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compression,external stress applied to an object or substance, tending to cause a decrease in volume (see pressurepressure,
in mechanics, ratio of the force acting on a surface to the area of the surface; it is thus distinct from the total force acting on a surface. A force can be applied to and sustained by a single point on a solid.
..... Click the link for more information. ). Gases can be compressed easily, solids and liquids to a very small degree if at all. Water, for example, is practically incompressible, thus making it especially useful for hydraulic machineshydraulic machine,
machine that derives its power from the motion or pressure of water or some other liquid. Hydraulic Engines
Water falling from one level to a lower one is used to drive machines like the water wheel and the turbine.
..... Click the link for more information. . According to the kinetic-molecular theory of gaseskinetic-molecular theory of gases,
physical theory that explains the behavior of gases on the basis of the following assumptions: (1) Any gas is composed of a very large number of very tiny particles called molecules; (2) The molecules are very far apart compared to their sizes,
..... Click the link for more information. , when the molecules of a gas are brought close enough together by compression, the gas (under certain conditions of temperature) undergoes liquefactionliquefaction,
change of a substance from the solid or the gaseous state to the liquid state. Since the different states of matter correspond to different amounts of energy of the molecules making up the substance, energy in the form of heat must either be supplied to a substance
..... Click the link for more information. . This principle is applied commercially to several gases, including liquid oxygen and the so-called bottled gas (a mixture of hydrocarbons) used as a fuel. Boyle's law deals with the decrease in the volume of a gas in relation to the increase of pressure upon it (see gas lawsgas laws,
physical laws describing the behavior of a gas under various conditions of pressure, volume, and temperature. Experimental results indicate that all real gases behave in approximately the same manner, having their volume reduced by about the same proportion of the
..... Click the link for more information. ). The ability or the degree to which an internal-combustion engine reduces the volume of its fuel mixture preparatory to firing is called its compression. Also, a region of high pressure in a fluid is called a compression; thus sound waves are said to propagate at compressions and rarefactions (regions of low pressure) of their medium, such as air.
(in Russian, kompressiia), a force effect on a gaseous body that reduces the volume occupied by the gas and increases its pressure and temperature. Compression is achieved by means of compressors and during the operation of internal combustion engines and other equipment.
Russian technical literature usually uses the word szhatie instead of kompressiia, although the meaning of the former is more general, since it also covers solids and volume reduction upon cooling.
The standard Unix compression utilty is called compress though GNU's superior gzip has largely replaced it. Other compression utilties include pack, zip and PKZIP.
When compressing several similar files, it is usually better to join the files together into an archive of some kind (using tar for example) and then compress them, rather than to join together individually compressed files. This is because some common compression algorithms build up tables based on the data from their current input which they have already compressed. They then use this table to compress subsequent data more efficiently.
See also TIFF, JPEG, MPEG, Lempel-Ziv Welch, "lossy", "lossless".
Web Content Compression FAQ.
Usenet newsgroups: news:comp.compression, news:comp.compression.research.