is the masterpiece of the school. The artistic activity at Avignon greatly influenced French painting in the late 15th and 16th centuries. See also Gothic art.
Aviles, Pedro Menendez de See Pedro Menendez de Aviles
Avogadro's number Number of units in one mole of any substance (defined as its molecular weight in grams), equal to 6.0221367 x 10 23 . The units may be electrons, atoms, ions, or molecules, depending on the nature of the substance and the character of the reaction (if any). See also Avogadro's law; law of mass action; stoichiometry.
avoidance, zone of Region of apparent absence of galaxies near the plane of the Milky Way Galaxy. Millions of external galaxies are in this region of the sky but are obscured by interstellar dust within the Milky Way. The zone’s obscuration can be penetrated at the infrared portion of the ELECTROMAGNETIC SPECTRUM. See also INFRARED ASTRONOMY.
avoidance behaviour Type of activity, exhibited by animals exposed to adverse stimuli, in which the tendency to flee or to act defen¬ sively is stronger than the tendency to attack. Vision is the sense that most often produces avoidance behaviour (e.g., small birds react to the sight of an owl), but sound (e.g., a warning cry) may do so as well.
Avon, River or Lower Avon River, southwestern England. Rising in Gloucestershire, it flows 75 mi (121 km) southwest through Bristol and into the Bristol Channel at Avonmouth, Bristol’s port. Below Bristol it has cut through a limestone ridge to form Clifton Gorge, noted for its sus¬ pension bridge.
Avon, River or Upper Avon River, central England. Rising in Northamptonshire, it flows 96 mi (154 km) southwest into the River Sev¬ ern at Tewkesbury. It is known for its scenic beauty, notably in the Vale of Evesham. Important towns along it include Stratford, where William Shakespeare was born.
of the laurel family, a tree native
avocado Fruit of Persea americana, to the Western Hemisphere from Mexico south to the Andean regions.
Avocados are extremely variable in shape, size, and colour (green to dark purple). The outer skin may be thin, or coarse and woody. The greenish or yellowish flesh has a buttery consis¬ tency and a rich, nutty flavour. In some varieties the flesh contains as much as 25% unsaturated oil. Avo¬ cados are the principal ingredient of the Mexican sauce guacamole. They provide thiamine, riboflavin, and vitamin A.
avocet Va-v9-,set\ Any of several large shorebirds (genus Recurviros- tra ) with boldly contrasting plum¬ age, long bluish legs, and a long black bill upturned at the tip. Avocets inhabit fresh and salt marshes that have areas of open shallow water and mudflats, and feed by sweeping the partly open bill in the shallows. They often wade together to corral min¬ nows and crustaceans. Four avocet species are found in temperate and tropical regions worldwide. The American avocet is about 18 in. (45 cm) long, including the bill.
Avogadro's law ^a-vo-'ga-drozX Statement that, under the same con¬ ditions of TEMPERATURE and PRESSURE, equal volumes of different GASes
contain an equal number of molecules (see Avogadro's number). First pro¬ posed by the Italian scientist Amedeo Avogadro (1776-1856) in 1811, it became accepted c. 1860. From the law, it follows that the volume occu¬ pied by one mole of gas (at standard conditions of 32 °F [0 °C] and 1 atmosphere of pressure) is the same for all gases (0.791 cubic feet [22.4 litres]).
Avocado (Persea americana).
S.A. SCIBOR-SHOSTAL
American avocet (Recurvirostra ameri¬ cana)
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AWACS Va-waks\ (Airborne Warning and Control System)
Mobile, long-range radar surveillance-and-control centre for air defense. Used by the U.S. Air Force since 1977, AWACS is mounted in a specially modified Boeing 707 aircraft, with its main radar antenna affixed to a rotat¬ ing dome. It can detect, track, and identify low-flying aircraft at a distance of 200 nautical mi (370 km) and high-level targets at much greater dis¬ tances. It can also track maritime traffic and operate in any weather. The computer system can assess enemy action and track the location and avail¬ ability of any aircraft within range. Operators of its secure communications system can guide friendly aircraft against enemy planes.
Awakening, Great See Great Awakening
Awolowo Va-'wii-lo-.woV Obafemi (b. March 6, 1909, Ikenne, Colony and Protectorate of Southern Nigeria—d. May 9, 1987, Ikenne) Nigerian nationalist politician and leader of the Yoruba ethnic group. While studying law in London, he wrote the influential Path to Nigerian Freedom (1947) and laid the basis for the first Yoruba political party, the Action Group. As premier of Nigeria’s Western Region (1954-59), he worked to improve education, social services, and agriculture. He remained a major figure in national politics but never won high elective office.
ax Hand tool used for chopping, splitting, chipping, and piercing. Stone Age hand axes originated in simple stone implements that acquired wooden hafts, or handles, about 30,000 bc. Copper-bladed axes appeared in Egypt about 4000 bc and were followed by axes with blades of bronze and eventually iron. The development of the iron-bladed felling ax in the Middle Ages made possible the vast forest clearances of Europe, North and South America, and elsewhere. Though the ax has lost much of its historic role to powered saws and other machinery, it remains a widely used tool with many uses.
axiology or value theory Philosophical theory of value. Axiology is the study of value, or goodness, in its widest sense. The distinction is commonly made between intrinsic and extrinsic value—i.e., between that which is valuable for its own sake and that which is valuable only as a means to something else, which itself may be extrinsically or intrinsically valuable. Many different answers have been given to the question “What is intrinsically valuable?” For hedonists, it is pleasure; for pragmatists, it is satisfaction, growth, or adjustment; for Kantians, it is a good will. Plu- ralists such as G.E. Moore and William David Ross assert that there are any number of intrinsically valuable things. According to subjective theories of value, things are valuable only insofar as they are desired; objective theories hold that there are at least some things that are valuable inde¬ pendently of people’s interest in or desire for them. Cognitive theories of value assert that ascriptions of value function logically as statements of fact, whereas noncognitive theories assert that they are merely expres¬ sions of feeling (see emotivism) or prescriptions or commendations (see prescriptivism). According to naturalists, expressions such as “intrinsically good” can be analyzed as referring to natural, or non-ethical, properties, such as being pleasant. Moore famously denied this, holding that “good” refers to a simple (unanalyzable) non-natural property. See also fact-value distinction; naturalistic fallacy.
axiom In mathematics or logic, an unprovable rule or first principle accepted as true because it is self-evident or particularly useful (e.g., “Nothing can both be and not be at the same time and in the same respect”). The term is often used interchangeably with postulate, though the latter term is sometimes reserved for mathematical applications (such as the postulates of Euclidean geometry). It should be contrasted with a theorem, which requires a rigorous proof.
axiomatic method In logic, the procedure by which an entire science or system of theorems is deduced in accordance with specified rules by logical deduction from certain basic propositions (axioms), which in turn are constructed from a few terms taken as primitive. These terms may be either arbitrarily defined or conceived according to a model in which some intuitive warrant for their truth is felt to exist. The oldest examples of axiomatized systems are Aristotle’s syllogistic and Euclidean geometry. Early in the 20th century, Bertrand Russell and Alfred North Whitehead attempted to formalize all of mathematics in an axiomatic manner. Schol¬ ars have even subjected the empirical sciences to this method, as in J. H. Woodger’s The Axiomatic Method in Biology (1937) and Clark Hull’s Principles of Behavior (1943).