n between D and P, we obtain a spark, as
previously explained, which may be timed to take place by adjusting the
position of cam C on side shaft relatively to the position of piston.

It may be said that the position of the magneto-igniter is immaterial;
it will be fixed in different positions on different types of engines,
and so long as the operating mechanism is simple and effective, _i.e._,
as direct as is practicable, it works well, and requires little
attention. The timing of the spark will be dealt with in the chapter on
Cams and Valve Settings.

CHAPTER VI

GOVERNING

The devices for governing the speed of the engine may be divided,
broadly speaking, into two classes--the inertia or hit and miss
governor, and the centrifugal. Of the latter type we will give an
instance first. In figs. 23 and 24 the governor gear is shown
diagrammatically, consisting of a couple of weights WW suspended from a
vertical spindle. These fly apart when caused to revolve by the bevel
wheel gearing BB, and raise the sleeve S to a greater or lesser extent.
A recess in the latter engages a lever arm L, through which the vertical
movement of the sleeve S is converted into a horizontal movement of the
sleeve T. The latter is carried by the valve lever P, and is virtually a
roller which engages with one or other of the steps of the cam C,
according to the speed of the engine. The object of this arrangement is
to keep the ratio of air to gas uniform throughout all variations of
load. The gas and air valve are shown as both being operated by the same
lever P, the accurate timing of the latter being obtained by means of
set screws.

[Illustration: FIG. 23.]

[Illustration: FIG. 24.]

Messrs Dougill & Co.'s engines are fitted with a step down cam and
governor such as this. The centrifugal governor is often arranged so
that instead of the charge being merely reduced in volume, the whole
charge is cut out, and no explosion whatever takes place. (In this
respect the same results are obtained as when a hit and miss governor is
used, and the latter form therefore is to be preferred, especially on
small engines, where the difference between the indicated power and the
brake power is always, even under the best conditions, fairly great.)

In this case the governor lever only operates the gas valve; the air
valve being opened on every charging or suction stroke, whether gas is
admitted or not.

Another application of the centrifugal