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,
between which there is no communication by which steam or any other elastic
fluid can pass. A casing set beside the cylinder contains the valves, by
means of which the steam which impels the piston is admitted and withdrawn,
as the piston commences its motion in each direction. The upper steam box
B, is divided into three compartments by two valves. Above the upper steam
valve V, is a compartment communicating with the steam pipe B. Below the
lower valve E is another compartment communicating with a pipe called the
eduction pipe, which leads downwards from the cylinder to the condenser, in
which vessel the steam is condensed by a jet of cold water. By the valve V,
a communication may be opened or closed between the boiler and the top of
the cylinder, so as to permit or prevent a supply of steam from the one to
pass to the other. By the valve E a communication may be open or closed
between the top of the cylinder and the condenser, so that the steam in the
top compartment of the cylinder may either be permitted to escape into the
condenser, or may be confined to the cylinder. The continuation of the
steam pipe leads to the lower steam box B', which, like the upper, is
divided into three compartments by two valves V' and E', and the action of
the lower valves is in all respects the same as that of the upper.

98. _Q._--Are all these valves connected together so that they act
simultaneously?

_A._--The four valves V, E, V', E' are connected by rods to a single handle
H, which handle is moved alternately up and down by means of pins or
tappets, placed on the rod which works the air pump. When the handle H is
pressed down, the levers in connexion with it