e of 100 lbs.
pressure steam we get much more _work_.

HOW THE CUT-OFF IS MANAGED.

[Illustration: FIG. 27.--A slide-valve with "lap."]

[Illustration: FIG. 28.]

Look at Fig. 27. Here we have a slide-valve, with faces much wider than
the steam ports. The parts marked black, P P, are those corresponding to
the faces of the valves shown in previous diagrams (p. 54). The shaded
parts, L L, are called the _lap_. By increasing the length of the lap we
increase the range of expansive working. Fig. 28 shows the piston full
to the left; the valve is just on the point of opening to admit steam
behind the piston. The eccentric has a throw equal to the breadth of a
port + the lap of the valve. That this must be so is obvious from a
consideration of Fig. 27, where the valve is at its central position.
Hence the very simple formula:--Travel of valve = 2 x (lap + breadth of
port). The path of the eccentric's centre round the centre of the shaft
is indicated by the usual dotted line (Fig. 28). You will notice that
the "angle of advance," denoted by the arrow A, is now very
considerable. By the time that the crank C has assumed the position of
the line S, the eccentric has passed its dead point, and the valve
begins to travel backwards, eventually returning to the position shown
in Fig. 28, and cutting off the steam supply while the piston has still
a considerable part of its stroke to make. The steam then begins to work
expansively, and continues to do so until the valve assumes the position
shown in Fig. 27.

If the valve has to have "lead" to admit steam _before_ the end of the
stroke to the other side of the piston, the _angle of advance_ must be
increased, and the eccentric centre line would lie on the line E^2.
Therefore--total angle of advance = angle for _lap_ and angle for
_lead_.

LIMIT OF EXPANSIVE WORKING.

Theoretically, by increasing the _lap_ and cutting off the steam earlier
and earlier in the stroke, we should economize our power more and more.
But in practice a great difficulty is met with--namely, that _as the
steam expands its temperature falls_. If the cut-off occurs early, say
at one-third stroke, the great expansion will reduce the temperature of
the metal walls of the cylinder to such an extent, that when the next
spirt of steam enters from the other end a considerable proportion of
the steam's energy will be lost by cooling. In such a case, the
difference in temperature between admitted steam and