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Educational resources: Sample articles
 oil paint
Painting medium in which ground pigment (colour) is bound with oil, usually linseed. It has the advantage of being slow to dry and therefore reworkable. Oil paint can be applied with a brush or a palette knife to a prepared ground, usually stretched canvas.
Oil paint was used for decorating houses as early as the 5th century, but its artistic application was gradually adopted and refined by artists such as the 15th-century Flemish painter Jan van Eyck. Passing from Flanders to Italy, oil paint quickly overtook tempera as the standard medium because of its flexibility and luminosity. Since the 16th century oil paint has been considered first among painting media, although acrylic paint may prove to be a rival because of its quick drying time.
Techniques of the old masters
One of the most elaborate techniques using oil paints, favoured by the old masters, employed the building up of colours in layers from dark to lighter tones. The paint was applied in a way that allowed the lower layers to show through the upper layers to a certain extent. Transparent glazes were also added to give further gradations of tone. The old masters had to rely on extensive knowledge of the different chemical qualities of the paints because, if they were not carefully applied, the layers of paint could damage those around or next to them over a period of time.
Netherlandish technique
Oil painting first developed in a distinct form in the Netherlands and Germanic lands during the 14th century, and with van Eyck attained a brilliance that led to the belief that he possessed some secret process. His method, and the Netherlandish method in general, was to paint transparently on a white gesso ground, the picture being smoothly finished piece by piece, with a luminous enamel-like result. The method was taken up in Italy, Antonello da Messina being a pioneer.
Renaissance techniques
In Venice, Giovanni Bellini rivalled the brilliance of Flemish technique. A combination of a tempera underpainting with a final glaze of oil colour was a Renaissance technique used by Michelangelo. In the 16th and 17th centuries there was a further development of technique, and a desire to obtain greater depth and three-dimensional effect. The transparent method, still in essence that of the tempera painter, was replaced by a more elaborate process. The picture was first painted in monochrome, and on this basis the light parts were painted with thick opaque colour, the shadows being painted thinly. Successive glazes of transparent colour gave richness. The new phase of oil painting flourished in Venice, and Titian represents its perfection. Rembrandt, Rubens, and Velázquez give individual variants of what may be called the classic method.
Modern techniques
A decisive change came with the 19th century, when painters such as the Impressionists abandoned the old-master process of building up a picture in stages, in favour of a direct mode of painting. In part this was due to the practice of painting from nature, which made swiftness of achievement necessary.
Oil painting has remained particularly responsive to individual treatment. For instance, the later work of Paul Cézanne, with its application of transparent colour (almost like watercolour) to represent delicate modifications of light on the form of an object, can be contrasted with the heavily loaded paint of Vincent van Gogh, who used the medium with an emotional violence.
 mutation
In biology, a change in the genes produced by a change in the DNA that makes up the hereditary material of all living organisms. It can be a change in a single gene or a change that affects sections of chromosomes. In the process of DNA replication, which takes place before any cell divides, the two halves of DNA separate and new halves are made. Because of specific base pairing, the inherited information is copied exactly. Despite this, rarely, a mistake occurs and the sequence of bases is altered. This changes the sequence of amino acids in a protein. This is mutation, the raw material of evolution. The consequences of mutation are varied. Due to the redundancy built into genetic code many mutations have no effect upon DNA functions. Genes describe how to make proteins. As a result of mutation a protein may not be produced, may be produced but act abnormally, or remain fully functional. Only a few mutations improve the organism's performance and are therefore favoured by natural selection. Mutation rates are increased by certain chemicals and by ionizing radiation.
Common mutations include the omission or insertion of a base (one of the chemical subunits of DNA); these are known as point mutations. Larger-scale mutations include removal of a whole segment of DNA or its inversion within the DNA strand. Not all mutations affect the organism, because there is a certain amount of redundancy in the genetic information. If a mutation is 'translated' from DNA into the protein that makes up the organism's structure, it may be in a non-functional part of the protein and thus have no detectable effect. This is known as a neutral mutation, and is of importance in molecular clock studies because such mutations tend to accumulate gradually as time passes. Some mutations do affect genes that control protein production or functional parts of protein, and most of these are lethal to the organism.
 chromatography
Greek chromos 'colour'
Technique for separating or analysing a mixture of gases, liquids, or dissolved substances. This is brought about by means of two immiscible substances, one of which (the mobile phase) transports the sample mixture through the other (the stationary phase). The mobile phase may be a gas or a liquid; the stationary phase may be a liquid or a solid, and may be in a column, on paper, or in a thin layer on a glass or plastic support. The components of the mixture are adsorbed or impeded by the stationary phase to different extents and therefore become separated. The technique is used for both qualitative and quantitive analyses in biology and chemistry.
In paper chromatography, the mixture separates because the components have differing solubilities in the solvent flowing
through the paper and in the chemically bound water of the paper.
In thin-layer chromatography, a wafer-thin layer of adsorbent medium on a glass plate replaces the filter paper. The mixture separates because of the differing solubilities of the components in the solvent flowing up the solid layer, and their differing tendencies to stick to the solid (adsorption). The same principles apply in column chromatography.
In gas–liquid chromatography, a gaseous mixture is passed into a long, coiled tube (enclosed in an oven) filled with an inert powder coated in a liquid. A carrier gas flows through the tube. As the mixture proceeds along the tube it separates as the components dissolve in the liquid to differing extents or stay as a gas. A detector locates the different components as they emerge from the tube. The technique is very powerful, allowing tiny quantities of substances (fractions of parts per million) to be separated and analysed.
Preparative chromatography is carried out on a large scale for the purification and collection of one or more of a mixture's constituents; for example, in the recovery of protein from abattoir wastes.
Analytical chromatography is carried out on far smaller quantities, often as little as one microgram (one-millionth of a gram), in order to identify and quantify the component parts of a mixture. It is used to determine the identities and amounts of amino acids in a protein, and the alcohol content of blood and urine samples. The technique was first used in the separation of coloured mixtures into their component pigments.
 ballot
Italian ballotta, diminutive of balla, 'a ball'
The process of voting in an election. In political elections in democracies ballots are usually secret: voters indicate their choice of candidate on a voting slip which is then placed in a sealed ballot box. Ballot rigging is a term used to describe elections that are fraudulent because of interference with the voting process or the counting of votes. Under the Athenian and other democracies in ancient Greece small balls were used for secret voting; this method was known as psephisma (from psephos, a pebble, the original instrument of balloting). It was used on all questions affecting the status of individuals. These were of two kinds: lawsuits, and proposals of ostracism against a politician, which carried a sentence of exile for ten years. In Rome the system of open suffrage allowed so much corruption and intimidation that a series of laws (139–107 BC) prescribed the use of the ballot for all business transacted in the popular assemblies.
Secret voting at elections of members of Parliament was advocated by English reformers in the early 19th century; it was included in the draft of the Reform Bill of 1832, a bill on the subject having been introduced by Daniel O'Connell in 1830. It was also among the demands of Chartism (a mainly working class democratic movement), and frequent attempts were made to introduce it between 1832 and 1872. The secret ballot was first used in England in 1870, in the London School Board elections. Two years later, William Forster's Ballot Act introduced it into parliamentary and municipal elections. Almost all countries holding elections now use the secret ballot.
 adhesive
Substance that sticks two surfaces together. Natural adhesives (glues) include gelatin in its crude industrial form (made from bones, hide fragments, and fish offal) and vegetable gums. Synthetic adhesives include thermoplastic and thermosetting resins, which are often stronger than the substances they join; mixtures of epoxy resin and hardener that set by chemical reaction; and elastomeric (stretching) adhesives for flexible joints. Superglues are fast-setting adhesives used in very small quantities.
Natural water-based adhesives
Typical natural substances used for water-based adhesives are starch, casein, tree exudates, skin, and bones. Starch, usually provided from maize, potato, tapioca, or sago, is extracted from the vegetable matter by disintegration and extraction with cold water. Dextrin is prepared from starch by roasting it in the presence of acid or by acid hydrolysis. Its adhesive properties vary widely with the source of the starch and the degree of hydrolysis.
Casein is prepared from milk by precipitating its protein with acid or rennet. After further purification the casein is dissolved in a solution of alkali or urea to form a strong adhesive.
Among tree exudates, natural rubber latex is obtained from incisions in the bark of Hevea brasiliensis. Gum arabic (acacia) and gum tragacanth are solid exudates from Acacia leguminosae and Astragalus leguminosae respectively. These
natural substances (except latex) are brittle when dry and support mould growth when wet. Adhesives are formulated from them by the additions of water, plasticizers, fungicides, tack, and wetting agents. Their adhesive action is due to the formation of physico-chemical bonds and penetration to give mechanized keying. Setting and hardening of this type of adhesive depends on loss of water; therefore they are mainly used for joining porous substrates such as paper, board, and wood. As a group these natural adhesives are less waterproof and strong, but cheaper than the synthetic-resin-based adhesives.
Natural non-water-based adhesives
Bitumen, derived from asphalt; shellac, produced by parasitic tree insects; and resin, an exudate from pine
trees, are examples of naturally occurring resins used both as hot melt adhesives and as spirit-based cements. The marine bacterium Shewanell colwellii secretes a natural glue, PAVE (polysaccharide adhesive viscous exopolymer), in large quantities. Because PAVE can be used in wet conditions and is resistant to sea water, it was being developed commercially in 1994 as a sealant for ships' hulls.
Synthetic adhesives
Many modern adhesives are based on the increasing number of synthetic resins available. Thanks to the chemists' closer control over these polymers, a wider variety of type and nomenclature of adhesives has evolved. Definitions, common uses, and the names of some resins used in these adhesives are listed below.
Solution adhesives are resins dissolved in a volatile organic solvent, used for bonding porous materials. They include natural and synthetic rubbers, nitrocellulose, polyvinyl acetate, and polymethyl methacrylate.
Emulsion adhesives are resins dispersed in an aqueous base, used for bonding porous materials. Natural and synthetic rubbers, polyvinyl acetate, and polymethyl methacrylate are used.
Contact adhesives are emulsions or solutions formulated to bond impervious materials. Both faces are covered with adhesive and allowed to dry before being brought into contact. They are used for bonding plastics, sticking rubber shoe soles, and self-seal envelopes.
Pressure-sensitive adhesives are used on tapes or sheet material, sometimes with a nonstick backing paper. Applied pressure forms a bond. Some adhesives are formulated for ease and cleanliness of removal (permanently tacky adhesive), and some to give a permanent bond. The materials used include modified natural and synthetic rubbers and polyisobutylene. Thermoplastic adhesives are solventless adhesives, softened by the application of heat before bonding. They may be remelted after bonding and are used for high-speed packaging, labelling, and unsewn bookbinding. Polyamides, polyvinyl acetate, and its copolymers are used.
Thermosetting adhesives are solventless, and are cured by heat to form a bond that, once cured, cannot be resoftened by heat. They are used for exterior plywood and for bonding brake linings to shoes. They are made from phenolic and epoxy groups of resins.
Two-part or chemical-cure adhesives consist of resin and hardener, which are mixed together shortly before use and set by chemical action without the necessity for the application of heat. They are used for bonding aluminium alloys in the aircraft industry and varied domestic applications. Resins used include epoxypolyamide and resorcinol.
Structural adhesives are adhesives of high strength, toughness, and creep resistance, used for bonding load-bearing members. They are mainly confined to chemical-cure and thermosetting adhesives.
 sonnet
Genre of 14-line poem of Italian origin introduced to England by English poet Thomas Wyatt in the form used by Italian poet Petrarch and followed by English poets John Milton and William Wordsworth; English playwright and poet William Shakespeare wrote 14-line sonnets consisting of three groups of four lines (quatrains) and two final rhyming lines (a couplet), following ] the rhyme scheme abab cdcd efef gg.
The sonnet was very popular in Elizabethan literature and some of the finest lyric poetry of the period was written in the sonnet form. The sonnet sequence enjoyed a vogue during the 1590s, when several remarkable collections appeared, including Astrophel and Stella (1591) by English poet Sir Philip Sidney, Delia (1592) by English poet Samuel Daniel, and Amoretti (1595) by English poet Edmund Spenser. It was during this period that Shakespeare wrote his sonnet sequence.
Sonnet form
In a Petrarchan sonnet, the difference in the rhyme scheme of the first eight lines (the octet) and the last six (the sestet) reflected a change in mood or direction. The rhyme scheme generally follows the pattern abba abba cdcdcd or cdecde). The first four lines of a Shakespearean sonnet set out the theme or 'argument'; the next two quatrains develop variations on this theme (sometimes with a sharp change of mood between the two quatrains); and the final couplet presents a resolution. The sonnet form has continued into 20th-century literature; of particular note are examples by the English World War I poet Wilfred Owen, in which he uses formal sonnet forms to discipline the portrayal of extreme horror and explicit violence.
Shakespeare's sonnets
The identity of the man and woman addressed in Shakespeare's sonnets (if they are more than mere literary conventions) is still uncertain, though many ingenious solutions have been proposed. Though each sonnet can be read as an independent poem, the sequence develops a broad narrative. In the first group (1–126) the poet begins by expressing his love and affection for the young man. Gradually a note of uncertainty enters: the young man seems not to return the poet's love and is even disloyal. By the end of this group, however, the poet and the young man are reconciled. In the second group (127–54) the poet addresses a woman who is now known as the 'Dark Lady'. This relationship is more complex and troubled: the poet frequently accuses the Dark Lady of unfaithfulness (with the young man of the first group as well as others) and the tone of several of the sonnets is bitter and cynical.
 map projection
Way of showing the Earth's spherical surface on a flat piece of paper. The most common approach has been to redraw the Earth's surface within a rectangular boundary. The main weakness of this is that countries in high latitudes are shown disproportionately large. The most famous cylindrical projection is the Mercator projection, which dates from 1569. Although it gives an exaggerated view of the size of northern continents, it is the best map for navigation because a constant bearing appears as a straight line.
In 1973 German historian Arno Peters devised the Peters projection in which the countries of the world retain their relative areas. In other projections, lines of longitude and latitude appear distorted, or the Earth's surface is shown as a series of segments joined along the Equator. In 1992 the US physicist Mitchell Feigenbaum devised the optimal conformal projection, using a computer program designed to take data about the boundary of a given area and calculate the projection that produces the minimum of inaccuracies.
The theory behind traditional map projection is that, if a light were placed at the centre of a transparent Earth, the surface features could be thrown as shadows on a piece of paper close to the surface. This paper may be flat and placed on a pole (azimuthal or zenithal), or may be rolled around the Equator (cylindrical), or may be in the form of a tall cone resting on the Equator (conical). The resulting maps differ from one another, distorting either area or direction, and each is suitable for a particular purpose. For example, projections distorting area the least are used for distribution maps, and those with least distortion of direction are used for navigation charts.
 Ribbentrop–Molotov pact
or Nazi–Soviet pact
Non-aggression treaty signed by Germany and the USSR on 23 August 1939. The pact is named after the German foreign minister Joachim von Ribbentrop and Russian foreign minister Vyacheslav Molotov, working under German Nazi dictator Adolf Hitler and Soviet dictator Joseph Stalin respectively. Under the terms of the treaty both countries agreed to remain neutral and to refrain from acts of aggression against each other if either went to war. Secret clauses allowed for the partition of Poland – Hitler was to acquire western Poland, Stalin the eastern part. On 1 September 1939 Hitler invaded Poland. The pact ended when Hitler invaded Russia on 22 June 1941 during World War II.
Nazi aims
The two sides had their own reasons for the pact. The Nazis wanted to ensure that their planned takeover of Poland would not be opposed by the USSR. Hitler did not want to ally himself to his ideological enemy in the long term; he simply wanted to buy himself time as he pursued his policy of Lebensraum, a theory that he had developed for the expansion of Germany into Eastern Europe. Hitler always intended to attack the USSR eventually, but with a Nazi–Soviet pact Germany was able to operate in a controlled environment in Europe, safe in the knowledge that the USSR would not interfere with their expansion plans or war with France and the UK.
Soviet aims
The USSR was able to fulfil some of its goals as well. Offended by the negative attitude of the Allies towards the USSR Stalin was able to strike fear into the UK and France. Stalin felt no reason to help the British or French in their increasingly dangerous
struggle with Hitler. The British and French had provided help in 1920 for the Tsarist 'Whites' during the Russian civil war (1918–21) to try to overthrow the Soviet Bolshevik regime. Although the USSR was eventually allowed into the League of Nations in 1934, they never felt fully accepted or trusted by the western nations. Stalin negotiated with the UK and France in the summer of 1939, but the British and French would not accept Soviet demands to annex the Baltic States or to be allowed to send troops into Poland if Germany attacked the Poles. Stalin felt abandoned by the British and French and, aware that the German policy of Lebensraum threatened Soviet territory, he concluded that the USSR would be better off negotiating a pact with Germany than waiting for the UK and France to accept his terms.
Reaction to the Nazi–Soviet pact
The Ribbentrop–Molotov pact was a pact of convenience that suited both sides at the time. It was an unexpected product of a specific moment in history when two nations were able to offer each other something that they could not obtain elsewhere, while still maintaining mutual dislike and distrust. However, the pact shocked the world. The idea that the strongly anticommunist German Nazis and the equally strongly anti-Nazi Soviet communists would sign what amounted to an alliance was totally unexpected. The USSR was diplomatically isolated by the world's other major powers, the UK, France, and the USA. Many politicians in the UK and France had been prepared to pursue a policy of appeasement of Hitler because he was so vehemently against communism. The Soviets were feared in the UK and France as a threat to democracy and capitalism. A pact of cooperation between Germany and the USSR overturned the certainties of Europe and threatened the balance between the European powers.
 input device
Device for entering information into a computer. Input devices include keyboards, joysticks, mice, light pens, touch-sensitive screens, scanners, graphics tablets, speech-recognition devices, and vision systems. The input into an electronic system is usually
through switches or sensors. Compare with an output device.
Keyboards, the most frequently used input devices, are used to enter instructions and data via keys. There are many variations on the layout and labelling of keys. Extra numeric keys may be added, as may special-purpose function keys, whose effects can be defined by programs in the computer.
The graphics tablet is an input device in which a stylus or cursor is moved, by hand, over a flat surface. The computer can keep track of the position of the stylus, enabling the operator to input drawings or diagrams into the computer. The joystick signals to a computer the direction and extent of displacement of a hand-held lever.
Light pens resemble ordinary pens and are used to indicate locations on a computer screen. With certain computer-aided design (CAD) programs, the light pen can be used to instruct the computer to change the shape, size, position, and colours of sections of a screen image.
Scanners produce a digital image of a document for input and storage in a computer, using technology similar to that of a photocopier. Small scanners can be passed over the document surface by hand; larger versions have a flat bed, like that of a photocopier, on which the input document is placed and scanned.
Input devices that are used commercially – for example, by banks, postal services, and supermarkets – must be able to read and capture large volumes of data very rapidly. Such devices include document readers for magnetic-ink character recognition (MICR), optical character recognition (OCR), and optical mark recognition (OMR); mark-sense readers; bar-code scanners; magnetic-strip readers; and point-of-sale (POS) terminals. Punched-card and paper-tape readers were used in earlier commercial applications but are now obsolete.
 trigonometry
Branch of mathematics that concerns finding lengths and angles in triangles. In a right-angled triangle the sides and angles are related by three trigonometric ratios: sine, cosine, and tangent. Trigonometry is used frequently in navigation, surveying, and simple harmonic motion in physics.
Using trigonometry, it is possible to calculate the lengths of the sides and the sizes of the angles of a right-angled triangle as long as one angle and the length of one side are known, or the lengths of two sides. The longest side, which is always opposite to the right angle, is called the hypotenuse. The other sides are named depending on their position relating to the angle that is to be found or used: the side opposite this angle is always termed opposite and that adjacent is the adjacent. So the following trigonometric ratios are used:
sine = opposite/hypotenuse
cosine = adjacent/hypotenuse
tangent = opposite/adjacent
Using trigonometry it is possible to measure the height of a tall object from a vantage point at a known distance. The following example shows the calculation of the height of a tree, from a point 22 m from its base. The angle at this point to the top of the tree is 33°:
Using the trigonometric ratio tangent  = opposite/adjacent:
tangent 33° = h/22
h = 22 × tan 33°, so h = 22 × 0.6494 = 14.29 (to two decimal places)
The height of the tree is 14.29 m. beyond 90° The trigonometric ratios have a meaning for any angle, not just angles between 0° and 90°. These ratios (or functions) can be expressed as trigonometric graphs, using angles of any size. The sine rule and cosine rule are both used to calculate unknown angles or sides in triangles where there is not a right angle.
Three-dimensional and spherical trigonometry
The methods of elementary trigonometry can be used to solve problems in three dimensions by considering triangles in different planes that have a side in common. This may also involve the use of a dropping perpendicular, that is the envisaging of an imaginary line from a point above the base that will fall vertically to the base. Spherical triangles can be solved using the trigonometric functions, though the formulae are not the same as those employed for plane triangles.
History of trigonometry
Trigonometry arose out of the study of astronomy, and was originated by the Greek astronomer Hipparchus. It was also known to early Hindu and Arab mathematicians. Ptolemy the Alexandrian astrologer greatly extended the subject and German astronomer Regiomontanus made it a science independent of astronomy much later on, when he began compiling trigonometric tables in 1467.
 percussion instrument
Musical instrument played by being struck with the hand or a beater, crashed, shaken, or scraped. Percussion instruments can be divided into those that can be tuned to produce a sound of definite pitch, such as the timpani, tubular bells, glockenspiel, xylophone, and piano, and those of indefinite pitch, including the bass drum, tambourine, triangle, cymbals, castanets, and gong.
The timpano is a large hemispherical bowl of metal with a membrane stretched across the rim, affixed and tuned by screwtaps or with a pedal mechanism; tubular or orchestral bells are vertically suspended on a frame and struck at the top end with mallets. These instruments were often used by Witold Lutoslawski in his orchestral works; the glockenspiel is a small keyboard of aluminium alloy keys played with small beaters; the xylophone has hardwood, rather than metal, bars, and is played using hard-headed beaters to give a distinctive 'pop' to the sound.
The snare drum is a shallow double-sided drum on the underside of which gut coils or metal springs are secured by a clamp, and which rattle against the underside when the drum is beaten, while the bass drum produces the loudest sound (unpitched) in the orchestra; the tambourine has a wooden hoop with a membrane stretched across it, and has metal discs suspended in the rim to make a jangling sound; a triangle is formed from a suspended triangular-shaped steel bar, played by striking it with a separate bar of steel – the sound produced can be clearly perceived even when played against a full orchestra; cymbals are two brass dishes struck together; castanets are two hollow shells of wood struck together; and the gong is a suspended disc of metal struck with a soft hammer.
 diffraction
The spreading out of waves when they pass through a small gap or around a small object, resulting in some change in the direction of the waves. In order for this effect to be observed, the size of the object or gap must be comparable to or smaller than the wavelength of the waves. Diffraction occurs with all forms of progressive waves – electromagnetic, sound, and water waves – and explains such phenomena as the ability of long-wave radio waves to bend around hills more easily than short-wave radio waves.
The wavelength of visible light ranges from about 400 to 700 nanometres, several orders of magnitude smaller than radio waves. The gap through which light travels must be extremely small for diffraction to be observed. The slight spreading of a light beam
through a narrow slit causes the different wavelengths of light to interfere with each other, producing a pattern of light and dark bands. A diffraction grating is a plate of glass or metal ruled with close (some diffraction gratings have from 2,000 to 7,000 lines per inch), equidistant, parallel lines used for separating a wave train such as a beam of incident light into its component frequencies. White light passing through a grating will be separated into its constituent colours. Red light is diffracted more as it has a longer wavelength; blue light is diffracted less as it has a shorter wavelength.
The wavelength of sound is between 0.5 m/1.6 ft and 2.0 m/6.6 ft. When sound waves travel through doorways or between buildings they are diffracted significantly, so that the sound is heard round corners.
The regularly-spaced atoms in crystals diffract X-rays, and by exploiting this fact the structure of many substances has been elucidated, including that of proteins.
 Pill, the
Commonly used term for the contraceptive pill, based on female hormones. The combined pill, which contains synthetic hormones similar to oestrogen and progesterone, stops the production of eggs, and makes the mucus produced by the cervix hostile to sperm. It is the most effective form of contraception apart from sterilization, being more than 99% effective.
The minipill or progesterone-only pill prevents implantation of a fertilized egg into the wall of the uterus. The minipill has a slightly higher failure rate, especially if not taken at the same time each day, but has fewer side effects and is considered safer for long-term use. Possible side effects of the Pill include migraine or headache and high blood pressure. More seriously, oestrogen-containing pills can slightly increase the risk of a clot forming in the blood vessels. This risk is increased in women over 35 if they smoke. Controversy surrounds other possible health effects of taking the Pill. The evidence for a link with cancer is slight (and the Pill may protect women from some forms of cancer). Once a woman ceases to take it, there is an increase in the likelihood of conceiving identical twins.
In the UK, the Family Planning Association (FPA) runs the national Contraceptive Education Service (CES), which provides information on all aspects of sexual health and contraception both to the general public and health professionals.
 pilgrimage
Journey to sacred places inspired by religious devotion. For Hindus, the holy places include Varanasi and the purifying River Ganges; for Buddhists, the places connected with the crises of Buddha's career; for the ancient Greeks, shrines such as those at Delphi and Ephesus; for Jews, the Western Wall or Wailing Wall in Jerusalem; for Muslims, Mecca and Medina; and for Roman Catholics, Lourdes in France, among others. Pilgrimages are usually undertaken as opportunities to reflect upon and deepen one's religious faith, or to earn religious merit.
Among Christians, pilgrimages were common by the 2nd century and, as a direct result of the growing frequency and numbers of
pilgrimages, there arose numerous hospices catering for pilgrims, the religious orders of knighthood, and the Crusades. The great centres of Christian medieval pilgrimages were Jerusalem, Rome, the tomb of St James of Compostela in Spain, and the shrine of St Thomas à Becket in Canterbury, England.
Pilgrimage often involved ascetic practices (restrictions on diet or sexuality) but also sightseeing, and was a major factor in the exchange of ideas between regions and cultures. The motives for pilgrimage varied from penance for sin and concern for the afterlife to the desire for benefits in the present life, such as fertility, healing, or luck. Pilgrimage is not an essential feature of the Christian religion, and is mostly undertaken by members of the Roman Catholic and High Church Anglican churches. The most popular sites are those where visions of the Virgin Mary have been seen or miracles recorded.
The three major centres of pilgrimage in medieval England were Canterbury; Bury St Edmunds, Suffolk; and Walsingham, Norfolk. Walsingham is still a site of pilgrimage each Easter.
There are a number of sacred mountains in China, some Buddhist, some Taoist, which are major centres of pilgrimage. In Japan, pilgrimage to sacred mountains could bring benefits in both Shinto and Buddhist belief systems. In Shintoism, Mount Fuji is regarded as the supreme home of the gods. Pilgrims usually ascend this snow-capped mountain on foot. Alternatively, pilgrims would visit sites associated with Buddhist saints, or make a round of any of several series of temples: the 33 sanctuaries of Kwannon or the 25 temples of the Pure Land school, for example.
Pilgrimage is not an obligatory part of Judaism, now that the Temple of Jerusalem no longer exists, but many Jews visit the Western Wall or Wailing Wall, the last remaining portion of the Temple since its destruction in AD 70.
As required by the fifth of the Five Pillars of Islam, all Muslims try to make pilgrimage at least once in their lives to Mecca in Saudi Arabia, to the Kaaba, the first House of God on earth, and the birthplace of the Prophet Muhammad. They may also visit Medina where the first Muslim community was established, and Jerusalem, the site of Muhammad's miraculous journey to heaven.
As well as the River Ganges and the city of Varanasi (Benares), Hindus make pilgrimage to Vrindavan, the birthplace of Krishna, and Ayodhya, the birthplace of Rama.
Buddhist sites include the Buddha's birthplace at Lumbini; the Deer Park at Sarnath, near Benares, where he preached his first sermon; the bodhi tree near Buddh Gaya in Bihar, under which he attained enlightenment; the city of Varanasi, where he founded the Sangha (monastic community); and the place of his death, Kusinagara in Uttar Pradesh. The Temple of the Tooth at Kandy, Sri Lanka, contains what is believed to be a tooth of the Buddha, and there is a site of the Buddha's footprint on top of Sri Pada Mountain.
The city of Amritsar in the Punjab was founded by the fourth guru of Sikhism, Guru Ram Das, as a focus for the Sikh community, and the Golden Temple there was built by his successor, Guru Arjan Dev. Amritsar has become the centre of Sikh pilgrimage, particularly during the gurpurbs, the festivals that commemorate the Sikh gurus.
The two most holy places for Baha'is are the tomb of Baha'u'llah near Acre and the shrine of the Bab, on the side of Mount Carmel in Haifa, both in Israel.
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