In the first place, an amount of steam is saved in each revolution of the engine equivalent to the capacity of the steam ports, and clearance between the ends of the cylinder and piston, which in many cases is fully 5 per cent. of the total steam used, in addition to transferring the steam necessary to heat the cylinder after being cooled by communication with the condenser.

In the second place, with the ordinary slide valve, provided the vacuum is good and steam ports large enough, there is nothing or very little to compress in the low-pressure cylinder, and thus severe shocks are imparted to the several joints and moving parts. But, by the adoption of this arrangement of valve, the steam is taken from the other end of the cylinder, after performing all the work possible, and compressed nearly up to initial pressure, using the work stored in the piston for this purpose, so that there is a far less shock consequent on changing the direction of the motion of the piston, and more revolutions (the inventor asserts) can be got out of the engines with the same indicated horse-power.

As the change of pressure from the one side of the piston to the other passes through the stages-(1) release by communication opening; (2) compressing steam transferred; (3) exhaust opens to condenser; (4) steam at initial pressure enters-it is obvious that the engine turns the dead points with far less shock, than in

the ordinary arrangement of slide valve, where it is sometimes necessary to give great lead to the low-pressure valve, especially on the bottom centre, to arrest the momentum of the vertically moving parts. As, however, in the use of this valve, the work stored in the piston and other moving parts is used to compress steam taken from the opposite end of the cylinder, and is not thrown away against the steam admitted by the lead of the valve, as in the ordinary arrangement, no lead of the valve is necessary as here used, and the engine turns the centre without knocking, even if the brasses are rather slack.

A further advantage in the use of this valve lies in its being lighter than the ordinary double-ported slide valve, and taking up less space in depth, which enables it to be more easily overhauled. By its use, the friction on the cylinder port face is greatly reduced by the fact, that one port is always exposed to the pressure in the cylinder, thereby tending to reduce that on the valve face.

Piston Valve. As we remarked before, no thoroughly satisfactory plan has yet been devised for relieving the ordinary slide valve from the pressure of steam upon its back surface; consequently, we find that now with the very high steam pressures carried in marine and other boilers (about 100 lbs. for compound, 150 lbs. for triple, and 180 to 200 lbs. on the square inch for quadruple-expansion engines), it has become absolutely necessary, in the case of large engines, to adopt another form of valve for admitting and exhausting the steam from the high-pressure cylinders. This form of valve is known as the piston valve, and we illustrate one fitted with Thom's "trick arrangement for

producing sufficient compression on the exhausting side of the piston to turn the dead points smoothly, and to economise steam as explained above. It consists of two pistons, P1, P2, of cast iron, connected together by a cast-iron pipe and fixed to the valve spindle, VS, as shown. These pistons fit and work in liners, L1, L2, inserted into the ends of a cylindrical chamber, CC, bolted to the side, or to the end of the high-pressure steam cylinder. They are suitably packed with expanding rings, R. R2, Rg, R, like locomotive piston rings, which are frequently made of bronze. The pistons, P1, P2, open and shut the two steam ports, SP1, S P2, from the steam chest, and the exhaust port, EP, in the same way as an ordinary slide valve does; but the area of steam port is much larger than that of an ordinary slide valve of the same size across, and there is no unbalanced pressure owing to its cylindrical form, hence no frictional wear and tear except that due to the pressure of the expansion rings, R1, Rg R, R. To prevent these expansion rings from springing

outwards beyond the bore of the liners, L, L, when they come opposite the steam ports, SP1, SP, the faces of the steam ports

in the liners are cast diagonally so W. With piston valves, the clearance spaces are naturally rather larger than with the ordinary slide valve, but this is minimised as much as possible by keeping the pistons, P1, P2, as far apart as practicable. The trick pipe or passage, for communicating with both ends of the valve, in order to effect the necessary compression when the piston comes near the ends of the cylinder, is seen at, TP. In large vertical piston valves, there is a small balance piston with cylinder (similar to BP, p. 234), fitted to the upper end of the valve spindle, in order to support the weight of the valve, valve spindle, eccentric rods, and links. Piston valves can be made lighter than ordinary slide valves, but they are essentially single-ported valves, and have therefore as a rule a longer travel than doubleported slides. One would have expected that with piston valves, much lighter valve spindles, links, and rods could have been used than with ordinary slide valves doing the same work; but this does not appear to be the case in practice. Piston valves are sometimes fitted with expansion piston valves inside the main valves. Their construction is similar to the latter, but the arrangement is somewhat complicated and difficult to get at for adjustment and for repairs. All the latest Transatlantic Liners, as well as many of the best passenger steamers, have been fitted with piston valves. The only objection or disadvantage usually urged against these valves, is their first cost, which naturally prevents their being commonly fitted to the low-pressure cylinder of compound, or of multiple expansion engines.

Reversing Link Motion.-The reversing of an engine which has ordinary slide or piston valves, is most easily affected by means of a combination of links and rods, which is known as "link motion." In order that an engine may work in both directions, the eccentric which moves the valve must always remain at a given fixed angle in advance of the crank; and evidently with one eccentric fixed on the shaft, the position of this advance is dependent upon the direction of rotation, and is not the same when the engine rotates in either way. The arrangement by which the desired object is attained with link motion is as follows:-Two eccentrics are fitted to the crank shaft side by side, the one being set in such a position relative to that of the crank, as to control the valve properly when the engine is going in one direction, and the other in a position to control the valve when the engine is rotating in the opposite direction. These eccentrics are connected by separate eccentric-rods and straps to the ends of a link, in which a block connected to the

*We have already described and illustrated the reversing motion for an Oscillating Engine in Lecture XIX., and Joy's Valve Gear in Lecture XX.

valve-rod is fixed, and which is capable of sliding from end to end. When the link is drawn to the one side, the block being stationary, comes into line with one of the eccentric-rods, and the valve is worked by that eccentric to which this rod is attached. If the link be pushed over to the other side, the other eccentric comes into play with the valve, and the engine rotates in the opposite direction. If the link be placed in such a position that the block which is connected to the valve stands in the middle of the link, then the engine stops, since the valve is thereby placed in mid-position, and simply travels to and fro over the valve port face, a distance nearly equal to the lap plus the lead. The construction of these links is very various. One form is shown in the following figure.* It is a very simple form, and

It consists of a

has been used for small engines for many years. flat piece of iron with a circular slot in it, in which a block attached to the valve-rod is fitted. This block is able to slide lengthwise along the link. The link has two snugs formed on one side for the attachment of the eccentric-rods; and also a