A term applied by practical machinists to an arrangement of parallel bars, by which the alternating rectilinear motion of a piston rod is made to work harmoniously with the alternating curvilinear motion of a rocking beam. As the beam of all engines vibrates upon a centre, of course it performs portions of a circle with each of its extremities; and as the rod of a piston is required to move up and down in a straight line, it cannot be immediately attached to the end of the beam; hence the necessity of the intervening mechanism called the parallel motion. There are many methods of effecting this motion in general use; and ingenuity may devise many more of equal or superior merit. In single engines of the old construction, where the action was apull at both ends of the beam (at the one end by the weight of the lump rod, and, at the other, by the downstroke of the piston), a chain was affired to the upper part of the curved ends of the beam, and to the pump and piston rods, which answered the purpose very well, and is still much used for similar purposes; but in double acting engines, where the piston rod pushes upward, as well as pulls downward, some other mode of action is required.

The first plan employed by Bolton and Watt was to place a toothed sector on the end of the beam, the length of the radius being equal to the distance between the axis of the beam, and a vertical line passing through the centre of the piston rod; and on the upper part of the piston rod was placed a rack, which acted upon the sector, and forming a tangent to it, preserved the rectilinear motion of the piston rod throughout the stroke. A much superior method of effecting this was afterwards devised, to which the name of parallel motion more justly belongs; it consisted of an arrangement of parallel rods moving on circular axes, the principle of which may be thus briefly explained: - If a bar be so confined by other bars that the motion of the end a, in a right line, causes the other end b to describe a certain curve, it follows, on the other hand, the motion of b in the curve will cause a to describe a right line. To apply this to the case before us, let a b c represent the beam of an engine at the highest point of the stroke; a d its position at the middle of the stroke; and af its lowest position: c g and b h are two side rods, suspending the bar g h, parallel toabc;gkla right line, in which the bar g moves in a groove; then, when the end of the beam c is at d, the end g of the bar g h will be at k; and as g h is parallel to a d, the other end h of the bar g h will be at m; and when c arrives at f, g will be at I, and h at n; the point h, therefore, will have described a curve, in a right line, passed through the points g; if, therefore, the groove in which the head of the piston rod moved be taken away, and the end h of the bar g h be jointed to a radius bar, describing a circular arc passing through the points hmn, then the end g, of g h, to which the piston rod is attached, will move through the points gkl, and the whole path of the piston rod will differ very little from a right line.

The small deviation from a right line arises from the circumstance, that the curve described by the end h is not exactly a circular arc when g moves in a strictly right line. To find the length and centre of motion of the radius bar with any distance in the compasses, and on the points hmn, describe arcs intersecting each other; and through the points of intersection draw lines cutting in o; then o n will be the length of the radius bar, and o its centre of motion.

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The parallel motion in general use in steam-boats is represented in the foregoing diagram. The length of the radius bar and the centre of motion may easily be found, as in the former case, by supposing the piston rod to move in a right line, and finding three points, through which a point in the side rod (assumed at pleasure) would pass in the highest, lowest, and middle position of the piston rod; then a circle, which passes through these points, will give the radius and centre sought; and the point assumed in the side bar will be the point for its connexion with the radius bar. a be, part of the beam; egand b h side rods; g the point of junction of the piston to the side rod eg; and m c the radius bar.

In portable engines without a beam, the cross on the head of the piston rod has usually on its ends friction wheels running between guides; but we prefer the parallel motion introduced in Lloyd's portable engine, described hereafter, as it affords a convenient method of working the air pump and cold water pump. The principle of the parallel motion in this engine will be understood by reference to the following diagram. abc represent a bar corresponding to half the beam of an engine, c kf the path of the piston rod, and b k the radius rod; now the radius rod, and the two portions of the beam a b, and b c being respectively equal, if a move in a right line towards g, c will move in the line c k, and if a be connected to a rocking bar a e, which, from its length, or it small angular motion, describes an are ga, differing but little from a right line; and a side bar or strap b h, and the parallel bar h e being added, the centre of b h will be the point of suspension for the rod of the air pump, and the rod of the cold water pump may be suspended from the parallel bar h e.

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