therefore, of the links and the piston rod is of the form of a knuckle or
bend at some parts of the stroke.

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.

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