ey run
while the packing space is being turned out. Where many oscillating engines
are made, a lathe with four centres is used, which makes the use of
straight edges in setting out the trunnions superfluous.

724. _Q._--Will you explain how the slide valve of a marine engine is set?

_A._--Place the crank in the position corresponding to the end of the
stroke, which can easily be done in the shop with a level, or plumb line;
but in a steam vessel another method becomes necessary. Draw the transverse
centre line, answering to the centre line of the crank shaft, on the sole
plate of the engine, or on the cylinder mouth if the engine be of the
direct action kind; describe a circle of the diameter of the crank pin upon
the large eye of the crank, and mark off on either side of the transverse
centre line a distance equal to the semi-diameter of the crank pin. From
the point thus found, stretch a line to the edge of the circle described on
the large eye of the crank, and bring round the crank shaft till the crank
pin touches the stretched line; the crank may thus be set at either end of
its stroke. When the crank is thus placed at the end of the stroke, the
valve must be adjusted so as to have the amount of lead, or opening on the
steam side, which it is intended to give at the beginning of the stroke;
the eccentric must then be turned round upon the shaft until the notch in
the eccentric rod comes opposite the pin on the valve lever, and falls into
gear: mark upon the shaft the situation of the eccentric, and put on the
catches in the usual way. The same process must be repeated for going
astern, shifting round the eccentric to the opposite side of the shaft,
until the rod again falls into gear. In setting valves, regard must of
course be had to the kind of engine, the arrangements of the levers, and
the kind of valve employed; and in any general instructions it is
impossible to specify every modification in the procedure that
circumstances may render advisable.

725. _Q._--Is a similar method of setting the valve adopted when the link
motion is employed.

_A._--Each end of the link of the link motion has the kind of motion
communicated to it that is due to the action of the particular eccentric
with which that end is in connection. In that form of the link motion in
which the link itself is moved up or down, there is a different amount of
lead for each different position of the link, since to raise or lower the
link is t