hich a stuffing or
packing of plaited hemp is forced, which, pressing on the one side against
the interior of the stuffing box, and on the other side against the piston
rod, which is smooth and polished, prevents any leakage in this situation.
The packing of this stuffing box is forced down by a ring of metal
tightened by screws. This ring, which accurately fits the piston rod, has a
projecting flange, through which bolts pass for tightening the ring down
upon the packing; and a similar expedient is employed in nearly every case
in which packing is employed.

103. _Q._--In what way is the piston rod connected to the great beam?

_A._--The piston rod is connected to the great beam by means of two links,
one at each side of the beam shown at _f g_, (fig. 21.) These links are
usually made of the same length as the crank, and their purpose is to
enable the end of the great beam to move in the arc of a circle while the
piston rod maintains the vertical position. The point of junction,
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. T