THE STEAM-ENGINE. 481 



man required after the slides are first adjusted. The slides receive the pres- 

 sure necessary to keep them in steam-tight contact with the surfaces of the 

 nozzles from the steam itself, which acts behind them. 



The eduction and steam slides heing independent of each other, they may 

 be adjusted so that the engine shall work expansively in any required degree ; 

 and this may be accomplished either by working the slides by separate 

 mechanism, or by a single eccentric. 



One of the advantages claimed by the patentees for these slides is, that the 

 engines are secured from the accidents which arise from the accumulation of 

 water within the steam-cylinder. If such a circumstance should occur, the 

 action of the piston will press the water against the faces of the steam-slides, 

 and the play allowed to them by their connexion with the rods which move 

 them permits their faces to be raised from the surfaces of the nozzles, so that 

 the water collected in the cylinder shall be driven into the steam-pipe, and sent 

 back thence to the boiler. 



Of the cocks or valves which are opened and closed by the motion of an 

 axis passing through their centre, the throttle-valve, whether worked by hand 

 or by the governor, is an example. But the most common form for cocks is 

 that of a cylindrical or slightly conical plug, fig. 37, inserted in an aperture of 



Fig. 37. 



corresponding magnitude passing across the pipe or passage which the cock is 

 intended to open or close. One or more holes are pierced transversely in the 

 cock, and when the cock is turned so that these holes run in the direction of 

 the tube, the passage through the tube is opened ; but when the passage through 

 the cock is placed at right angles to the tube, then the sides of the tube stop 

 the ends of the passage in the cock, and the passage through the tube is ob- 

 structed. The simple cock is designed to open or close the passage through 

 a single tube. When the cock is turned, as in fig. 38, so that the passage 

 through the cock shall be at right angles to the length of the tube, then the 

 passage through the tube is stopped ; but when the cock is turned from that 

 position through a quarter of a revolution, as in fig. 39, then the passage through 



Pig. 38. Tig. 39. 



the cock takes the direction of the passage through the tube, and the cock is 

 opened, and the passage through the tube unobstructed. In such a cock the 

 passage may be more or less throttled by adjusting the position of the cock, 

 so that a part of the opening in it shall be covered by the side of the tube. 

 VOL,. II. 31 



