What is "parallel motion"

Parallel \Par"al*lel\, a. [F. parall[`e]le, L. parallelus, fr. Gr. ?; para` beside + ? of one another, fr. ? other, akin to L. alius. See Alien.]

1. (Geom.) Extended in the same direction, and in all parts equally distant; as, parallel lines; parallel planes.

   Revolutions . . . parallel to the equinoctial.
   --Hakluyt.

   Note: Curved lines or curved planes are said to be parallel when they are in all parts equally distant.

2. Having the same direction or tendency; running side by side; being in accordance (with); tending to the same result; -- used with to and with.

   When honor runs parallel with the laws of God and our country, it can not be too much cherished.
   --Addison.

3. Continuing a resemblance through many particulars; applicable in all essential parts; like; similar; as, a parallel case; a parallel passage. --Addison. Parallel bar.

   1. (Steam Eng.) A rod in a parallel motion which is parallel with the working beam.

   2. One of a pair of bars raised about five feet above the floor or ground, and parallel to each other, -- used for gymnastic exercises. Parallel circles of a sphere, those circles of the sphere whose planes are parallel to each other. Parallel columns, or Parallels (Printing), two or more passages of reading matter printed side by side, for the purpose of emphasizing the similarity or discrepancy between them. Parallel forces (Mech.), forces which act in directions parallel to each other. Parallel motion.

      1. (Mach.) A jointed system of links, rods, or bars, by which the motion of a reciprocating piece, as a piston rod, may be guided, either approximately or exactly in a straight line.
         --Rankine.

      2. (Mus.) The ascending or descending of two or more parts at fixed intervals, as thirds or sixths.

         Parallel rod (Locomotive Eng.), a metal rod that connects the crank pins of two or more driving wheels; -- called also couping rod, in distinction from the connecting rod. See Illust. of Locomotive, in App. -- Parallel ruler, an instrument for drawing parallel lines, so constructed as to have the successive positions of the ruling edge parallel to each other; also, one consisting of two movable parts, the opposite edges of which are always parallel.

         Parallel sailing (Naut.), sailing on a parallel of latitude.

         Parallel sphere (Astron. & Geog.), that position of the sphere in which the circles of daily motion are parallel to the horizon, as to an observer at either pole.

         Parallel vise, a vise having jaws so guided as to remain parallel in all positions.

The parallel motion is a mechanical linkage invented by the Scottish engineer James Watt in 1784 for the double-acting Watt steam engine. It allows a rod moving straight up and down to transmit motion to a beam moving in an arc, without putting sideways strain on the rod.