rk the
intersection of these lines with the circle of the crank shaft; measure
with a pair of compasses the chord of the arc intercepted between either of
these points, and the diameter which is at right angles with the crank, and
the diameters being first marked on the shaft itself, then by transferring
with the compasses the distance found in the diagram, and marking the
point, the eccentric may at any time be adjusted without difficulty.

[Illustration: Fig. 45.]

522. _Q._--Will you describe the structure and arrangement of the feed
pumps of locomotive engines?

_A._--The feed pumps of locomotives are generally made of brass, but the
plungers are sometimes made of iron, and are generally attached to the
piston, cross head, though in Stephenson's engines they are worked by rods
attached to eyes on the eccentric hoops. There is a ball valve, fig. 45,
between the pump and the tender, and two usually in the pipe leading from
the pump to the boiler, besides a cock close to the boiler, by which the
pump may be shut off from the boiler in case of any accident to the valves.
The ball valves are guided by four branches, which rise vertically, and
join together at the top in a hemispherical form. The shocks of the ball
against this cap have in some cases broken it after one week's work, from
the top of the cage having been flat, and the branches not having had their
junction at the top properly filleted. These valve guards are attached in
different ways to the pipes; when one occurs at the junction of two pieces
of pipe it has a flange, which along with the flanges of the pipes and that
of the valve seat are held together by a union joint. It is sometimes
formed with a thread at the under end, and screwed into the pipe. The balls
are cast hollow to lessen the shock, as well as to save the metal. In some
cases where the feed pump plunger has been attached to the cross head, the
piston rod has been bent by the strain; and that must in all cases occur,
if the communication between the pump and boiler be closed when the engine
is started, and there be no escape valve for the water.

523. _Q._--Are none but ball valves used in the feed pump?

_A._--Spindle valves have in some cases been used instead of ball valves,
but they are more subject to derangement; but piston valves, so contrived
as to shut a portion of water in the cage when about to close, might be
adopted with a great diminution of the shock. Slide valves might be
a