into a socket with jaws, through
the holes of which a cross head passes, which is embraced between the jaws
by the small end of the connecting rod, while the ends of the cross head
move in guides. Between the piston rod clutch and the guide blocks, the
feed pump rod joins the cross head in some engines.

511. _Q._--What kind of guides is employed for the end of the piston rod?

_A._--The guides are formed of steel plates attached to the framing,
between which work the guide blocks, fixed on the ends of the cross head,
which have flanges bearing against the inner edges of the guides. Steel or
brass guides are better than iron ones: Stephenson and Hawthorn attach
their guides at one end to a cross stay, at the other to lugs on the
cylinder cover; and they are made stronger in the middle than at the ends.
Stout guide rods of steel, encircled by stuffing boxes on the ends of the
cross head, would probably be found superior to any other arrangement. The
stuffing boxes might contain conical bushes, cut spirally, in addition to
the packing, and a ring, cut spirally, might be sprung upon the rod and
fixed in advance of the stuffing box, with lateral play to wipe the rod
before entering the stuffing box, to prevent it from being scratched by the
adhesion of dust.

512. _Q._--Is any provision made for keeping the connecting rod always of
the same length?

_A._--In every kind of locomotive it is very desirable that the length of
the connecting rod should remain invariable, in spite of the wear of the
brasses, for there is a danger of the piston striking against the cover of
the cylinder if it be shortened, as the clearance is left as small as
possible in order to economize steam. In some engines the strap encircling
the crank pin is fixed immovably to the connecting rod by dovetailed keys,
and a bolt passes through the keys, rod, and strap, to prevent the
dovetailed keys from working out. The brass is tightened by a gib and
cutter, which is kept from working loose by three pinching screws and a
cross pin or cutter through the point. The effect of this arrangement is to
lengthen the rod, but at the cross head end of the rod the elongation is
neutralized by making the strap loose, so that in tightening the brass the
rod is shortened by an amount equal to its elongation at the crank pin end.
The tightening here is also effected by a gib and cutter, which is kept
from working loose by two pinching screws pressing on the side of the