here 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?

_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
the end of the stroke, and thus obviates what would otherwise be an
injurious shock upon the machine. When the crank approaches the lowest part
of its throw, and at the same time the piston is approaching the top of the
cylinder, the motion of the crank becomes nearly horizontal, or, in other
words, the piston is only advanced through a very short distance, for any
given distance measured on the circle described by the crank pin. Since,
then, the velocity of rotation of the crank is nearly uniform, it will
follow that the piston will move very slowly as it approaches the end of
the stroke; and the piston is brought to a state of rest by this gradually
retarded motion, both at the top and