cessary space in the valve-box. The area, or transverse section of the valve-box, should be rather more than double the magnitude of the upper surface of the valve, in order to allow a sufficiently free passage for the steam, and the play of the valve should be such as to allow it to rise from its seat to a height not less than one fourth of the diameter of its upper surface.

The valves coming under this class are sometimes formed as spheres or hemispheres resting in a conical seat, and in such cases they are generally closed by their own weight, and opened by the pressure of the fluid which passes through them.

(132.) One of the advantages attending the use of slides, compared with the other form of valves, is the simplicity with which the same slide may be made to govern several passages, so that a single motion with a slide may perform the office of two or more motions imparted to independent valves.

In most modern engines the passage of the steam to and from the cylinder is governed by slides of various forms, some of which we shall now explain.

(133.) In figs.47. and 48. is represented a slide-valve contrived by Mr. Murray of Leeds.

A B is a steam-tight case attached to the side of the cylinder; E F is a rod, which receives an alternate motion, upwards and downwards, from the eccentric, or from whatever other part of the engine is intended to move the slide. This rod, passing through a stuffing-box, moves the slide & upwards and downwards. s is the mouth of the steam pipe coming from the boiler; T is the mouth of a tube or pipe leading to the condenser; H is a passage leading to the top, and I to the bottom, of the cylinder. In the position of the slide represented in fig. 47., the steam coming from the boiler through s passes through the space н to the top of the cylinder, while the steam from the bottom of the cylinder passes through the space I into the tube T, and goes to the condenser. When the rod

Fig. 48.

EF is raised to the position represented in fig. 48., then the passage H is thrown into communication with the tube T, while the passage I is made to communicate with the tube s. Steam, therefore, passes from the boiler through 1 below the piston, while the steam which was above the piston, passing through H into T, goes to the condenser. Thus the single slide G performs the office of the four valves described in (116.).

(134.) The slide G has always steam of a full pressure behind it, while the steam in front of it escaping to the condenser, exerts but little pressure upon it. It is therefore always forcibly pressed against the surfaces in contact with which it moves, and is thereby maintained steam-tight. Indeed this pressure would rapidly wear the rubbing surfaces, unless they were made sufficiently extensive, and hardened so as to resist the effects of the friction. Where fresh water is used, as in land boilers, the slide may be made of hardened steel; and in the case of marine boilers, it may be constructed of gun-metal. In this and all other contrivances in which the apertures by which the steam is admitted to and withdrawn from the piston are removed to any considerable distance from the top and bottom of the cylinder, there is a waste of steam, for the steam consumed at each stroke of the piston is not only that which would fill the capacity of the cylinder, but also the steam which fills the passage between the slide & and the top or bottom of the cylinder. Any arrangement which would throw the passages H and on the other side of the slide G, that is, between s and G, instead of being, as they are, between G and the top and bottom of the cylinder, would remove this defect. This is accomplished by a slide, which is usually called the D valve, because, being semi-cylindrical in its form, and hollow, its cross section resembles the letter D. This slide, which is that which at present is in most general use, is represented in figs. 49, 50.; E is the rod by which the slide is moved, pass

ing through a stuffing-box F; GG is the slide represented by a vertical section, a a being a passage in it extending from the top to the bottom; s is the mouth of the great steam pipe

Fig. 51.

coming from the boiler; P is the pipe leading to the condenser; TH is a hollow space formed in the slide always in communicawith the steam

tion

pipe s, and consequently always filled with steam from the boiler. A transverse

section of the slide and

cylinder is represented in fig. 51., where a represents the top of the passage marked a in fig. 49. In the position of the slide represented in fig. 49., the steam filling the space TH has access to the top of the cylinder, but is excluded from the bottom. The steam which was below the piston, passing up the passage a, escapes through the tube P to the condenser. When the piston has descended, the rod E moves the slide downwards, so as to give it the position represented in fig. 50. The steam in TH has now access to the bottom of the cylinder, while the steam above the piston passing through P escapes to the condenser. In this way the operation of the piston is continued and the steam consumed at each stroke only exceeds the capacity of the cylinder by what is necessary to fill the passages between the slide and the cylinder.

In a slide constructed in this manner, the steam filling the space Tн has a tendency to press the slide back, so as to break the contact of the rubbing surfaces, and thereby to cause the steam to leak from the space T H to the back of the slide. This is counteracted by the packing x, at the back of the slide.

In engines of very long stroke, the extent of the rubbing surfaces of slides of this kind renders it difficult to keep

them in steam-tight contact and to insure their uniform wear. In such cases, therefore, separate slides, upon the same principle, are provided at the top and bottom of the cylinder, moved, however, by a single rod of communication.

(135.) In slides, as we have here described them, the same motion which admits steam to either end of the cylinder, withdraws it from the other end. Such an arrangement is only compatible with the operation of a cylinder which works without expansion; for in such a cylinder the full flow of steam to the piston is only interrupted for a moment during the change of position of the slide. But if the steam act expansively, it would be necessary to move the slide, so as to stop its flow to one end of the cylinder, without at the same time obstructing the escape of steam from the other end to the condenser. It would therefore be necessary that the slide should close the passage leading to the cylinder at one end, without at the same time obstructing the communication between the passage from the cylinder to the condenser at the other end. On the arrival of the piston, however, at the bottom of the cylinder, it would be necessary immediately to put the lower passage to the cylinder in communication with the steam pipe, and the upper passage in communication with the condenser. This would necessarily suppose two motions of the slide as well as some modifications in its length. Let the length of the slide be such that when the passage to the top of the cylinder is stopped, the lower part of the slide shall not reach the passage to the lower part of the cylinder; and let such a provision be made in the mechanism by which the rod E governing the slide is driven that it shall receive two motions during the descent of the piston, the first to be imparted to it at the moment the steam is to be cut off, and the second just before the termination of the stroke. Let the position of the slide, at the commencement of the stroke, be represented in fig. 52., and let it be required that the steam shall be cut off at one half of the stroke. When the piston has made half the stroke, the rod governing the slide is moved downwards, so as to throw the slide into the position represented in fig. 53. The passage between the steam pipe and the cylinder is

now stopped at both ends; but the passage from the bottom of the cylinder to the condenser remains open. During the remainder of the stroke, therefore, the steam in the cylinder

works expansively. As the piston approaches the bottom of the cylinder, another motion is imparted to the rod governing the slide, by which the latter is thrown into the position represented in fig. 54. Steam now flows below the piston while the steam above it passes to the condenser. In a similar manner, by two motions successively imparted to the slide during the ascent of the piston, the steam may be cut off at half stroke; and it is evident that by regulating the time at which these motions are given to the slide, the steam may be worked expansively, to any required extent.

It is easy to conceive various mechanical means by which, in the same engine, the point at which the steam is cut off may be regulated at pleasure.

In cases where the motion of the piston is very rapid, as in locomotive engines, it is desirable that the passages to and from the cylinder should be opened very suddenly. This is difficult to be accomplished with any form of slide consisting of a single aperture; but if, instead of admitting the steam to the cylinder by a single aperture, the same magnitude of opening were divided among several apertures, then a proportionally less extent of motion in the slide would clear the passage for the steam, and consequently greater suddenness of opening would be effected.