105. Q.—­What is the parallel motion?

A.—­The parallel motion is an arrangement of jointed rods, so connected together that the divergence from the vertical line at any point in the arc described by the beam is corrected by an equal and opposite divergence due to the arc performed by the jointed rods during the stroke; and as these opposite deviations mutually correct one another, the result is that the piston rod moves in a vertical direction.

106. Q.—­Will you explain the action more in detail?

A.—­The pin, fig 21, which passes through the end of the beam at f has a link f g hung on each side of the beam, and a short cross bar, called a cross head, extends from the bottom of one of these links to the bottom of the other, which cross head is perforated with a hole in the middle for the reception of the piston rod.  There are similar links b d at the point of the main beam, where the air pump rod is attached.  There are two rods d g connecting the links b d with the links f g, and these rods, as they always continue parallel to the main beam throughout the stroke, are called parallel bars.  Attached to the end of these two rods at d are two other rods c d, of which the ends at c are attached to stationary pins, while the ends at d follow the motion of the lower ends of the links b d.  These rods are called the radius bars.  Now it is obvious that the arc described by the point d, with c as a centre, is opposite to the arc described by the point g with d as a centre.  The rod d g is, therefore, drawn back horizontally by the arc described at d to an extent equal to the versed sine of the arc described at g, or, in other words, the line described by the point g becomes a straight line instead of a curve.

[Illustration:  Fig. 21.]

107. Q.—­Does the air pump rod move vertically as well as the piston rod?

A.—­It does.  The air pump rod is suspended from a cross head, passing from the centre of one of the links b d to the centre of the other link, on the opposite side of the beam.  Now, as the distance from the central axis of the great beam to the point b is equal to the length of the rod c d, it will follow that the upper end of the link will follow one arc, and the lower end an equal and opposite arc.  A point in the centre of the link, therefore, where these opposite motions meet, will follow no arc at all, but will move up and down vertically in a straight line.

108. Q.—­The use of the crank is to obtain a circular motion from a reciprocating motion?