Online Encyclopedia

COMPRESSION

Online Encyclopedia
Originally appearing in Volume V06, Page 813 of the 1911 Encyclopedia Britannica.
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COMPRESSION, in astronomy, the deviation of a heavenly body from the spherical form, called also the " ellipticity." It is numerically expressed by the ratio of the differences of the axes to the major axis of the spheroid. The compression or " flattening " of the earth is about 1/298, which means that the ratio of the equatorial to the polar axis is 298:297 (see EARTH, FIGURE OF THE). In engineering the term is applied to the arrangement by which the exhaust valve of a steam-engine is made to close, shutting a portion of the exhaust steam in the cylinder, before the stroke of the piston is quite complete. This steam being compressed as the stroke is completed, a cushion is formed against which the piston does work while its velocity is being rapidly reduced, and thus the stresses in the mechanism due to the inertia of the reciprocating parts are lessened. This compression, moreover, obviates the shock which would otherwise be caused by the admission of the fresh steam for the return stroke. In internal combustion engines it is a necessary condition of economy to compress the explosive mixture before it is ignited: in the Otto cycle, for instance, the second stroke of the piston effects the compression of the charge which has been drawn into the cylinder by the first forward stroke.
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Additional information and Comments

Compression can also relate to data. The compression of data relates to the removal of redundencies in the binary representation of a set of information. Common compression algorithms include Jpeg, Mpeg and MP3. MP3 is considered more as the subtraction of irrelevent data as opposed to true compression. This would give us two definitions of compression. 1) A compression where a set of data could be transformed into a smaller (cardinally-speaking) set where it is possible to re-instate the data to its original form, and 2) A compression where irrelevant information is subtracted but it would not be possible to re-instate the original set but where no measureable loss of information is percieved. Perfect compression of data would be the compression of data into the minimum possible transmittable bit set, that can be reconstituted into its original form. Perfect compression has so far eluded mankind. Although this is not a mathematically proven fact, it is widely accepted that perfect compression is not possible. Perfect compression is equivalent to perfect secrecy, the only model of perfect secrecy that exists today is the VERNAM cipher, which is not perfect compression.
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