104. _Q._--But what compels the top of the piston rod to maintain the vertical position?
_A._--Some engines have guide rods set on each side of the piston rod, and eyes on the top of the piston rod engage these guide rods, and maintain the piston rod in a vertical position in every part of the stroke. More commonly, however, the desired end is attained by means of a contrivance called 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.
107. _Q._--Does the air pump rod move vertically as well as the piston rod?