112 résultats pour "pressure"

• Blood Pressure.

• Rock (mineral).

  pressure mineral assemblage. In a sense, the rock is stuck in a state that is characteristic of its earlier high-temperature and high-pressure environment. Thus,metamorphic rocks carry with them information about the history of temperatures and pressures to which they were subjected. The size, shape, and distribution of mineral grains in a rock are called the texture of the rock. Many metamorphic rocks are named for their main texture. Textures giveimportant clues as to how the rock formed. As t...

• Circulatory System.

  C Additional Functions In addition to oxygen, the circulatory system also transports nutrients derived from digested food to the body. These nutrients enter the bloodstream by passingthrough the walls of the intestine. The nutrients are absorbed through a network of capillaries and veins that drain the intestines, called the hepatic portal circulation.The hepatic portal circulation carries the nutrients to the liver for further metabolic processing. The liver stores a variety of substances, suc...

• Weather.

  hours, and the snow can be much deeper in places where the wind piles it up in drifts. Extraordinarily deep snows sometimes accumulate on the upwind side ofmountain slopes during severe winter storms or on the downwind shores of large lakes during outbreaks of polar air. VI WIND Wind is the horizontal movement of air. It is named for the direction from which it comes—for example, a north wind comes from the north. In most places near theground, the wind speed averages from 8 to 24 km/h (from 5...

• Turbine.

  steam turbine has supplanted the reciprocating engine as a prime mover in large electricity-generating plants and is also used as a means of jet propulsion. Steam turbines are used in the generation of nuclear power and in nuclear ship propulsion. They operate with fuel-fired boilers for power generation. In cogenerationapplications requiring both process heat (heat used in an industrial process) and electricity, steam is raised at high pressure in the boiler and extracted from the turbineat th...

• Heart.

  situation. A Cardiac Cycle Although the right and left halves of the heart are separate, they both contract in unison, producing a single heartbeat. The sequence of events from the beginning ofone heartbeat to the beginning of the next is called the cardiac cycle. The cardiac cycle has two phases: diastole, when the heart’s chambers are relaxed, and systole,when the chambers contract to move blood. During the systolic phase, the atria contract first, followed by contraction of the ventricles. T...

• Physical Chemistry - chemistry.

  by the system in the form of the flow of electrical currents, formation of surfaces and changes in surface tension, changes in volume or pressure, and formation ordisappearance of chemical species. B Chemical Kinetics This field studies the rates of chemical processes as a function of the concentration of the reacting species, of the products of the reaction, of catalysts and inhibitors, ofvarious solvent media, of temperature, and of all other variables that can affect the reaction rate. It is...

• Diving (underwater).

  compensator (or control) device (BCD or BC), which the diver wears as a vest. By adding air to the BCD, the diver becomes more buoyant and rises. By releasing air,the diver becomes less buoyant and sinks. With minor adjustments of air, the diver can achieve neutral buoyancy. A third hose attaches to pressure gauges that diversuse to monitor how much air remains in the tank. A fourth hose attaches to a backup breathing device called an alternate air source, or octopus. Divers also wear a belt w...

• Heat (physics) I INTRODUCTION Heat Loss from a House A thermograph shows the large amount of heat lost through a house's windows during winter.

  Theoretically, the molecules of a substance would exhibit no activity at the temperature termed absolute zero. See Molecule. III TEMPERATURE SCALES Five different temperature scales are in use today: the Celsius scale, known also as the Centigrade scale, the Fahrenheit scale, the Kelvin scale, the Rankine scale, andthe international thermodynamic temperature scale ( see Thermometer). The Celsius scale, with a freezing point of 0° C and a boiling point of 100° C, is widely used throughout the...

• Meteorology.

  to find the corresponding relative humidity and dew-point temperature. III SPECIAL METEOROLOGICAL INSTRUMENTS Meteorologists have developed several sophisticated instruments that measure multiple physical characteristics of the air simultaneously and at more than one location.The most important of these special instruments are radiosondes, Doppler radar, and weather satellites. A Radiosonde A radiosonde measures air temperature, air pressure, and humidity from the earth’s surface up to an alt...

• First Aid.

  IV SEVERE BLEEDING The presence of blood over a considerable area of a person’s body does not always indicate severe bleeding. The blood may ooze from multiple small wounds or besmeared, giving the appearance of more blood than is actually present. The rate at which blood is lost from a wound depends on the size and kind of blood vesselruptured. Bright red, spurting blood indicates injury to an artery while welling or steadily flowing, dark red blood indicates injury to a vein. Welling or spur...

• Fossil Fuels.

  water molecules. This crystalline solid is known as gas hydrate. Because technology for the commercial extraction of gas hydrates has not yet been developed, this typeof fossil fuel is not included in most world energy resource estimates. However, in February 2007 the U.S. Department of Energy completed the drilling of a well toretrieve core samples of gas hydrates found in the Prudhoe Bay region of Alaska’s North Slope. By identifying the nature of these gas hydrates, the Energy Departmenthoped...

• Hydrogen - chemistry.

  Hydrogen gas does not usually react with other chemicals at room temperature. That is, it does not split into two hydrogen atoms to combine with other chemicals. Thebond between the hydrogen atoms is very strong and can only be broken with a large amount of energy. However, when heated with a flame or a spark, hydrogen gaswill react violently with oxygen in the air to produce water in the following reaction: 2H2 + O 2 → 2H 2O This chemical equation shows that two hydrogen molecules (H 2) and o...

• Steam Engine.

  wear caused by this pressure, steam-engine valves are frequently made in a cylindrical form entirely enclosing the piston, so that the pressure is equal all around thevalve and friction is minimized. The development of this type of valve is attributed to the American inventor and manufacturer George Henry Corliss. In other types ofslide valves, the moving portion of the valve is designed so that steam does not press directly on the back of the valve. The linkage between the piston of the engine...

• Ear. I INTRODUCTION Ear, organ of hearing and balance. Only vertebrates, or animals

  line or rotates in any direction. Each canal also contains sensory areas with sensory hair cells that project into a cone-shaped cap of gelatin. Two of the semicircularcanals are in a vertical position and are used to detect vertical movement, such as jumping or falling. The third canal is horizontal and detects horizontal movement,such as turning or spinning. The action of the canals depends on the inertia of the fluid inside. When the motion of the body changes, the fluid lags behind, causing...

• Stress (psychology).

  blood flow is diverted from the internal organs and skin to the brain and muscles. Breathing speeds up, the pupils dilate, and perspiration increases. This reaction issometimes called the fight-or-flight response because it energizes the body to either confront or flee from a threat. Another part of the stress response involves the hypothalamus and the pituitary gland, parts of the brain that are important in regulating hormones and many otherbodily functions. In times of stress, the hypothal...

• Uranus (planet) - astronomy.

  V COMPOSITION AND STRUCTURE A Interior of Uranus Uranus contains mostly rock and water, with hydrogen and helium (and trace amounts of methane) in its dense atmosphere. Astronomers believe that Uranus, likeNeptune, formed from the same material—principally frozen water and rock—that composes most of the planet’s moons. As the planet grew, pressures andtemperatures in the planet’s interior increased, heating the planet’s frozen water into a hot liquid. Uranus probably has a relatively small roc...

• Surgery.

  III SURGICAL PROCEDURES Surgical procedures are classified as optional, required, elective, urgent, and emergent based on the patient’s medical condition. Optional surgery consists of operationsthat are not required but which the patient chooses to undergo as with some types of cosmetic surgery. Required surgery is performed when only surgery will correct aproblem—such as cataracts—but the surgery can be delayed for a period of weeks or months. Elective surgical procedures usually involve cond...

• Submarine.

  B Propulsion Modern submarines use either diesel-electric or nuclear power to drive the sub's propeller and to provide internal electric power. Diesel-electric power emerged as themost efficient propulsion system for submarines in the early 20th century, following unsuccessful attempts to use steam or gasoline power. While on the surface, thesubmarine uses a diesel engine to drive the propeller and generate electricity. When submerged, a battery-driven electrical motor takes over for propulsion...

• Aerospace Medicine - astronomy.

  medically as aeroembolism and popularly as the bends, leads to confusion, paralysis, or neurocirculatory collapse. The most characteristic symptoms of the bends arepain in the large joints resulting from pressure of the gas on tendons and nerves, together with spasm of the blood vessels. Preflight inhalation of pure oxygen toeliminate nitrogen from the system has proved valuable as a preventive measure. Rapid decompression, resulting from accidental failure at high altitudes of thepressure withi...

• Sound I INTRODUCTION Sound, physical phenomenon that stimulates the sense of hearing.

  A fundamental law of harmony states that two notes an octave apart, when sounded together, produce a pleasant-sounding combination. Other combinations of notescan also be pleasing. Physically, an interval of a fifth consists of two notes, the frequencies of which bear the arithmetical ratio 3 to 2, and a major third, the ratio 5 to4. Fundamentally, the law of harmony states that two or more notes sound pleasant when played together if their frequencies bear small, whole number ratios; if thefreq...

• Coronary Heart Disease.

  when a patient is at rest may indicate that the blood supply of the heart is not normal, and the ECG can often detect damage from a previous heart attack. In anexercise stress test, an ECG is recorded while a patient is performing physical activity such as walking on a treadmill or riding a stationary bicycle. As the intensity ofexercise increases, the doctor looks for specific changes in the ECG that indicate the heart is not getting enough oxygen. In cardiac catheterization, a long, thin, flex...

• Star (astronomy) - astronomy.

  absorbing the missing colors of light. For example, the set of dark lines made by hydrogen includes a dark red line, the set of dark lines made by sodium includes a pairof dark yellow lines, and the set of dark lines made by iron includes lines of nearly every color. Each element in the gaseous outer layer of a star produces its ownparticular pattern of dark spectrum lines, depending on the temperature and pressure of the gas. Astronomers have observed spectrum lines, or spectra, for hundredsof...

• Earthquake.

  III CAUSES Most earthquakes are caused by the sudden slip along geologic faults. The faults slip because of movement of the Earth’s tectonic plates. This concept is called theelastic rebound theory. The rocky tectonic plates move very slowly, floating on top of a weaker rocky layer. As the plates collide with each other or slide past eachother, pressure builds up within the rocky crust. Earthquakes occur when pressure within the crust increases slowly over hundreds of years and finally exceeds...

• Cell (biology) - biology.

  proteins, or other proteins required by the cell. While relatively simple in construction, prokaryotic cells display extremely complex activity. They have a greater range of biochemical reactions than those found in theirlarger relatives, the eukaryotic cells. The extraordinary biochemical diversity of prokaryotic cells is manifested in the wide-ranging lifestyles of the archaebacteria andthe bacteria, whose habitats include polar ice, deserts, and hydrothermal vents—deep regions of the ocean un...

• Bicycle.

  narrow as possible to minimize weight and interference with pedaling. F Specialty Bicycles Many other types of bicycles are designed for special purposes. Although not as common as standard single-rider racing, touring, mountain, or recreational bicycles,they nonetheless have significant niches. Recumbent bicycles are bicycles on which the rider sits upright as if in a chair, with legs and feet stretched out in front. Because they support the rider’s lower back,recumbents allow the rider far mo...

• Rocket.

  the vacuum of space, however, demonstrated that this belief was not true. In fact, rockets produce more thrust in the vacuum of space than on Earth. Air pressure andfriction with the air reduce a rocket’s thrust by about 10 percent on Earth as compared to the rocket’s performance in space. B Thrust and Efficiency Thrust is a measurement of the force of a rocket, or the amount of “push” exerted backward to move a rocket forward. Thrusts vary greatly from rocket to rocket.Engineers measure thrust...

• Rocket - astronomy.

  the vacuum of space, however, demonstrated that this belief was not true. In fact, rockets produce more thrust in the vacuum of space than on Earth. Air pressure andfriction with the air reduce a rocket’s thrust by about 10 percent on Earth as compared to the rocket’s performance in space. B Thrust and Efficiency Thrust is a measurement of the force of a rocket, or the amount of “push” exerted backward to move a rocket forward. Thrusts vary greatly from rocket to rocket.Engineers measure thrust...

• Neptune (planet) - astronomy.

  The gaseous atmosphere of Neptune contains hydrogen, helium, and about 3 percent methane. It extends about 5,000 km (about 3,000 mi) above the planet’s ocean.Light reflected from Neptune’s deep atmosphere is blue, because the atmospheric methane absorbs red and orange light but scatters blue light. In 1998 astronomersalso identified molecules of methyl in Neptune’s atmosphere. Methyl molecules each contain one carbon atom and three hydrogen atoms. Methyl molecules are knownas hydrocarbon radical...

• Jupiter (planet) - astronomy.

  Beneath the supercritical fluid zone, the pressure reaches 3 million Earth atmospheres. At this depth, the atoms collide so frequently and violently that the hydrogenatoms are ionized—that is, the negatively charged electrons are stripped away from the positively charged protons of the hydrogen nuclei. This ionization results in asea of electrically charged particles that resembles a liquid metal and gives rise to Jupiter’s magnetic field. This liquid metallic hydrogen zone is 30,000 to 40,000 k...

• Executive Branch.

  Popular pressure for increased regulation of businesses increased in the early 20th century. This pressure led to laws such as the Pure Food and Drug Acts, and the1906 Meat Inspection Act. Pressure for reform also led to creation of the Federal Trade Commission in 1914 to prevent unfair business practices. Worldwide depressionin the 1930s brought new demands that government take a more activist role in regulating businesses, protecting workers, and ensuring national economic stability. Aspart of...

• Maris Breaks Home Run Record.

  Bothered by an injured hip, Mantle was removed from the lineup in mid-September with 54 long balls, leaving Maris alone in the home-run derby. The pressureincreased as a result of a July 17 ruling by Major League Baseball Commissioner Fred Frick that the record would have to be matched or bettered within 154 games,the number of games played in the 1927 season. In 1961, for the first time, American League teams played 162 games. Frick, a friend of Ruth's, said that if therecord was broken after 1...

• Armenian Massacres .

  Some of the Armenian revolutionaries and others hoped that the massacres would provoke the intervention of the European powers (Britain, France, Austria-Hungary,and Germany). Although the leaders of the European powers publicly condemned the actions of the sultan, they failed to intervene. Mutual rivalries and suspicions, aswell as the imprecise terms of Article 61 of the Treaty of Berlin, helped produce this inaction. But these bloody episodes soon paved the way for the rise of a newnationalist...

• Carbon - chemistry.

  Graphite is black and slippery and conducts electricity. In graphite, the atoms form planar, or flat, layers. Each layer is made up of rings containing six carbon atoms.The rings are linked to each other in a structure that resembles the hexagonal mesh of chicken wire. Each atom has three sigma bonds (with 120° between any two ofthe bonds) and belongs to three neighboring rings. The fourth electron of each atom becomes part of an extensive pi bond system. Graphite conducts electricity,because th...

• Alaska - geography.

  depression surrounded by highlands and have the coldest winter and hottest summer temperatures in Alaska. Once the Kuskokwim River passes through theKuskokwim Mountains, it forms the southern edge of a vast lake-studded alluvial plain bounded on the north by the Yukon River. This water-logged lowland is a majorsummer nesting area for birds. Fairbanks is the major city in this region, while Fort Yukon is the major community in the Yukon Flats and Bethel the largest settlementon the Lower Kuskokwi...

• Alaska - USA History.

  depression surrounded by highlands and have the coldest winter and hottest summer temperatures in Alaska. Once the Kuskokwim River passes through theKuskokwim Mountains, it forms the southern edge of a vast lake-studded alluvial plain bounded on the north by the Yukon River. This water-logged lowland is a majorsummer nesting area for birds. Fairbanks is the major city in this region, while Fort Yukon is the major community in the Yukon Flats and Bethel the largest settlementon the Lower Kuskokwi...

• Alternative Medicine.

  The field of chiropractic was founded by David Daniel Palmer in the 1890s. He believed that joint subluxation, or a partial dislocation, is a causal factor in disease andthat removal of the subluxation by thrusting on the bony projections of the vertebrae restores health. In addition to manipulating and adjusting bone and tissue, particularly in the spinal column, chiropractors use a variety of manual, mechanical, and electrical treatments.Chiropractors are most widely recognized for providing d...

• Water - chemistry.

  flourides in drinking water has been found to reduce tooth decay. See Fluorine. Seawater contains, in addition to concentrated amounts of sodium chloride, or salt, many other soluble compounds, as the impure waters of rivers and streams areconstantly feeding the oceans. At the same time, pure water is continually lost by the process of evaporation, and as a result the proportion of the impurities that givethe oceans their saline character is increased. See Ocean and Oceanography. VII WATER...

• Stroke.

  IV DIAGNOSIS The symptoms typical of a stroke can also be caused by other conditions, including brain tumors, various infections, and overdoses of certain drugs. A patient that hasstrokelike symptoms may undergo a battery of imaging techniques to rule out other medical problems and confirm that a stroke has occurred. The key imagingtechnique used in diagnosing strokes is computed tomography (also known as CT or CAT scanning), which employs X rays to obtain images of the internal structures oft...

• Eye. I INTRODUCTION Eye, light-sensitive organ of vision in animals. The eyes of

  Several structures, not parts of the eyeball, contribute to the protection of the eye. The most important of these are the eyelids, two folds of skin and tissue, upper andlower, that can be closed by means of muscles to form a protective covering over the eyeball against excessive light and mechanical injury. The eyelashes, a fringe ofshort hairs growing on the edge of either eyelid, act as a screen to keep dust particles and insects out of the eyes when the eyelids are partly closed. Inside the...

• Whale - biology.

  III BEHAVIOR OF WHALES Studies of whales in captivity have taught scientists much about the complex social behavior of whales. Since the late 1980s, advances in the use of satellite trackingsystems have also broadened opportunities for scientists to observe how whales behave in the wild. A Swimming and Diving Whales swim by making powerful up-and-down movements of the tail flukes, which provide thrust. The power comes from body muscles that flex the lower spine upand down in a wavelike motion...

• French Revolution.

  B2 Assembly of Notables and Estates-General To pressure the parlements into accepting the plan, Calonne decided to gain prior approval of it from an Assembly of Notables—a group of hand-picked dignitaries hethought would sympathize with his views. But Calonne had badly miscalculated. Meeting in January 1787, the assembly refused to believe that a financial crisis reallyexisted. They had been influenced by Necker’s argument that state finances were sound and suspected that the monarchy was only...

• French Revolution .

  B2 Assembly of Notables and Estates-General To pressure the parlements into accepting the plan, Calonne decided to gain prior approval of it from an Assembly of Notables—a group of hand-picked dignitaries hethought would sympathize with his views. But Calonne had badly miscalculated. Meeting in January 1787, the assembly refused to believe that a financial crisis reallyexisted. They had been influenced by Necker’s argument that state finances were sound and suspected that the monarchy was only...

• California - geography.

  The Basin and Range province is an arid area of mountain ranges, basins, and deserts. In California it is represented primarily by parts of the Great Basin and SonoranDesert sections. Within the Great Basin lies Death Valley, whose lowest elevation, 86 m (282 ft) below sea level, is the lowest point in the Western Hemisphere. TheSonoran Desert section is characterized by numerous flat plains separated by low but rugged ranges. It includes the extensive Mojave, or Mohave, Desert. Also in thisprov...

• California - USA History.

  The Basin and Range province is an arid area of mountain ranges, basins, and deserts. In California it is represented primarily by parts of the Great Basin and SonoranDesert sections. Within the Great Basin lies Death Valley, whose lowest elevation, 86 m (282 ft) below sea level, is the lowest point in the Western Hemisphere. TheSonoran Desert section is characterized by numerous flat plains separated by low but rugged ranges. It includes the extensive Mojave, or Mohave, Desert. Also in thisprov...

• France - country.

  In both the Paris and Aquitaine basins, fertile soils derived from limestone and wind-deposited dust, called loess, have supported prosperous agriculture since ancienttimes. Other lowlands in France are scattered and relatively small. They include the Alsace Plain in the east, bordering Germany, the valley of the Rhône River in thesoutheast, and the Languedoc Plain along the Mediterranean coast. A2 Uplands France contains several regions of uplands, the worn down remains of ancient mountain sys...

• Wars of Yugoslav Succession.

  of Yugoslav succession was over. In January 1992 members of the European Community (EC; after 1993 the European Union, EU) recognized Slovenia’s independencealong with that of Croatia. The United States and other countries did so shortly thereafter. IV THE WAR IN CROATIA As the Yugoslav army withdrew from Slovenia in July 1991, a second and far more serious conflict erupted in Croatia. But the road to war in Croatia began more than ayear earlier. In April and May 1990 the Croatian Democratic U...

• Wars of Yugoslav Succession .

  of Yugoslav succession was over. In January 1992 members of the European Community (EC; after 1993 the European Union, EU) recognized Slovenia’s independencealong with that of Croatia. The United States and other countries did so shortly thereafter. IV THE WAR IN CROATIA As the Yugoslav army withdrew from Slovenia in July 1991, a second and far more serious conflict erupted in Croatia. But the road to war in Croatia began more than ayear earlier. In April and May 1990 the Croatian Democratic U...

• Sun - astronomy.

  A The Sun’s Place in the Milky Way The Milky Way Galaxy contains about 400 billion stars. All of these stars, and the gas and dust between them, are rotating about a galactic center. Stars that arefarther away from the center move at slower speeds and take longer to go around it. The Sun is located in the outer part of the galaxy, at a distance of 2.6 × 10 17 km (1.6 × 10 17 mi) from the center. The Sun, which is moving around the center at a velocity of 220 km/s (140 mi/s), takes 250 million y...

• Prints and Printmaking I INTRODUCTION Prints and Printmaking, pictorial images that can be inked onto paper, and the art of creating and reproducing them.

  Bewick’s The SkylarkBritish engraver Thomas Bewick’s The Skylark is part of his History of British Birds (2 vols., 1797 and 1804). Bewick wasthe first artist to demonstrate the full potential of wood engraving and is renowned for his fine natural history illustrations.Each illustration shows some of the bird’s natural habitat.Folio Society, London/Bridgeman Art Library, London/New York Historically, the wood engraving was chiefly used for illustrations in magazines and books. It is similar to th...