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?

_A._--That is the object of it, and it accomplishes its object in a very
perfect manner, as it gradually arrests the velocity of the piston towards