PYRONOMICS. 101 
More frequently, however, a sliding-valve is made use of for this purpose, 
as in our representation (fig. 27) ; it is delineated on a larger scale in figs. 
30 and 31. The steam from the boiler enters through the pipe Z, into a 
space separated into two parts by a slide, and communicating by the 
pipes, D and E, with the cylinder. The middle space, m, into which the 
steam enters from the pipe Z, is entirely shut off from the upper part, d, and 
the lower, a: the two latter are in communication with the condenser as 
well as with each other by means of the cavity under the slide. If now the 
latter have the position represented in fig. 30, the steam will enter from m 
through D into the lower part of the cylinder, and the steam above the 
piston is drawn out through E towards d, through the slide towards a, and 
finally into the condenser. In the other position (fig. 31) the steam enters 
from m into the upper part of the cylinder through E, and the steam under 
the piston flows through D to a, thence to the condenser. Pl. 19, fig. 82, 
exhibits the slide-valve as seen in the direction of Z. | 
In all cases the arrangement for admitting steam into the upper and 
lower part of the cylinder, must be kept in operation by the engine itself, 
whether a slide-valve or a cross-cock be employed. This is done by means 
of the ‘governor, the most important part of which is the excentric circular 
disk represented at y in fig. 27. This is attached to the axis of the fly- 
wheel, the centre of the disk not coinciding, however, with the centre of 
the axis (figs. 33 and 34). About the periphery of the disk is laid a ring. 
prolonged on one side into a rod, whose end fits at T into the arm of a lever 
working about a fixed axis. As the central point of the excentric disk is 
always at an equal distance from the point T, then, during a half revolution 
of the principal axis, the lever arm at T’ must pass from the position in fig. 
33 to that in fig. 34, and back again when the revolution is completed. 
Thus the point T describes an arc, whose chord is equal to the diameter of 
the circle described by the central point of the excentric disk (during each 
rotation of the principal axis). As shown by fig. 32, the fixed axis, F, of 
the lever arm passes through the whole breadth of the machine. To this 
axis are attached two perfectly equal and parallel lever arms, N, on either 
side of the receiver, containing the slide-valve. Fig. 32 represents these 
foreshortened ; figs. 33 and 34 exhibit only one of them, but in its true 
shape. A vertical rod, M, is attached to each of these lever arms; these 
rods being connected above by a horizontal cross-head bar, Q, to the middle 
of which is attached the rod R; to the latter the slide-valve is fastened. It 
is evident that the motion of the lever arms, N, must produce a rise and 
fall of the cross-head, Q, by means of the rods, M, and thereby elevate and 
depress the valves themselves. 
Other applications of the steam-engine are to steamboats and locomotives. 
As, however, the principle is the same in all, being only modified for the 
special purpose, it is unnecessary to consider them here, especially as we 
shall have occasion to describe them minutely in another part of our 
work. 
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