have contributed much to its collections. The museum also conducts research and maintains a publishing plant. Many educational opportunities are offered by its library, school-service department, and special programs.

Chicago Natural History Museum: see Field Museum of Natural History.

American Museum of Natural History, incorporated in New York City in 1869 to promote the study of natural science and related subjects. Buildings on its present site were opened in 1877. Among the buildings later added were the Hayden Planetarium (opened 1935) and the Roosevelt Memorial building (completed 1936). In 2000 the museum opened the Rose Center for Earth and Space, which contains a new, state-of-the-art Hayden Planetarium. The museum maintains exhibitions in all branches of natural history, including anthropology and ecology. As a result of its wide explorations and research programs, it has acquired specimens and data of great value. Resources are derived from endowment, grants from the city, and a membership fund. Among the facilities for study are an extension library; illustrated lectures; publications; programs for young people; a special school service whereby the museum cooperates with city schools; circulating exhibits; habitat groups of animals and plants; a mineral and gem collection; an unrivaled assemblage of skeletons of extinct animals, especially dinosaurs; and replicas of invertebrates in glass. In 1995 the museum opened its extensively renovated dinosaur halls, making it the world's largest exhibit of its kind.

Process that results in adaptation of an organism to its environment by means of selectively reproducing changes in its genotype. Variations that increase an organism's chances of survival and procreation are preserved and multiplied from generation to generation at the expense of less advantageous variations. As proposed by Charles Darwin, natural selection is the mechanism by which evolution occurs. It may arise from differences in survival, fertility, rate of development, mating success, or any other aspect of the life cycle. Mutation, gene flow, and genetic drift, all of which are random processes, also alter gene abundance. Natural selection moderates the effects of these processes because it multiplies the incidence of beneficial mutations over generations and eliminates harmful ones, since the organisms that carry them leave few or no descendants. Seealso selection.

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Principle of physics according to which the energy of interacting bodies or particles in a closed system remains constant, though it may take different forms (e.g., kinetic energy, potential energy, thermal energy, energy in an electric current, or energy stored in an electric field, in a magnetic field, or in chemical bonds [see bonding]). With the advent of relativity physics in 1905, mass was recognized as equivalent to energy. When accounting for a system of high-speed particles whose mass increases as a consequence of their speed, the laws of conservation of energy and conservation of mass become one conservation law. Seealso Hermann von Helmholtz.

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or law of conservation

In physics, the principle that certain quantities within an isolated system do not change over time. When a substance in an isolated system changes phase, the total amount of mass does not change. When energy is changed from one form to another in an isolated system, there is no change in the total amount of energy. When a transfer of momentum occurs in an isolated system, the total amount of momentum is conserved. The same is true for electric charge in a system: charge lost by one particle is gained by another. Conservation laws make it possible to predict the macroscopic behaviour of a system without having to consider the microscopic details of a physical process or chemical reaction.

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Planned management of a natural resource or of a particular ecosystem to prevent exploitation, pollution, destruction, or neglect and to ensure the future usability of the resource. Living resources are renewable, minerals and fossil fuels are nonrenewable. In the West, conservation efforts date to 17th-century efforts to protect European forests in the face of increasing demands for fuel and building materials. National parks, first established in the 19th century, were dedicated to the preservation of uncultivated land not only to provide a safe haven to wildlife but to protect watershed areas and help ensure a clean water supply. National legislation and international treaties and regulations aim to strike a balance between the need for development and the need to conserve the environment for the future.

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Maintenance and preservation of works of art, their protection from future damage, deterioration, or neglect, and the repair or renovation of works that have deteriorated or been damaged. Research in art history has relied heavily on 20th- and 21st-century technical and scientific advances in art restoration. Modern conservation practice adheres to the principle of reversibility, which dictates that treatments should not cause permanent alteration to the object.

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(1933–42) U.S. unemployment program. One of the earliest New Deal programs, it was established to relieve unemployment during the Great Depression by providing national conservation work primarily for young unmarried men. Recruits lived in semimilitary work camps and received $30 a month as well as food and medical care. Projects included planting trees, building flood barriers, fighting forest fires, and maintaining forest roads and trails. It employed a total of 3 million men during its existence.

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In jurisprudence and political philosophy, a system of right or justice common to all humankind and derived from nature rather than from the rules of society, or positive law. The concept can be traced to Aristotle, who held that what was “just by nature” was not always the same as what was “just by law.” In one form or another, the existence of natural law was asserted by the Stoics (see Stoicism), Cicero, the Roman jurists, St. Paul, St. Augustine, Gratian, St. Thomas Aquinas, John Duns Scotus, William of Ockham, and Francisco Suárez. In the modern period, Hugo Grotius insisted on the validity of natural law even on the assumption that God does not exist, and Thomas Hobbes defined a law of nature as “a precept of general rule found out by reason, by which a man is forbidden to do that which is destructive of his life.” Hobbes attempted to construct an edifice of law by rational deduction from a hypothetical “state of nature” and a social contract of consent between rulers and subjects. John Locke departed from Hobbes in describing the state of nature as an early society in which free and equal men observe the natural law. Jean-Jacques Rousseau postulated a savage who was virtuous in isolation and actuated by two principles “prior to reason”: self-preservation and compassion. The authors of the U.S. Declaration of Independence refer only briefly to “the Laws of Nature” before citing equality and other “unalienable” rights as “self-evident.” The French Declaration of the Rights of Man and of the Citizen asserts liberty, property, security, and resistance to oppression as “imprescriptible natural rights.” Interest in the concept of natural law declined dramatically in the 19th century, partly as a result of skeptical attacks by Jeremy Bentham and other proponents of utilitarianism; it was revived in the mid-20th century in light of the crimes committed by the Nazi regime during World War II. Skepticism of natural law and natural rights remained strong, however, and later writers almost invariably talked of human rights rather than natural rights.

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Colourless, highly flammable gaseous hydrocarbon consisting primarily of methane and ethane. It may also contain heavier hydrocarbons, carbon dioxide, hydrogen, hydrogen sulfide, nitrogen, helium, and argon. It commonly occurs in association with crude oil (see petroleum). Natural gas is extracted from wells drilled into the Earth. Some natural gas can be used as it comes from the well, without any refining, but most requires processing. It is transported either in its natural gaseous state by pipeline or, after liquefaction by cooling, by tankers. Liquefied natural gas occupies only about 1/600 of the volume of the gas. It has grown steadily as a source of energy since the 1930s.

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Any of the systems (e.g., the Lamaze method) of managing birth without drugs or surgery. All begin with classes to teach pregnant women about the birth process, including when to push and what breathing and relaxation techniques to use at which stage. The goal is to reduce fear and muscle tension, which can increase the pain of labour, and to make the mother an active participant in the process. The father or another partner usually attends the classes with the mother and coaches her during the birth. Seealso midwifery; obstetrics and gynecology.

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or natural arch

Naturally created arch formation resembling a bridge. Most are erosion features that occur in sandstone or limestone. Some are formed by the collapse of part of a cavern roof. Others may be produced by entrenched rivers eroding through meander necks to form cutoffs. Still others are produced by exfoliation (separation of successive thin shells) and may be enlarged by wind erosion.

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Leading scientific society in Britain and the oldest national scientific society in the world. Founded in 1660, its early members included Robert Hooke, Christopher Wren, Isaac Newton, and Edmond Halley. It has long provided an impetus to scientific thought and research in the U.K., and its achievements have become internationally famous. The society's Philosophical Transactions, the oldest scientific periodical in continuous publication, has published papers since 1665. The society awards several prizes, the most prestigious being the Copley Medal. At the beginning of the 21st century, the society had some 1,300 fellows and 130 foreign members.

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National monument, southeastern Utah, U.S. Comprising three large natural bridges carved by two winding streams, it was established in 1908. The largest bridge, Sipapu, is 222 ft (68 m) high and spans 261 ft (80 m). Pictographs were carved on another of the bridges, Kachina, by early cliff dwellers.

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Major centre of research and education on the natural sciences, established in New York City in 1869. It pioneered in staging field expeditions and creating dioramas and other lifelike exhibits showing natural habitats and their plant and animal life. Its research collections contain tens of millions of specimens, and its fossil and insect collections are among the largest in the world. It conducts research in anthropology, astronomy, entomology, herpetology, ichthyology, invertebrate biology, mammalogy, mineralogy, ornithology, and vertebrate paleontology, and it maintains permanent research stations in The Bahamas and the U.S. states of New York, Florida, and Arizona. It also contains one of the world's largest planetariums.

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