sound barrier

Sharp rise in aerodynamic drag that occurs as an aircraft approaches the speed of sound. At sea level the speed of sound is about 750 miles (1,200 km) per hour, and at 36,000 feet (11,000 metres) it is about 650 miles (1,050 km) per hour. The sound barrier was formerly an obstacle to supersonic flight. If an aircraft flies at somewhat less than sonic speed, the pressure waves (sound waves) it creates outspeed their sources and spread out ahead of it. Once the aircraft reaches sonic speed the waves are unable to get out of its way. Strong local shock waves form on the wings and body; airflow around the craft becomes unsteady, and severe buffeting may result, with serious stability difficulties and loss of control over flight characteristics. Generally, aircraft properly designed for supersonic flight have little difficulty in passing through the sound barrier, but the effect on those designed for efficient operation at subsonic speeds may become extremely dangerous. The first pilot to break the sound barrier was Chuck Yeager (1947), in the experimental X-1 aircraft.

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or barrier penetration

In physics, the passage of a particle through a seemingly impassable energy barrier. Though a particle's energy may be too low to surmount a barrier in classical physics, the particle may still cross the barrier as a consequence of its quantum-mechanical wave properties. An important application of this phenomenon is in the operation of the scanning tunneling microscope.

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Extensive complex of coral reefs, shoals, and islets in the Pacific Ocean, off the northeastern coast of Australia. The largest deposit of coral in the world, it extends for more than 1,250 mi (2,000 km) along the coast of Queensland and has an area of some 135,000 sq mi (350,000 sq km). The reef has been formed over millions of years from the skeletons of a mass of living marine organisms. In addition to at least 300 species of hard coral, marine life includes anemones, worms, gastropods, lobsters, crayfish, prawns, crabs, and a variety of fishes. Encrusting red algae form the purplish red algal rim that is one of the reef's characteristic features. A major tourist attraction, nearly all of it is within Great Barrier Reef National Park; the reef was designated a UNESCO World Heritage site in 1981.

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A hedge is a line of closely spaced shrubs and bushes, planted and trained in such a way as to form a barrier or to mark the boundary of an area. Hedges, especially those used to separate a road from adjoining fields or one field from another, and of sufficient age to incorporate small trees, are also frequently known as hedgerows. It is also a simple form of Topiary.

Many hedgerows separating fields from lanes in England,Ireland and the Low Countries are estimated to have been in existence for more than seven hundred years. The root word of 'hedge' is much older: it appears in Old English, in German (Hecke), and Dutch (haag) to signify 'enclosure', as in the name of the Dutch city The Hague, or more formal 's Gravenhage, meaning The Count's hedge. Charles the Bald is recorded as complaining in 864, at a time when most official fortifications were constructed of wooden palisades, that some unauthorized men were constructing haies et fertés – tightly interwoven hedges of hawthorns.

The nineteenth century Great Hedge of India was probably the largest example of a hedge used as a barrier. It was planted and used to collect taxes by the British.


If hedges are not maintained or only trimmed repeatedly, gaps tend to form at the base over many years. In essence, hedgelaying consists of cutting most of the way through the stem of each plant near the base, bending it over and interweaving it between wooden stakes. This also encourages new growth from the base of each plant. Originally, the main purpose of hedgelaying was to ensure the hedge remained stock-proof. Some side branches were also removed and used as firewood.

The maintenance and laying of hedges in such a way as to form an impenetrable barrier for farm animals is a skilled art. In Britain there are many local hedgelaying traditions, each with a distinct style. Hedges are still being laid today as they are not only beautiful and functional but they also help wildlife and protect against soil erosion.

Dating ancient hedges

Hedges that have existed for hundreds of years are colonised by additional species. This may be useful to determine the age of the hedge. Hedgerow dating suggests that one new species is attracted to each 27 metre section of hedge every 100 years. However, results are mixed and the technique is controversial.

Hedges in gardening

Hedges, both clipped and unclipped, are often used as ornament in the layout of gardens. Typical woody plants for clipped hedges include privet, hawthorn, beech, yew, leyland cypress, hemlock, arborvitae, barberry, box, holly, oleander, lavender, etc. An early 20th century fashion was for tapestry hedges, using a mix of golden, green and glaucous dwarf conifers, or beech and copper beech. Unclipped hedges take up more space, generally at a premium in modern gardens, but compensate by flowering. Rosa multiflora is widely used as a dense hedge along median (central) strips of dual-carriageway roads, such as parkways in the United States. In mild climates, more exotic flowering hedges are formed, using Ceanothus, Hibiscus or Camellia.

Hedges of clipped trees forming avenues are a feature of 16th century Italian gardens such as the Boboli Gardens in Florence, and of formal French gardens in the manner of André Le Nôtre, e.g. at Versailles. The 'hedge on stilts' of clipped hornbeams at Hidcote Manor Garden, Gloucestershire, is famous and has sometimes been imitated.

Hedges below knee height are generally thought of as borders. Elaborately shaped and interlaced borders forming knot gardens or parterres were fashionable in Europe during the 16th and early 17th centuries. Generally they were appreciated from a raised position, either the windows of a house, or a terrace.

Clipped hedges above eye level may be laid out in the form of a labyrinth or garden maze. Few such mazes survived the change of fashion towards more naturalistic plantings in the 18th and 19th centuries, but many were replanted in 20th century restorations of older gardens. An example is behind the Governor's Palace, Colonial Williamsburg, Virginia.

Hedges and pruning can both be used to enhance a garden's privacy, as a buffer to visual pollution and to hide fences. A hedge can be aesthetically pleasing, as in a tapestry hedge, where alternate species are planted at regular intervals to present different colours or textures.

See also

External links


  • Alan Brooks, Elizabeth Agate: Hedging, British Trust for Conservation Volunteers, ISBN 0946752176



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