prestressed concrete Concrete reinforced by either pretensioning or posttensioning, allowing it to carry a greater load or span a greater dis¬ tance than ordinary reinforced concrete. In pretensioning, lengths of steel wire or cables are laid in the empty mold and stretched. The concrete is placed and allowed to set, and the cables are released, placing the con¬ crete into compression as the steel shrinks back to its original length. In posttensioning, the steel in the concrete is stretched after the curing pro¬ cess. Prestressing places a concrete member in compression; these com¬ pressive stresses counteract the tensile bending stresses of an applied load. The process was developed by the French engineer Eugene Fressinet in the early 20th century.
Pretoria \pri-'tor-e-9\ City (pop., 2005 est.: metro, area, 1,282,000), administrative capital of the Republic of South Africa. Founded in 1855, it became the capital of the Transvaal in 1860, the administrative capital of South Africa in 1910, and a city in 1931. In 1899, during the South African War (1899-1902), Winston Churchill was imprisoned there until his escape. Pretoria is primarily a seat of government, and most people are employed in the service sector. It is also an important rail centre, with an industrial economy based on iron and steel. Its educational institutions include the University of South Africa (1873) and the University of Pre¬ toria (1908). Pretoria became part of the Tshwane Metropolitan Munici¬ pality in 2000. See also Bloemfontein; Cape Town.
Pretorius Xpro-'tii-re-iBsV Andries (Wilhelmus Jacobus) (b. Nov. 27, 1798, near Graaff-Reinet, Cape Colony—d. July 23, 1853, Magalies- berg, Transvaal) Boer leader in the Great Trek. Pretorius’ s forces defeated the Zulu at Blood River in 1838 and at Magono in 1840. In 1842 he led an unsuccessful fight with the British over the annexation of Natal. Fol¬ lowing British annexation of Transvaal in 1848, his forces again attacked and were again defeated. In 1852 he participated in the Sand River Con¬ vention, where Transvaal independence was recognized. He led negotia¬ tions for independence of the Orange River Sovereignty, finally guaranteed by the Bloemfontein Convention of 1854. His son Marthinus Wessel Pretorius (1819-1901) was the first president of the South Afri¬ can Republic (1857, 1864, 1869) and president of the Orange Free State (1859-63). After British annexation of the Transvaal, Marthinus joined insurgent Boer leaders and helped win recognition of its independence. He was a member of the ruling triumvirate until the election of Paul Kruger as president in 1883.
preventive medicine Efforts toward disease prevention in the com¬ munity and the individual. It covers patient interviews and testing to detect risk factors; sanitary measures in homes, communities, and medical facili¬ ties; patient education; and diet and exercise programs as well as preven¬ tive drugs and surgery. It has three levels: primary (e.g., prevention of coronary heart disease in a healthy person), secondary (e.g., prevention of heart attack in a person with heart disease), and tertiary (e.g., preven¬ tion of disability and death after a heart attack). The first is by far the most economical. Important advances in preventive medicine include vac¬ cination (see vaccine), antibiotics, diagnostic imaging, and recognition of psychological factors. See also epidemiology, immunology, industrial medi¬ cine, QUARANTINE.
Prevert \pra-'ver\, Jacques (-Henri-Marie) (b. Feb. 4, 1900, Neuilly-sur-Seine, France—d. April 11, 1977, Omonville-la-Petite) French poet and screenwriter. A shop worker, he began writing after his military service. Influenced by the Symbolist movement, he renewed the ancient tradition of oral poetry, creating “song poems” about Paris street life, collected in the anthology Words (1945). Many were put to music and became extremely popular. His excellent screenplays for Marcel Carne include The Children of Paradise (1945), Spectacle (1951), and Things and Other Things (1972). He also worked for French television and on animated films for children.
Previn \'pre-vin\, Andre (George) orig. Andreas Ludwig Pri-
win (b. April 6, 1929, Berlin, Ger.) German-born U.S. pianist, composer, and conductor. He fled Nazi persecution with his family and moved to Los Angeles in 1939. He orchestrated and arranged music for MGM in the 1940s and ’50s and thereafter scored films for several studios. Mean¬ while he had become a noted classical and jazz pianist and started con¬ ducting. He served as principal conductor of the Houston Symphony, London Symphony, Pittsburgh Symphony, Los Angeles Philharmonic, and Royal Philharmonic orchestras. He also composed a symphony, con¬ certos, and the opera A Streetcar Named Desire (1998), as well as popu¬ lar songs. He was knighted in 1996.
Prevost d'Exiles \pra-,vo-dag-'zel\, Antoine-Francois, Abbe
known as Abbe Prevost (b. April 1, 1697, Hesdin, France—d. Nov. 25, 1763, Chantilly) French novelist. From an early age Prevost alternated between enlistments in the army and entries into the religious life, which he eventually fled in 1728. Subsequent years were marked by numerous love affairs and debt. His fame rests entirely on one work— Manon Les- caut (1731), a sentimental novel about a young man of good family who ruins his life for a courtesan, which became the basis of two famous operas, Jules Massenet’s Manon and Giacomo Puccini’s Manon Lescaut.
PRI See Institutional Revolutionary Party
Priam \'pri-3m\ In Greek mythology, the last king of Troy. He succeeded his father Laomedon as king and gradually expanded Troy’s control over the Hellespont. By his wife, Hecuba, he had many children, including Hector and Paris. He reigned during the Trojan War; in its final year he lost 13 sons, three of whom were killed by Achilles in a single day. Hec¬ tor’s death broke his spirit, and he went humbly to Achilles to ask for the corpse. When Troy fell, Achilles’ son Neoptolemus killed the elderly Priam on an altar.
Priapus \pri-'a-p3s\ Greek god of animal and vegetable fertility. He was represented in a caricature of the human form, grotesquely misshapen, with an enormous phallus. The ass was sacrificed in his honor, probably because it symbolized lecherousness and was associated with the god’s sexual potency. His father was Dionysus and his mother was either a local nymph or Aphrodite. In Hellenistic times the worship of Priapus spread throughout the ancient world, and he was adopted as the god of gardens.
Pribilof Vpri-bo-.lofN Islands Group of islands, southeastern Bering Sea, Alaska, U.S. It includes St. Paul, St. George, and three islets, and it lies about 300 mi (500 km) west of the mainland. Control of the islands was transferred from Russia to the U.S. with the Alaska Purchase (1867). The islands are hilly and treeless with no harbours. They are breeding grounds from April to November for most of the world’s fur seals. Commercial harvesting of seals was discontinued in 1986. The islands are also home to enormous numbers of birds and both blue and white phases of Arctic foxes. The indigenous population is made up of Aleuts.
price Amount of money that has to be paid to acquire a given good, ser¬ vice, or resource. Operating as a measure of value, prices perform a sig¬ nificant economic function, distributing the scarce supply of goods.
© 2006 Encyclopaedia Britannica, Inc.
1546 I Price ► Priestley
services, and resources to those who most want them through the adjust¬ ments of supply and demand. Prices of resources are called wages, interest, and rent. This system, known as the price mechanism, is based on the principle that only by allowing prices to move freely will the supply of any given commodity match demand. If supply is excessive, prices will be low and production will be reduced; this will cause prices to rise until there is a balance of demand and supply. If supply is inadequate, prices will be high, prompting an increase in production that in turn will lead to a reduction in prices until supply and demand are in equilibrium. A totally free price mechanism does not exist in practice; even in free-market economies, monopolies or government regulation may limit the efficiency of price as a determinant of supply and demand. In centrally planned economies, the price mechanism may be supplanted by centralized gov¬ ernment control. Attempts to operate an economy without a price mecha¬ nism usually result in surpluses of unwanted goods, shortages of desired products, black markets, and stunted economic growth.