levers are
employed; but it is better to connect the valve rod to the link of the link
motion without the intervention of levers. The pins of the eccentric lever
in the old engines used to wear quickly; Stephenson used to put a ferule of
brass on these pins, which being loose, and acting like a roller,
facilitated the throwing in and out of gear, and when worn could easily be
replaced, so that there was no material derangement of the motion of the
valve from play in this situation.

518. _Q._--What is the arrangement of a starting lever?

_A._--The starting lever travels between two iron segments, and can be
fixed in any desired position. This is done by a small catch or bell crank,
jointed to the bottom of the handle at the end of the lever, and coming up
by the side of the handle, but pressed out from it by a spring. The smaller
arm of this bell crank is jointed to a bolt, which shoots into notches,
made in one of the segments between which the lever moves. By pressing the
bell crank against the handle of the lever the bolt is withdrawn, and the
lever may be shifted to any other point, when, the spring being released,
the bolt flies into the nearest notch.

519. _Q._--In what way does the starting handle act on the machinery of the
engine to set it in motion?

_A._--Its whole action lies in raising or depressing the link of the link
motion relatively with the valve rod. If the valve rod be attached to the
middle of the link, the valve will derive no motion from, it at all, and
the engine will stop. If the attachment be slipped to one end of the link
the engine will go ahead, and if slipped to the other end it will go
astern. The starting handle merely achieves this change of position.

520. _Q._--Will you explain the operation of setting the valve of a
locomotive?

_A._--In setting the valves of locomotives, place the crank in the position
answerable to the end of the stroke of the piston, and draw a straight
line, representing the centre line of the cylinder, through the centres of
the crank shaft and crank pin. From the centre of the shaft describe a
circle with the diameter equal to the throw of the valve; another circle to
represent the crank shaft; and a third circle to represent the path of the
crank pin. From the centre of the crank shaft, draw a line perpendicular to
the centre line of the cylinder and crank shaft, and draw another
perpendicular at a distance from the first equal to the amount of the lap