from side to side, and by turning which more or less the opening through
the pipe will be more or less closed. The extent to which this valve is
opened or closed is determined by the governor, D, the balls of which, as
they collapse or expand, move up or down a collar on the governor spindle,
which motion is communicated to the throttle valve by suitable rods and
bell-cranks. The governor, it will be seen, consists substantially of two
heavy balls attached to arms fixed upon an upright shaft, which is kept in
revolution by means of a cord driven by a pulley on the fly wheel shaft.
The velocity with which the balls of the governor revolve being
proportional to that of the fly wheel, it will follow, that if by reason of
too rapid a supply of steam, an undue speed be given to the fly wheel, and
therefore to the balls, a divergence of the balls will take place to an
extent corresponding to the excess of velocity, and this movement being
communicated to the throttle valve it will be partly closed (see fig. 1),
the supply of steam to the engine will be diminished, and the velocity of
its motion will be reduced. If, on the other hand, the motion of the engine
is slower than is requisite, owing to a deficient supply of steam through
B, then the balls, not being sufficiently affected by centrifugal force,
will fall towards the vertical spindle, and the throttle valve, C, will be
more fully opened, whereby a more ample supply of steam will be admitted to
the cylinder, and the speed of the engine will be increased to the
requisite extent.

97. _Q._--The piston must be made to fit the cylinder accurately so as to
prevent the passage of steam?

_A._--The piston is accurately fitted to the cylinder, and made to move in
it steam tight by a packing of hemp driven tightly into a groove or recess
round the edge of the piston, and which is squeezed down by an iron ring
held by screws. The piston divides the cylinder into two compartments,