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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 1/2 9 8, which means that the ratio of the equatorial to the polar axis is 2 9 8:297 (see Figure of the Earth). 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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