=739. Is a device for varying the opening of a main valve= (connected to
rod ~A~) by the pressure on the small piston, which moves it against the
tension of a spring.
=740, 741, 742, & 743. Types of centrifugal governors,= of which
numerous varieties are in use.
_Pumping engines_ may be governed by allowing the pressure of water in
the rising main to accumulate in a stand pipe or equivalent device
until it stops the engine by excess of pressure. To prevent such an
engine running away a catch is used, kept open by the pressure of
water; when the pressure falls below a certain point the catch is
released and closes the throttle valve.
_Steam engines_ may also be safeguarded in the same way by a catch
which is released and closes the throttle valve when the governor
becomes fully expanded.
=744. Screw and nut device,= to control the travel of any machine, such
as a lift, by reversing the belt or throwing out a catch after any
specified number of revolutions, the travel being adjusted by the stop
nuts.
=745. The cataract= is one of the oldest governing devices. It consists
essentially of a vessel which is filled with water by one stroke of the
engine, and empties itself through an adjustable orifice during the
return stroke, the valve motion being prevented from reversing till the
water is all discharged.
=746. Gas engine governor.= Rod ~A~ has a reciprocating motion from the
engine, and the spur on lever ~B~ strikes the end of the gas valve slide
when brought in line with it by the motion of the governor, thus
supplying gas only when the governor falls to a certain point.
Differential Governor. See Nos. 556 and 557.