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, 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casing set beside the cylinder contains the valves, by means of which the steam which impels the piston is admitted and withdrawn, as the piston commences its motion in each direction.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 open the upper exhausting valve E, and the lower steam valve V’, and close the upper steam valve V and the lower exhausting valve E’.  On the other hand, when the handle H is pressed up it opens the upper steam valve V and the lower exhausting valve E’, and at the same time closes the upper exhausting valve E, and the lower steam valve V’.