mi (5.9 billion km) from the Sun (within the Kuiper belt), but its eccentric orbit brings it closer to the Sun than Neptune for 22 years during its 248- year orbit. Its axis is tipped 120°, so it rotates nearly on its side and “back¬ ward” (see retrograde motion) once every 6.39 days, locked synchronously with the orbit of its largest moon, Charon, discovered in 1978. Two addi¬ tional, small moons were discovered in 2005. Pluto has a diameter of about 1,455 mi (2,340 km), roughly two-thirds that of the Moon, and has less than 1% of Earth’s mass and only about 6% of its surface gravity. Its estimated average surface temperature is near -390 °F (-235 °C). Its thin atmosphere contains nitrogen, methane, and perhaps other heavier gases. Pluto is thought to be made of frozen gases with a significant fraction of rocky material. Its size, composition, and orbital location in the Kuiper belt have sparked debate over its classification as a major planet.
plutonium Radioactive (see radioactivity) metallic chemical element, chemical symbol Pu, atomic number 94. A member of the actinide series of transition elements, it is the most important transuranium element because of its use in certain types of nuclear reactors (see nuclear power) and in nuclear weapons. It is found in nature only in traces produced by natural neutron irradiation in uranium ores. It is produced artificially by neutron irradiation of uranium-238. Plutonium is a silvery metal that tarnishes in air; it is warm because of energy released in alpha decay. Its isotopes, all radioactive, are highly toxic radiological poisons (see radiation injury) because they give off alpha particles and are specifically absorbed by bone marrow.
Plymouth City (pop., 1999 est.: 255,800) in Devon, southwestern England. Located on Plymouth Sound southwest of London, the city was named Sudstone in Domesday Book (1086); its harbour is called Sutton Harbour. It was the port from which the English fleet sailed against the Spanish Armada in 1588. In 1690 its dockyard was built on the western bank of the River Tamar. During World War II Plymouth suffered bomb damage from air raids. The rebuilt city has some of the finest commer¬ cial, shopping, and civic centres in Britain and new bridges over the Riv¬ ers Plym and Tamar.
Plymouth City (pop., 2000: 51,701), southeastern Massachusetts, U.S. Located on Plymouth Bay, it was the site of the first permanent European settlement in New England, the Colony of New Plymouth, founded by the Pilgrims in 1620 and governed under the Mayflower Compact until 1691, when it became part of Massachusetts Bay Colony. Its seaside loca¬ tion and historic associations make Plymouth an outstanding summer resort. A tourist-based economy is supplemented by light industry, fish¬ ing, and cranberry growing. Historical attractions include Plimoth Plan¬ tation (a recreation of the original Pilgrim village) and many restored early colonial houses.
Plymouth Company (1606-09) Commercial trading company char¬ tered by the English crown to colonize the eastern coast of North America in present-day New England. Also called the Virginia Colony of Plymouth, its shareholders were merchants of Plymouth, Bristol, and Exeter. Its twin company was the more successful London Company. The Plymouth Com¬ pany established a colony on the coast of Maine in 1607 but soon aban¬ doned it. Inactive after 1609, it was reorganized under a new charter in 1620 as the Council for New England. See also chartered company.
plywood Manufactured panel made up of three or more thin plies (lay¬ ers) of wood. Each ply is laid down with its grain running perpendicular to the one before it. Like other glued-wood products, plywood provides a strong, inexpensive alternative to solid wood. It is widely used both in cabinetmaking (for chests, dressers, wardrobes, and tables) and in house construction (for walls, ceilings, subfloors, doors, and in forms for cast¬ ing concrete).
PMS See premenstrual syndrome
pneumatic device \nu-'ma-tik\ Any of various tools and instruments that generate and use compressed air. Examples include rock drills, pave¬ ment breakers, riveters, forging presses, paint sprayers, blast cleaners, and atomizers. Compressed-air power is flexible, economical, and safe. In general, pneumatic systems have relatively few moving parts, contribut¬ ing to high reliability and low maintenance costs.
pneumatic structure Membrane structure that is stabilized by the pres¬ sure of compressed air. Air-supported structures are supported by inter¬ nal air pressure. A network of cables stiffens the fabric, and the assembly is supported by a rigid ring at the edge. The air pressure within this bubble is increased slightly above normal atmospheric pressure and maintained by compressors or fans. Air locks are required at entrances to prevent loss
of internal air pressure. Air-supported membranes were first devised by Walter Bird in the late 1940s and were soon put to use as covers for swimming pools, temporary warehouses, and exhibition buildings. Air- inflated structures are supported by pressurized air within inflated build¬ ing elements that are shaped to carry loads in a traditional manner. Pneumatic structures are perhaps the most cost-effective type of building for very long spans.
pneumococcus V.nu-mo-'ka-kosX Spheroidal bacterium ( Streptococcus pneumoniae ) that causes human diseases including pneumonia, sinusitis, ear infection, and meningitis. Usually occurring in the upper respiratory tract, this gram-positive (see gram stain) coccus is often found in a chain configuration and surrounded by a polysaccharide capsule. Pneumococci are separated into types depending on the specific capsular polysaccha¬ ride formed. Their disease-causing ability resides in the capsule, which delays or prevents their destruction by cells in the bloodstream that nor¬ mally engulf foreign material.
pneumoconiosis X.nu-mo-.ko-ne-'o-sssV Any lung disease caused by inhalation of organic or inorganic dusts or chemical irritants, usually over time. Some dusts (e.g., silica minerals, asbestos) produce grave reactions in small quantities. Dust collects in the pulmonary alveoli, causing inflam¬ mation that scars lung tissue, reducing its elasticity. Chest tightness and shortness of breath may progress to chronic bronchitis and emphysema. Cigarette smoking worsens many types. The most common cause of severe disease is silica (see silicosis). Asbestos (see asbestosis), beryllium, and aluminum dusts can cause worse disease, often on brief exposure. Organic causes include mold spores (see allergy) and textile fibres. Chemi¬ cal irritants such as ammonia, acid, and sulfur dioxide are soon absorbed by the lung lining; their irritant effect can cause pulmonary edema, and they can lead to chronic bronchitis.
pneumonia \nu-'mon-y9\ Inflammation and solidification of lung tissue caused by infection, foreign particle inhalation, or irradiation but usually by bacteria. Mycoplasma pneumoniae is the most common cause in healthy individuals. The bronchi and alveoli may be inflamed. Coughing becomes severe and may bring up flecks of blood. It can be serious but is rarely fatal. Streptococcus pneumoniae is more common and generally more severe but usually affects only those with low resistance, especially in hospitals. A highly lethal form caused by Klebsiella pneumoniae is almost always con¬ fined to hospitalized patients with low immunity. Other bacterial pneumo¬ nias include Pneumocystis carinii pneumonia (rare except in AIDS) and Legionnaire disease. Most respond to antibiotic treatment. ViRUses set the stage for bacterial pneumonia by weakening the individual’s immune sys¬ tem more often than they cause pneumonia directly. Fungal pneumonia usually occurs in hospitalized persons with low resistance, but contami¬ nated dusts can cause it in healthy individuals. It can develop rapidly and may be fatal. X-ray treatment (see radiation therapy) of structures in the chest may cause temporary lung inflammation.
pneumothorax Condition in which air accumulates in the pleural sac, causing it to expand and thus compress the underlying lung, which may then collapse. (The pleural sac is a cavity formed by the two pleural membranes that line the thoracic cavity and cover the lungs.) Traumatic pneumothorax is the accumulation of air caused by penetrating wounds (knife stabbing, gunshot) or other injuries to the chest wall, after which air is sucked through the opening and into the pleural sac. Spontaneous pneumothorax is the passage of air into the pleural sac from an abnormal connection created between the pleura and the bronchial system as a result of tuberculosis or some other lung disease. The symptoms of spontane¬ ous pneumothorax are a sharp pain in one side of the chest and shortness of breath.