100 _ PILYSICS. 
is communicated to one end of the beam, and of course an opposite motion 
to the other end. This upward and downward or rectilineal motion is con- 
verted into a circular by the connecting rod P, and the winch Q; the axis 
of this winch is the principal axis of the machinery to be set in motion. 
About it turns also the great fly-wheel, X,which serves to maintain uniformity 
in the motion of the engine. This, however, is not quite sufficient. A 
continual diminution of the resistance to be overcome by the engine, with the 
same head of steam, must gradually produce an increasing, and finally exceed- 
ingly dangerous acceleration in the velocity of rotation of the fly. To set a 
certain limit of safety to the velocity of rotation, it becomes necessary to 
attach a valve to the steam-pipe, Z, whose increase or diminution of the 
aperture may restrain to a greater or less extent the admission of steam to 
the cylinder. The turning of this valve is effected by the engine itself, by 
means of an apparatus termed the regulator or governor. An endless string, 
i, is passed round the axis of the fly-wheel, and a vertical pulley, so that the 
motion of the former is communicated to the latter. To the axis of the 
pulley a conical or bevel-edged wheel is attached, whose teeth play in those 
of a similar conical wheel placed horizontally. The axis of the latter is 
prolonged into a rod, whose upper end carries the conical pendulum (or 
centrifugal regulator) V. The pendulum consists of two heavy balls, which 
are attached to the upper end of the vertical rod, hanging by two short rods, 
which are again connected by means of other rods with a collar, A, 
surrounding the vertical rod. If, now, this rod rotate rapidly, the two balls 
separate in consequence of the centrifugal force; by this separation the 
collar 4 is elevated, and with it the connected angular lever, rSa, turning 
about the axis, S. This motion draws the horizontal rod ab towards the 
right, which turns the angular lever, bcd, about the axis c, and this lever, 
being connected with the vertical rod ed, draws it downwards. Now, as e is 
the extreme end of a lever arm, by whose turning the valve in the pipe Z is 
turned, this valve will be closed during the depression of the rod de. 
Less steam enters, therefore, than before, and the rate of the engine is 
retarded. The converse takes place when the engine goes too slowly: the 
balls fall, and by means of the connecting lever work, open the throttle 
valve for an additional supply of steam. This system of levers is in our 
figure represented only by lines as, being placed on the front side of the 
engine, it is not really visible in a section. 
The alternate admission of the steam into the upper and lower parts of the 
cylinder, may be effected in various ways, among which the cross-cock 
(pl. 19, 2 figs. 29), is perhaps the simplest. This is a cock with two per- 
forations: the tube, K, leads to the boiler, C to the condenser, O to the 
upper and U to the lower part of the cylinder. When the cock has the 
position of the upper figure, the steam enters from the boiler into the 
upper part of the cylinder, and at the same time escapes from the lower 
part to the condenser. When the piston reaches the bottom of the cylinder, 
the cock is brought into the position of the lower figure by a quarter turn, 
by which means the steam can enter the lower part of the cylinder, and 
escape frem the upper. 
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