VALVES AND SLIDES. 



at the moments at which the piston reaches the top and bottom 

 of the cylinder, it is evident that they may be all worked by a 

 single lever connected with them by proper mechanism. When 

 the piston arrives at the top of the cylinder, this lever would be 

 made to open the valves s and c', and at the same time to close 

 the valves s' and c ; and when it arrives at the bottom of the 

 cylinder, it would be made to close the valves s and c', and to 

 open the valves s' and c. 



30. The methods of opening and closing the passages by means 

 of lids slipping over them called slides, are those most generally 

 used, and have infinitely various forms, although they differ one 

 from another but little in the principle of their action. One of 

 these expedients shown in fig. 17 18, will render the mode o 



their action easily under- 

 stood. A B is a steam-tight 



case attached to the side of 



the cylinder ; E r is a rod, 



which receives an'alternate 



motion, upwards and down- 

 wards, 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 G upwards and down- 

 wards, 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. 17, the steam coming from the boiler 

 through s passes through the space H 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 

 E F is raised to the position represented in fig. 18, 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 i below the piston, while the steam which 

 was above the piston, passing through H into T, goes to the con- 

 denser. Thus the single slide G performs the office of the four 

 valves described in 29. 



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