. Appleton's dictionary of machines, mechanics, engine-work, and engineering. t set parallel to the shaft of the drum, butobliquely to it, and is, moreover, somewhat curved, so as to close its edge progressively upon that of thefixed blade. The blade v may also be set between two guide-pieces, and have the necessary motiongiven to it by levers. PARALLEL MOTIONS. Tke following figures exhibit a variety of forms of parallel motions, suchas are employed to maintain the rectilineal direction of the piston-rod of a steam-engine, under theconstantly varying angular direction of the beam. Contrivance
. Appleton's dictionary of machines, mechanics, engine-work, and engineering. t set parallel to the shaft of the drum, butobliquely to it, and is, moreover, somewhat curved, so as to close its edge progressively upon that of thefixed blade. The blade v may also be set between two guide-pieces, and have the necessary motiongiven to it by levers. PARALLEL MOTIONS. Tke following figures exhibit a variety of forms of parallel motions, suchas are employed to maintain the rectilineal direction of the piston-rod of a steam-engine, under theconstantly varying angular direction of the beam. Contrivances of this kind are required in other cir-cumstances of the conversion of rotatory and alternating angular motion into rectilineal motion, and theconverse; but the absolute necessity there is of guiding the path of piston in the steam-engine, hascalled forth more attention to the principles and mechanism of parallel motions than would otherwise,in all probability, have been awarded to the subject for other purposes. In the first place, the principlemay be briefly lig. 3016. Given A B D a right angle: it can be demonstrated that if the end A of the right line A Ddescend from A to B along the liue A B, while the end D moves along the line B D produced, a point C iuthe middle of the line will describe the circle C G. Hence, if a beam A D has one end sliding in a grooveat D, and is connected or jointed at the middle C in a guide B C of half its length, this guide also mov-ing on a joint at B, then in every position of the beam the point C will describe the circle C G, and thepoint A of the beam will move in a straight line. Fig. 3017. In practice, it may be more convenient to have the end D of the beam fixed to the end PARALLEL MOTIONS. 463 of a movable bar, as D N, of some feet in length, than to slide in a groove ; for, though the arc describedby the end D will deviate a little from a straight line, yet the error produced thereby will be so verysmall that it can have n
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