so great as to neutralise much of the advantage due to the use of an expansion valve. The actual amount of expansion was consequently much less than that indicated by the fraction of the cut-off.

Expansion valve on back of main slide. To overcome this defect the gridiron valve was sometimes fitted to work on ports formed in the back of the main slide-valve, as shown in Fig. 128, the expansion valve being worked as before by a separate eccentric, and the cut-off regulated by the amount of travel of the expansion valve.

Another plan of fitting an expansion valve on the back of the main slide-valve is shown in Fig. 129. In this case the expansion valve was constructed of two separate but similar parts, connected together by a right- and left-handed screw, by means of which their distance from each other might be varied. In this arrangement the travel of the valve was constant, and the point of cut-off was regulated by the distance between the two plates that formed the valve, which could be varied as required by moving the wheel in connection.

So far as the distribution of the steam is concerned these valves, which worked directly on the backs of the main slides, had a great advantage over the expansion valves working in separate casings, as the clearance space that had to be filled with steam at each stroke was much diminished. It was, however, often found difficult to properly lubricate the working surfaces, especially in large engines, and the valves wore away rapidly, causing excessive stresses on the gear from

the great friction, and for this reason the most commonly employed expansion valve was that working in a separate casing, as in Fig. 127, which gave less trouble.

Reasons for abandonment of separate expansion valves.--The separate expansion valves when fitted were always troublesome to maintain in proper order. As steam pressures increased and larger ratios of cylinder were fitted in compound engines, the necessity for such valves passed away. It was found that most of the expansion capable of being usefully carried out could be obtained with the cut-off of the ordinary slide-valve in association with the stage expansion system, so that for a steam pressure of 90 lbs. the expansion valves were abandoned. The usual ratio of H.P. to L.P. cylinder-volumes with this pressure was 1 to 4, so that a fair amount of expansion was

FIG. 130.

obtainable within the limits of cut-off which could be given efficiently with the ordinary slide-valve.

With the modern triple and quadruple expansion engines there is still less necessity for separate expansion valves, and they are not fitted, as all the expansion required is obtainable by means of the link motion.

Independent linking-up gear.-All modern engines in the Royal Navy which are intended to work with considerable variations in the total power are provided with means for independently linking up the slide gears of the various cylinders. This gear enables the cut-off in any cylinder to be made earlier than that corresponding to the position of the main reversing gear. This latter gear, when moved by the starting

engine or wheel, alters the cut-off in the various cylinders to the same extent, but the independent linking-up gear enables various alterations of cut-off to be made in any cylinder, so that the total power at any speed may be more equally divided between the various cylinders, and any other desirable adjustments made. The effect of these changes on the distribution of power in the various cylinders is explained in Chapter XXVI.

To enable this to be effected, the reversing arms attached to the weigh-shaft are fitted with slots and sliding blocks to which the suspension rods leading to the links are attached. By moving these blocks the links are altered in position. As the alteration of the blocks takes some time, the angle of the slot is so arranged that in the astern position the slot is approximately perpendicular to the suspension rods, so that the position of the block in the slot does not affect the link in the astern position. This being so, the engines may be reversed without making any alterations in the independent linkingup gear. A sketch of this fitting is shown in Fig. 130. A nut and washer are provided on one side of the block pin, which when tightened up secure it in position. This nut is slacked back before any alterations in the position of the block are made.

Slide-jacket. The steam, after passing the regulating valve, enters the slide-jacket or casing, which is simply a rectangular or cylindrical box bolted to the cylinder, in which the slide-valve works. This slide casing is either cast in one with the cylinder, or bolted to it.

Slide-valve. The distribution of steam in each of the steam cylinders of an engine, involving the processes of admission, expansion, and finally exhaust into the receiver pipes of the succeeding cylinder or the condenser as the case may be, is now effected by the agency of

CYLINDER

S

a single valve, called the slidevalve.' The slide-valve is one of the most important parts of the engine, and on the skill and care exercised in its design and fitting, the satisfactory working of the machinery will greatly depend.

A section of an ordinary single-ported slide-valve is shown in Fig. 131.

In the cylinder face there are three passages, called 'ports,' marked respectively A, B, and c. A and B are the steam passages or ports, one leading to each end of the cylinder, and c is the exhaust port leading to the condenser. s is the slide-valve, which is rectangular in plan, and the hollow space, D, in its centre is called the exhaust cavity of the valve. The valve is shown in its central position, and it will be seen that it not only closes the steam ports, but overlaps the edges for some distance on the steam side. The object of this will be explained later.

FIG. 131.

The slide-valve has a flat face, and it works steamtight on the corresponding flat face of the cylinder. The casing around it is supplied with steam, while the exhaust cavity is connected either to the condenser, the reservoir, or the atmosphere, depending on the type of engine.

ports are generally arranged so that the ports are never wide open to the steam, so that A is not wide open. The reason for this will be explained later.

This

The valve now commences its return stroke, the piston and valve travelling in opposite directions, the next important phase being shown in Fig. 134, when the admission of steam to the cylinder is stopped by the steam edge of the valve closing the port A is called the instant of cut-off, and the remainder of the piston's motion in the direction of the arrow is caused by the expansion of the steam previously admitted. It will be noticed that the port n is still open to exhaust. The piston and valve now proceed still further in opposite directions until the piston has travelled nearly the whole of its stroke and the valve reaches the middle of its travel, as