In those engines which have to perform unequal work, and in which
sometimes a great drag is suddenly removed from the engine, some
contrivance is necessary to prevent the too rapid motion which would
ensue, to the great risk of damaging the engine; this is effected by
what is called the “governor;” a contrivance by which a part of the
steam is struck off when the action is too rapid, and again let on
when it has diminished. This arrangement is shown in fig. 3; the two
heavy iron balls swing round as the engine works, and the faster they
revolve the more they tend to separate, from the natural tendency to
fly off called “centrifugal force,” and in separating they bring the
other ends of the rods to which they are attached nearer together,
and so push up a collar, A, attached to the levers which turn off the
steam-tap; and as the action subsides the balls sink down together
and the collar also, the steam being thus turned on again. In order
that the pressure of the steam in the boiler may be known, a “gauge”
is used, which acts on the principle of the barometer, consisting of
a column of mercury which is pressed up by the force of the steam,
the height to which it rises indicating the pressure. With respect to
the details of the steam-engine, they are too various and complicated
to be enumerated or described here; but the motion--being regular,
continuous, and powerful--can be applied to almost any sort of work
by being adapted to the machine suitable for such work, and which
receives its motion from the steam-engine, the same as though it were
worked by water or by hand.