(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 In this and all other contrivances in which the apertures by which the steam is admitted to and withdrawn from the piston are removed to any considerable distance from the top and bottom of the cylinder, there is a waste of steam, for the steam consumed at each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 G and the top or bottom of the cylinder. Any arrangement which would throw the passages H and I 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, passing [Pg231] through a stuffing-box F; G G 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 coming from the boiler; P is the pipe leading to the condenser; T H is a hollow space formed in the slide always in communication with the steam 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 T H 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 T H 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. [Illustration: _Fig._ 49.] [Illustration: _Fig._ 50.] [Illustration: _Fig._ 51.]