contra Member of a counterrevolutionary force that sought to overthrow Nicaragua’s left-wing Sandinista government. The original contras had been National Guardsmen during the regime of Anastasio Somoza (see Somoza family). The U.S. Central Intelligence Agency played a key role in training and funding the group, whose tactics were decried by the inter¬ national human-rights community. In 1984 the U.S. Congress banned military aid to the contras; the efforts of the administration of U.S. presi¬ dent Ronald Reagan to circumvent the ban led to the Iran-Contra Affair. A general peace in the region was negotiated by Costa Rican president Oscar Arias SAnchez, and in 1990 Nicaraquan president Violeta Chamorro negotiated the contras’ demobilization. See also Daniel Ortega.
contraception Birth control by prevention of conception or impreg¬ nation. The most common method is sterilization. The most effective tem¬ porary methods are nearly 99% effective if used consistently and correctly. Many methods carry health risks; barrier devices and avoidance of inter¬ course during the most fertile period are safest. Hormonal contraceptives use estrogen and/or progesterone to inhibit ovulation. The “morning-after pill” (high-dose hormones) is effective even after intercourse. The most serious side effect of oral contraceptives is the risk of blood-clotting dis¬ orders. Intrauterine devices (IUDs) are placed inside the uterus and appear to cause a mild endometrial inflammation that either inhibits fertilization or prevents a fertilized egg from implanting. Certain types were taken off the market in the 1970s and ’80s when it was found that their side effects included a high incidence of pelvic inflammatory disease, ectopic pregnancy, and spontaneous septic abortion. Barrier devices, such as condoms, dia¬ phragms, cervical caps, female condoms (vaginal pouches), and vaginal sponges, prevent sperm from entering the uterus. Condoms also prevent sexually transmitted disease. Used with spermicides, condoms are nearly 100% effective. Fertility awareness techniques have evolved from keep¬ ing track of the menstrual cycle (the so-called “rhythm method”; see men¬ struation) to avoid intercourse around the time of ovulation; tracking body temperature and cervical mucus consistency can raise effectiveness to more than 80%. Experimental forms of birth control include an oral con¬ traceptive for men.
contract Agreement between two or more parties that creates for each party a duty to do something (e.g., to provide goods at a certain price according to a specified schedule) or a duty not to do something (e.g., to divulge an employer’s trade secrets or financial status to third parties). A party’s failure to honour a contract allows the other party or parties to bring an action for damages in a court of law, though arbitration may also be pursued in an effort to keep the matter confidential. In order to be valid, a contract must be entered into both willingly and freely. A contract that violates this principle, including one made with a legal minor or a per¬ son deemed mentally incompetent, may be declared unenforceable. A contract also must have a lawful objective.
contralto See alto
contrast medium Substance comparatively opaque to X-rays, which appears lighter on X-ray film and allows a body structure that does not normally contrast with its background to be seen clearly on the film. Common contrast media include barium sulfate and iodized organic com¬ pounds. They are given by the route that introduces them into the struc¬ ture to be examined—swallowed or as an enema for the digestive tract, inhaled for the respiratory tract, or injected for blood vessels and for organs and tissues they supply. Serious reactions to contrast media are not infrequent. See also diagnostic imaging.
contredanse See country dance
Contreras \kon-'tra-ras\, Battle of (Aug. 19-20, 1847) Decisive engagement between U.S. and Mexican troops in the Mexican War. On its march to Mexico City, the U.S. force under Winfield Scott was blocked by a Mexican force and detoured via a difficult road across lava beds held by another Mexican force. After an engagement that lasted less than 20 minutes, Scott’s troops gained control of several roads to Mexico City. They then captured the main Mexican army of Antonio Santa Anna at Churubusco.
contributory negligence In law, behaviour that contributes to one’s own injury or loss and fails to meet the standard of prudence that one should observe for one’s own good. Contributory negligence of the plain¬ tiff is frequently pleaded in defense to a charge of negligence. In Engish law and in the law of many U.S. states, if the plaintiff is shown to have contributed through negligence to his own injury, recovery may still be allowed, but provision is made for an equitable reduction of damages. Contributory negligence has been criticized by some authorities because it effectively excuses one party (the defendant) even though both were negligent.
control system Means by which a set of variable quantities is held constant or caused to vary in a prescribed way. Control systems are inti¬ mately related to the concept of automation but have an ancient history. Roman engineers maintained water levels in aqueducts by means of float¬ ing valves that opened and closed at appropriate levels. James Watt’s fly- ball governor (1769) regulated steam flow to a steam engine to maintain constant engine speed despite a changing load. In World War II, control- system theory was applied to anti-aircraft batteries and fire-control sys¬ tems. The introduction of analog and digital computers opened the way for much greater complexity in automatic control theory. See also Jac¬ quard LOOM, PNEUMATIC DEVICE, SERVOMECHANISM.
control theory Field of applied mathematics relevant to the control of certain physical processes and systems. It became a field in its own right in the late 1950s and early ’60s. After World War II, problems arising in engineering and economics were recognized as variants of problems in dif¬ ferential equations and in the calculus of variations, though they were not covered by existing theories. Special modifications of classical techniques and theories were devised to solve individual problems, until it was rec¬ ognized that these seemingly diverse problems all had the same math¬ ematical structure, and control theory emerged. See also control system.
convection Process by which heat is transferred by movement of a heated fluid such as air or water. Most fluids expand when heated. They become less dense and more buoyant, and so rise. The heated molecules eventually cool, become more dense, and sink. This repeated process sets up convection currents that account for the uniform heating of the air in a room or water in a kettle. Air convection can be forced by a fan, and water convection by a pump. Atmospheric convection currents can be set up by local heating effects such as solar radiation or contact with cold surfaces. Such currents are usually vertical and account for atmospheric phenomena such as clouds and thunderstorms.
convection See mass flow
Convention, National See National Convention
convention, political See political convention
convergence Mathematical property of infinite series, integrals on unbounded regions, and certain sequences of numbers. An infinite series is convergent if the sum of its terms is finite. The series Vi + l A + Vs + Vi6 + Via. + ... sums to 1 and thus is convergent. The harmonic series 1 + Vi + l A + l A + Vs + ... does not converge. An integral calculated over an inter¬ val of infinite width, called an improper integral, describes a region that is unbounded in at least one direction. If such an integral converges, the unbounded region it describes has finite area. A sequence of numbers converges to a particular number when the difference between successive terms becomes arbitrarily small. The sequence 0.9, 0.99, 0.999, etc., con¬ verges to 1.