II 



passage I is made to communicate with the tube S. Steam, therefore, passes 

 from the boiler through I 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 page 448. 



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 sieam-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 and withdrawn from the piston are removed to any considerable distance from 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 oif 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 the most general use, is represented in 

 figs. 30, 31 ; E is the rod by which the slide is moved, passing 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 conden- 

 ser ; 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. 32, where 

 a represents the top of the passage marked a in fig. 30. In the position of 

 the slide represented in fig. 30, the steam filling the space T H has access to 

 the top of the cylinder, but is excluded from the bottom. The steam which 



