Tull, Jethro (b. 1674, Basildon, Berkshire, Eng.—d. Feb. 21, 1741, Prosperous Farm, near Hungerford, Berkshire) British agronomist and inventor. He was educated at Oxford. Around 1701 he perfected a horse- drawn seed drill that economically sowed the seeds in neat rows, and later a horse-drawn hoe. He stressed the use of manure and the importance of breaking up the soil into small particles. Tull’s methods, though initially attacked, were eventually adopted by many large landowners, and they helped form the basis of modern agriculture.
Tulsa City (pop., 2000: 393,049), northeastern Oklahoma, U.S. On the Arkansas River, Tulsa originated in 1836 as a settlement of Creek Indian immigrants; white settlement began in 1882 after the arrival of the railway. The discovery of oil nearby in the early 20th century launched an oil and gas boom, and phenomenal growth followed. Many major oil companies now have plants and offices in the city. It is the head of navigation for the Arkansas River Navigation System, a waterway that stretches 440 mi (708 km) to the junction of the Arkansas and Mississippi rivers. The commercial and financial centre of a rich agricultural area, it is the seat of the University of Tulsa (1894) and Oral Roberts University (1965).
tumbleweed Plant that breaks away from its roots and is driven about by the wind as a light rolling mass, scattering seeds as it goes. Examples include pigweed (. Amaranth retroflexus, a widespread weed in the west¬ ern U.S.) and other amaranths, tumbling mustard, Russian thistle, the steppe plant Colutea arborea, and the grass Spinifex of Indonesian shores and Australian steppes.
Tumen Vtu-'monV River River, eastern Asia. It forms the northeastern frontier of North Korea with China and Russia. It originates on Mount
Paektu, North Korea’s highest mountain, and flows generally north and northeast before turning southeast to enter the Sea of Japan (East Sea). It is 324 mi (521 km) long but navigable for only 53 mi (85 km). Many historic battle sites lie along its banks.
tumour or neoplasm Mass of abnormal tissue that arises from nor¬ mal cells, has no useful function, and tends to grow. Cell abnormalities may include increased size or number or loss of characteristics that dif¬ ferentiate their tissue of origin. Cells in malignant tumours (see cancer) have a distorted size, shape, and/or structure. Less differentiated cells tend to grow faster. Malignant tumours invade tissues locally and spread (metastasize) in blood or lymph: the stronger the tendency to metastasize, the more malignant the tumour. Tumours may not cause pain until they press on or invade nerves. Both benign and malignant tumours can press on nearby structures, block vessels, or produce excess hormones, all of which can cause death. Benign tumours remain as a solid mass that can be removed by surgery if accessible; they can consist of various tissues and may become malignant; malignant tumours, though they may remain quiescent for a time, never become benign.
tuna Any of seven species (genus Thunnus, family Scombridae) of com¬ mercially valuable food fishes. Species range from the 80-lb (36-kg) alba- core to the bluefin tuna ( T. thynnus), which grows to 14 ft (4.3 m) long and weighs up to 1,800 lbs (800 kg). Tunas have a slender, streamlined body and a forked or crescent-shaped tail. They are unique among fishes in having a vascular system modified to maintain a body temperature above the water temperature. Though slow swimmers, they migrate long distances over all the world’s oceans. They eat fishes, squid, shellfish, and plankton.
tundra Treeless, level or rolling ground above the taiga in polar regions (Arctic tundra) or on high mountains (alpine tundra), characterized by bare ground and rock or by such vegetation as mosses, lichens, small herbs, and low shrubs. Animal species are limited by harsh environmen¬ tal conditions. In the Arctic tundra they include lemmings, the Arctic fox, the Arctic wolf, caribou, reindeer, and musk-oxen. In the alpine tundra many animals, including mountain sheep and wildcats, descend to warmer zones during winter. The climate of alpine tundra is more moderate and has a higher amount of rainfall than does Arctic tundra. The freezing cli¬ mate of the Arctic produces a layer of permanently frozen soil (perma¬ frost). An overlying layer of soil alternates between freezing and thawing with seasonal temperature variations. Alpine tundras have a freeze-thaw layer but no permafrost. Because Arctic tundras receive extremely long periods of daylight and darkness (lasting between one and four months), biological rhythms tend to be adjusted more to variations in temperature than to variations in sunlight. Arctic tundra covers about one-tenth of the earth’s surface. Alpine tundras begin above the timberline of spruce and firs. Because of the small number of plant and animal species and the fragility of the food chains in tundra regions, natural or mechanical dam¬ age to any element of the habitat affects the whole ecosystem.
tuned circuit Electrically conducting pathway containing both induc¬ tance and capacitance elements. When these elements are connected in series, the circuit presents low electrical impedance to alternating current of the same frequency as the resonance frequency of the circuit and high impedance to current of other frequencies. The circuit’s resonance fre¬ quency is determined by the values of inductance and capacitance. When the circuit elements are connected in parallel, the impedance is high at the resonance frequency and low at other frequencies. With their ability to pass only certain frequencies, tuned circuits are important in, for example, radio and television receivers.
Tung Ch'i-ch'ang See Dong Qichang Tung Chung-shu See Dong Zhongshu Tung-lin Academy See Dongun Academy Tung-Pei See Manchuria Tung-ting Hu See Dongting Hu
tungsten or wolfram Vwul-fromX Metallic chemical element, one of the transition elements, chemical symbol W, atomic number 74. Excep¬ tionally strong, white to grayish, and brittle, it has the highest melting point (6,170 °F [3,410 °C]), greatest high-temperature strength, and low¬ est thermal expansion coefficient of any metal. Its chief uses are in steels to increase hardness and strength and in lightbulb filaments (see incandes¬ cent lamp). It is also used in electrical contacts, rocket nozzles, chemical
© 2006 Encyclopaedia Britannica, Inc.
1948 I tungsten-halogen lamp ► Tunney
apparatus, high-speed rotors, and solar-energy devices. Tungsten is rela¬ tively inert, but compounds (in which it has various valences) are known. The most important, tungsten carbide, noted for its hardness, is used to increase the wear-resistance of cast iron and of tools’ cutting edges.
tungsten-halogen lamp See halogen lamp Tungusic languages See Manchu-Tungus languages
Tunguska event \tuq-'gu-sk3\ (June 30, 1908) Enormous aerial explo¬ sion that flattened about 500,000 ac (2,000 sq km) of pine forest near the Stony Tunguska River in central Siberia. Its energy is estimated to have been equivalent to that of about 15 megatons of TNT. Uncertain evidence suggests that a meteoroid (see meteor) 150-300 ft (45-90 m) in diameter or, less likely, a comet disintegrated in the atmosphere high above Earth’s surface, creating a fireball and blast wave but no crater. Eyewitnesses spoke of a fireball lighting the horizon, initially visible from about 500 mi (800 km) away, followed by trembling ground and hot winds strong enough to throw people down and shake buildings. The object’s vapor¬ ization scattered dust high into the atmosphere, causing abnormally bright nighttime skies in Siberia and Europe for some time afterward.