earlier cut off. If the cut off is desired to be later, the hand wheel
is so turned that the right and left hand screws will bring the valve
sections nearer together, thus practically taking off lap. Now this
may be done by hand or it may be done by the action of a governor.

[Illustration: FIG. 2.]

In the latter case the governor at each change of load turns the right
and left hand screws to add or take away lap, as the load demands an
earlier or later cut off; in other cases the governor moves a rack in
mesh with a gear by which the valve sections are brought closer
together or are separated. The difficulty with the case where the hand
wheel is turned by hand is that the cut off is fixed where you leave
it, and governing can only be at the throttle. For this reason
anywhere near full boiler pressure would not be obtained in the
cylinder of the engine. If the load was a constant one, and the cut
off could be fixed at about one-third, causing the throttle to open
its widest, very good results would be obtained, but there is no
margin left for governing.

If the load should increase at such a time the governor could not
control it under these conditions, and it would lead to a decrease in
speed unless the lap was again changed to give a later cut off. On
this account the general practice soon becomes to leave the cut off at
the later point and give range to the throttle, and we come back once
more to the plain slide valve cutting off at half stroke, and the only
gain there is, is in a quick port opening and quick cut off. But these
matters are more than offset by the wire drawing between the steam
pipe and chest, through the throttle, and the fact that there is added
to the friction of the engine the friction of this additional slide