ions 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
valve and a considerable liability to have a leaky valve.

In the case where the governor changes the position of the cut off
valve a greater decree of economy would result. In this engine, of
which the Lambertville engine is a type, the main valve is a long D
slide, with multiple ports at the ends through which the steam enters
the cylinders. It is operated from an eccentric on the crank shaft in
the usual manner. The cut off valve is also operated from the motion
on an eccentric fixed upon the crank shaft. The rod or stem of the cut
off valve passes through the main valve rod and slide. Upon the outer
end of the cut off valve rod are tappets fastened to engage with
tappets on the eccentric valve rod. Connection between the cut off
eccentric, therefore, and the cut off valve is only by means of the
engagement of these tappets.