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The 1903 Flyer was the first aircraft to make a successful, controllable, heavier-than-air flight. View the Flyer at different angles, the customized lightweight motor, and other related images for a complete picture of this breakthrough in aircraft…

A plant, Musa textilis, of the banana family that is valuable for its hard fiber (also known as Manila hemp) and native to Borneo and the Philippines. Abaca is one of the strongest of the hard fibers. Commercially, it is known as Manila hemp. The…

A family of adenosine triphosphate–binding cassette (ABC) transmembrane proteins that use energy to transport various molecules across extracellular and intracellular membranes (cytoplasmic membranes, endoplasmic reticulum, mitochondria, or…

A material of extreme hardness that is used to shape other materials by a grinding or abrading action. Abrasive materials may be used either as loose grains, as grinding wheels, or as coatings on cloth or paper. They may be formed into ceramic…

One of the major plant hormones that plays an important role in plant growth and development. Abscisic acid (ABA) is ubiquitous in vascular plants; the highest levels are found in young leaves and in developing fruits and seeds. Abscisic acid is…

The process whereby a plant sheds one of its parts. Leaves, flowers, seeds, and fruits are parts commonly abscised. Almost any plant part, such as very small buds and bracts to branches several inches in diameter, and scales or sheets of bark, may…

The temperature at which matter reaches a lowest energy state and all thermal activity theoretically ceases. Absolute zero is popularly conceived as the coldest possible temperature that a given system of matter, such as an atom, may reach.…

Either the taking up of energy from radiation by the medium through which radiation is passing, or the taking up of matter in bulk by other matter. A simple example of the absorption of energy is how sunlight warms a tree's leaves (see…

The net movement (transport) of water and solutes from outside an organism to its interior. The unidirectional flow of materials into an animal from the environment generally takes place across the alimentary tract, the lungs, or the skin, and in…

The reception of electromagnetic radiation by particles with electric charge, resulting in transference of the radiation's energy and momentum. Electromagnetic radiation is also referred to as electromagnetic waves, photons, and light. Photons are…

A mathematical entity consisting of a set of values (the carrier set) and a collection of operations that manipulate them. For example, the Integer abstract data type consists of a carrier set containing the positive and negative whole numbers and…

A class of the phylum Radiozoa in the Actinopoda. The kingdom Protozoa contains 18 phyla. One of the parvkingdoms (a hierarchical classification between kingdom and superphylum that is controversial and not officially recognized as such) is the…

An order of ectoparasitic annelid worms consisting of one species, Acanthobdella peledina, which feeds on the dermal tissues of freshwater salmonid fishes in the far regions of Northern Europe. The order Acanthobdellida is taxonomically odd in that…

A distinct phylum of helminths (parasitic worms), commonly known as the spiny- or thorny-headed worms. The phylum Acanthocephala comprises three classes: Archiacanthocephala, Palaeacanthocephala, and Eoacanthocephala (a suggested fourth class,…

A diverse group of early jawed fishes from the Paleozoic Era, usually classified as a distinct class of vertebrates. Most acanthodians have tiny body scales with a cross section like that of an onion, the layers reflecting incremental scale growth.…

A taxon of marine protozoans belonging to the Acantharea. Acanthometrida (also known as Arthracanthida) is a taxonomic grouping (sometimes considered as an order or subclass) placed within the Acantharea (Acantharia) taxon. All members are…

A taxon of arachnids (traditionally considered a subclass of the Arachnida), comprising the mites and ticks. The Acari (also Acarina) consist of two superorders of small to minute arachnids. They have two body regions (Fig. 1)—a headlike gnathosoma…

A mathematical technique, often done graphically, by which accelerations of parts of a mechanism are determined. In high-speed machines, particularly those that include cam mechanisms, inertial forces may be the controlling factor in the design of…

The technique of measuring the magnitude and direction of acceleration. Measurement of acceleration includes translational acceleration with tangential and radial components and angular acceleration. Acceleration Translation acceleration The…

A mechanical or electromechanical instrument that measures acceleration. The two general types of accelerometers measure either the components of translational acceleration or angular acceleration. Acceleration measurement Translational…

An impurity atom in a semiconductor which can accept or take up one or more electrons from the crystal and become negatively charged. An atom which substitutes for a regular atom of the material but has one less valence electron may be expected to…

A geminal diether (R1 = H). Ketals, considered a subclass of acetals, are also geminal diethers (R1 = C, aliphatic or aromatic). Acetals are (1) independent structural units or a part of certain biological and commercial polymers, (2) blocking or…

A naturally occurring quaternary ammonium cation ester, with the formula CH3(O)COC2H4N(CH3)3+, which plays a prominent role in nervous system function as a neurotransmitter. Acetylcholine (Fig. 1) is an organic compound released from certain…

An organic compound with the formula C2H2 or HCCH. The first member of the alkynes, acetylene is a gas with a narrow liquid range; the triple point is −81°C (−114°F). The heat of formation (ΔH°f ) is +227 kilojoules/mole, and acetylene is the most…

One of an important class of reactive organic compounds derived from acids via formal intermolecular dehydration; thus, acetic acid (1), on loss of water, forms acetic anhydride (2). Anhydrides of straight-chain acids containing from 2 to 12…

An order of actinopterygian (ray-finned) fishes represented by the sturgeons and paddlefishes. The order Acipenseriformes, the only extant order of about 10–12 orders of chondrosteans (subclass Chondrostei), probably originated from palaeonisciform…

A group of small, soft-bodied, ciliated worms with the form (“morph”) of acoel turbellarians. The Acoelomorpha is now ranked as a phylum, distinct from other turbellarian worms of the phylum Platyhelminthes. Its relation to other animals, however,…

An order of monocotyledonous angiosperms composed of a single genus, Acorus (sweet flag or sweet calamus), with two species widespread in the Northern Hemisphere. Formerly, Acorus was included in the aroid family, Araceae, but several lines of…

A method of nondestructive testing and materials characterization that uses mechanical waves moving through materials. It is similar to seismology, except in being concerned with the scale of engineering structures, such as aircraft, bridges, and…

An instrument that is sensitive to the interference of two or more acoustic waves. It provides information on acoustic wavelengths that is useful in determining the velocity and absorption of sound in samples of gases, liquids, and materials, and it…

Unwanted sound. Noise control is the process of obtaining an acceptable noise environment for people in different situations. These definitions and the words “unwanted” and “acceptable” suggest that criteria need to be established to determine when…

The net pressure exerted on a surface or interface by an acoustic wave. One might presume that the back-and-forth oscillation of fluid caused by the passage of an acoustic wave will not exert any net force on an object, and this is true for sound…

An autonomous undersea vehicle that can be launched from submarines, surface ships, or aircraft to attack enemy submarines and surface ships. An acoustic torpedo is a sophisticated weapon. Its main components are a guidance and control system, a…

The recording of sound waves in a two-dimensional pattern (the hologram) and the use of the hologram to reconstruct the entire sound field throughout a three-dimensional region of space. Acoustical holography, which first appeared in the 1960s in…

The science of sound, which in its most general form endeavors to describe and interpret the phenomena associated with motional disturbances from equilibrium of elastic media. An elastic medium is one such that if any part of it is displaced from…

The field of science and technology that is concerned with the diffraction of visible or infrared light (usually from a laser) by high-frequency sound in the frequency range of 50–2000 MHz. The term “acousto” is a historical misnomer; sound in this…

A viral disease of humans caused by the human immunodeficiency virus (HIV), which attacks and compromises the body's immune system. Individuals infected with the human immunodeficiency virus (HIV) proceed through a spectrum of stages that ultimately…

An induced state in which antigens originally regarded as foreign become regarded as self by the immune system. Tolerance can be induced (tolerization) in all of the cells of the immune system, including T cells (also known as T lymphocytes), the…

A subtaxon of burrowing barnacles in the subclass Thecostraca. All members of the Acrothoracica burrow into shells of mollusks and thoracican barnacles, echinoderm tests, bryozoans, dead coral, and limestone. The entrance to the burrow is slitlike.…

An order of inarticulated brachiopods. Together with Lingulida and Siphonotretida, Acrotretida is one of the three orders of inarticulated brachiopods currently classified in class Lingulata in subphylum Linguliformea (phylum Brachiopoda).…

An explosive, poisonous, flammable liquid, boiling at 77.3°C (171°F), partly soluble in water. The formula CH2 CH C N indicates it may be regarded as vinyl cyanide, and its systematic name is 2-propenonitrile. Acrylonitrile is prepared by…

An order of the class Anthozoa, commonly known as the sea anemones. Actiniaria contains approximately 1100 species. They are strictly marine, occurring from northern to southern polar latitudes and inhabiting regions that range from the intertidal…

The series of elements beginning with actinium (atomic number 89) and including thorium, protactinium, uranium, and the transuranium elements through the element lawrencium (atomic number 103). These elements, chemically similar, have a strong…

A chemical element, Ac, atomic number 89, and atomic weight 227.0. Actinium was discovered by A. Debierne in 1899. Milligram quantities of the element are available by irradiation of radium in a nuclear reactor. Actinium-227 is a beta-emitting…

A genus of gram-negative, immotile and nonspore-forming, oval to rod-shaped, often pleomorphic bacteria which occur as parasites or pathogens in mammals (including humans), birds, and reptiles. They are facultatively aerobic, capable of fermenting…

A heterogeneous collection of bacteria with diverse ecologies and physiologies. The initial definition was based on their branching filamentous cellular morphology during a stage of the growth cycle. The name actinomycete means “ray fungus,” and for…

An order of Heliozoia. These protozoans are characterized by stiff arms radiating from the central cell mass. Each arm is supported internally by an array of microtubules which terminate on the surface of nuclei. The organisms are found in…

A class of Sarcodina; or in some modern schemes a superclass (Actinopoda) within the phylum Sarcomastigophora. Some of these protozoans have more or less permanent pseudopodia, composed of a central shaft of microtubules surrounded by a thin…

A class of teleostom fishes commonly known as the ray-finned fishes, which are distinguished by the structure of the paired fins that are supported by dermal rays. The class Actinopterygii comprises the subclasses Cladistia [Polypteriformes (bichirs…

A powdered, granular, or pelleted form of amorphous carbon characterized by very large surface area per unit volume because of an enormous number of fine pores. Activated carbon is capable of collecting gases, liquids, or dissolved substances on the…

A technique in which a neutron, charged particle, or gamma photon is captured by a stable nuclide to produce a different, radioactive nuclide which is then measured. The technique is specific, highly sensitive, and applicable to almost every element…

The modification of sound fields by using additional sound transmitted from loudspeakers. The signals from the loudspeakers are controlled electronically by using digital signal processing techniques. Physical principles The physical principle…

The activity of a substance is a thermodynamic property that is related to the chemical potential of that substance. Activities are closely related to measures of concentration, such as partial pressures and mole fractions, and are more convenient…

A specific system or procedure for determining accurate manufacturing costs in order to achieve profitability goals. Traditional accounting systems allocate overhead costs by using a volume-oriented base, such as direct labor hours or direct…

A process in which a hydrogen atom in an organic compound is replaced by an acyl group (RCO where R = an organic group). The reaction involves substitution by a nucleophile (electron donor) at the electrophilic carbonyl group (CO) of a carboxylic…

A characteristic of an organism that makes it fit for its environment or for its particular way of life. Adaptation is a key biological process in organisms and can take many forms. Typically, adaptation refers to two mechanisms: (1) an adjustment…

A special type of nonlinear control system which can alter its parameters to adapt to a changing environment. The changes in environment can represent variations in process dynamics or changes in the characteristics of the disturbances.Nonlinear…

An approach to management of natural resources that emphasizes how little is known about the dynamics of ecosystems and that, as more is learned, management will evolve and improve. Adaptive management is a systematic strategy that seeks to improve…

The science of optical systems in which a controllable optical element, usually a deformable mirror, is used to optimize the performance of the system, for example, to maintain a sharply focused image in the presence of wavefront aberrations. A…

Signal processing is a discipline that deals with the extraction of information from signals. The devices that perform this task can be physical hardware devices, specialized software codes, or combinations of both. In recent years the complexity of…

A wing is the primary lift-generating surface of an aircraft. Adaptive wings—also known as smart, compliant, intelligent, morphing, controllable, and reactive wings—are lifting surfaces that can change their shape in flight to achieve optimal…

Habituation to a specific practice, such as drinking alcoholic beverages or using drugs (for example, heroin, cocaine, or amphetamines), and disorders characterized by the chronic use of a drug, alcohol, or similar substance. Addiction and addictive…

One of the four fundamental operations of arithmetic and algebra. The symbol + of addition is thought to be a ligature for “et,” used in a German manuscript of 1456 to denote addition. Its first printed appearance is in Johann Widman's Behennede und…

A type of defective virus particle in which part of the genetic material of papovavirus SV40 is encased within an adenovirus protein coat (capsid). Human adenoviruses require human cells for their propagation; however, papovavirus SV40 can serve as…

A family of double-stranded DNA viruses, collectively known as adenoviruses, which are icosahedrally symmetric, usually found in the respiratory tract of the host species, and often associated with respiratory diseases. Members of the Adenoviridae…

A material capable of fastening together two other materials by means of surface attachment. The terms glue, mucilage, mastic, and cement are synonymous with adhesive. In a generic sense, an adhesive is any material capable of fastening by means of…

The process of using an adhesive to manufacture an assembly. In this article, the adhesive-bonded assembly will be called an adhesive joint, and the materials to which the adhesive adheres will be called the adherends. The adhesive bonding process…

The removal or diminution of a magnetic field applied to a magnetic substance when the latter has been thermally isolated from its surroundings. The process concerns paramagnetic substances almost exclusively, in which case a drop in temperature of…

A thermodynamic process in which the system undergoing the change exchanges no heat with its surroundings. An increase in entropy or degree of disorder occurs during an irreversible adiabatic process. However, reversible adiabatic processes are…

The ratio of the current to the voltage in an alternating-current circuit.In terms of complex current I and voltageV, the admittance of a circuit is given by Eq. (1), and is related to the impedance of the circuit Z by Eq. (2). Y is a complex number…

A complex endocrine organ in proximity to the kidney. Adrenal gland tissue is present in all vertebrates from cyclostomes to placental mammals. The adrenal consists of two functionally distinct tissues: steroidogenic cells of mesodermal origin and…

Malfunctions of the paired adrenal glands. The adrenal glands can have congenital defects and can be damaged by infections and destructive tumors. Anencephaly, that is, gross underdevelopment of the brain in fetal life, leads to hypoplasia of the…

A process in which atoms or molecules move from a bulk phase (that is, solid, liquid, or gas) onto a solid or liquid surface. An example is purification by adsorption where impurities are filtered from liquids or gases by their adsorption onto the…

Processes for separation of gases based on the adsorption effect. When a pure gas or a gas mixture is contacted with a solid surface, some of the gas molecules are concentrated at the surface due to gas-solid attractive forces, in a phenomenon known…

The force exerted on a body whenever there is a relative velocity between the body and the air. There are only two basic sources of aerodynamic force: the pressure distribution and the frictional shear stress distribution exerted by the airflow on…

The force retarding an airplane, especially in supersonic flight, as a consequence of the formation of shock waves. Although the physical laws governing flight at speeds in excess of the speed of sound are the same as those for subsonic flight, the…

The applied science that deals with the dynamics of airflow and the resulting interactions between this airflow and solid boundaries. The solid boundaries may be a body immersed in the airflow (Fig. 1), or a duct of some shape through which the air…

A bacterial genus in the family Vibrionaceae comprising oxidase-positive, facultatively anaerobic, monotrichously flagellated gram-negative rods. The mesophilic species are A. hydrophila, A. caviae, and A. sobria; the psychrophilic one is A.…

A branch of engineering concerned primarily with the special problems of flight and other modes of transportation involving a heavy reliance on aerodynamics or fluid mechanics. The main emphasis in aeronautical engineering is on airplane and missile…

Aeronomy concentrates on the regions above the tropopause or upper part of the atmosphere. The region of the atmosphere below the tropopause is the site of most of the weather phenomena that so directly affect all life on the planet; this region has…

A suspension of small particles in a gas. The particles may be solid or liquid or a mixture of both. Aerosols are formed by the conversion of gases to particles, the disintegration of liquids or solids, or the resuspension of powdered material.…

Any of a group of secondary metabolites produced by the common molds Aspergillus flavus and A. parasiticus that cause a toxic response in vertebrates when introduced in low concentration by a natural route. The group constitutes a type of mycotoxin.…

A highly fatal insect-borne viral disease of horses and mules, and a mild, subclinical disease in donkeys and zebras. African horse sickness occurs in sub-Saharan Africa, but incursions outside Africa have been documented. Dogs may be affected by…

A major constituent of the cell walls of certain red algae, especially members of the families Gelidiaceae and Gracilariaceae. Extracted for its gelling properties, it is one of three algal polysaccharides of major economic importance, the others…

A variety of chalcedonic quartz that is distinguished by the presence of color banding in curved or irregular patterns (Fig. 1). Most agate used for ornamental purposes is composed of two or more tones or intensities of brownish-red, often…

A reaction in which suspended particles are aggregated or clumped. It occurs upon the admixture of another type of particle, a change in the composition of the suspending fluid, or the addition of a soluble agent that acts as a bridge between two or…

A substance that will cause a clumping of particles such as bacteria or erythrocytes. Of major importance are the specific or immune agglutinins, which are antibodies that will agglutinate bacteria containing the corresponding antigens on their…

Behavior that is intended to threaten or inflict physical injury or emotional harm on another person or organism. Aggression is a type of behavior (physical or verbal, or both) displayed by an individual who aims or is motivated to harm another…

An age-contingent reduction in physiological function and activity, with a concomitant increase in mortality rate and a diminution in reproductive rate. Aging is the process of becoming older, which is influenced by genetic and environmental…

Any disorder of recognition. Agnosias were named originally by Sigmund Freud in 1891 to denote disturbances in the ability to recognize and name objects. Usually the misrecognition involves one sensory modality, with primary sensation in that…

A large, rabbit-sized rodent, belonging to the family Dasyproctidae and inhabiting South America and Central America, which is well adapted for speed because of its elongated hind legs. The agouti is a large rodent that resembles the rabbit or hare…

Aircraft specifically designed or adapted from a general utility airframe for use in agriculture and forestry and for control of insect vectors of human, animal, and plant diseases. Agricultural aircraft (Fig. 1) have become an indispensable tool…

A discipline concerned with solving the engineering problems of providing food and fiber for the people of the world. These problems include designing agricultural machinery to work the soil and harvest the crops, as well as developing water…

The science that deals with the selection, breeding, nutrition, and management of domestic animals for economical production of meat, milk, eggs, wool, hides, and other animal products. The broad field of agricultural science focuses on two realms:…

The techniques and methods used in plowing, harrowing, planting, tilling, harvesting, drying, storing, and processing of agricultural crops. Agricultural soil and crop practices seek to keep the quality and fertility of soils at high levels so as to…

A model for the functioning of an agricultural system, with all inputs and outputs. An ecosystem may be as small as a set of microbial interactions that take place on the surface of roots, or as large as the globe. An agroecosystem may be at the…

The science and study of crops and soils. Agronomy is the umbrella term for a number of technical research and teaching activities (see illustration): crop physiology and management, soil science, plant breeding, and weed management frequently are…

The predicted effect of an electromagnetic vector or scalar potential in electron interference phenomena, in the absence of electric or magnetic fields on the electrons.The fundamental equations of motion for a charged object are usually expressed…

The hinged rear portion of an aircraft wing, moved differentially on each side of the aircraft to obtain lateral or roll control moments. The angular settings of the ailerons are controlled by the human or automatic pilot through the flight control…

The control of certain environmental conditions including air temperature, air motion, moisture level, radiant heat energy level, dust, various pollutants, and microorganisms.Comfort air conditioning refers to control of spaces to promote the…