left; and
the eccentric, which is set at an angle of 90 deg. in _advance_ of the
crank, is about to begin opening the left-hand port. By the time that C
has got to the position originally occupied by E, E will be horizontal
(Fig. 25)--that is, the eccentric will have finished its stroke towards
the left; and while C passes through the next right angle the valve will
be closing the left port, which will cease to admit steam when the
piston has come to the end of its travel. The operation is repeated on
the right-hand side while the piston returns.

[Illustration: FIG. 26.]

It must be noticed here--(1) that steam is admitted at full pressure
_all through_ the stroke; (2) that admission begins and ends
simultaneously with the stroke. Now, in actual practice it is necessary
to admit steam before the piston has ended its travel, so as to
_cushion_ the violence of the sudden change of direction of the piston,
its rod, and other moving parts. To effect this, the eccentric is set
more than 90 deg. in advance--that is, more than what the engineers call
_square_. Fig. 26 shows such an arrangement. The angle between E and
E^1 is called the _angle of advance_. Referring to the valve, you will
see that it has opened an appreciable amount, though the piston has not
yet started on its rightwards journey.

"LAP" OF THE VALVE--EXPANSION OF STEAM.

In the simple form of valve that appears in Fig. 24, the valve faces are
just wide enough to cover the steam ports. If the eccentric is not
_square_ with the crank, the admission of steam lasts until the very end
of the stroke; if set a little in advance--that is, given _lead_--the
steam is cut off before the piston has travelled quite along the
cylinder, and readmitted before the back stroke is accomplished. Even
with this lead the working is very uneconomical, as the steam goes to
the exhaust at practically the same pressure as that at which it entered
the cylinder. Its property of _expansion_ has been neglected. But
supposing that steam at 100 lbs. pressure were admitted till
half-stroke, and then suddenly cut off, the expansive nature of the
steam would then continue to push the piston out until the pressure had
decreased to 50 lbs. per square inch, at which pressure it would go to
the exhaust. Now, observe that all the work done by the steam after the
cut-off is so much power saved. The _average_ pressure on the piston is
not so high as in the first case; still, from a given volum