STEAM COCKS. 



communication with them, has a tendency to keep them by its 

 pressure in steam-tight contact with the surfaces in which the 

 apertures leading to the condenser are formed. These two slides 

 may be opened or closed whenever it is necessary. 



When the piston commences its descent, the upper steam slide 

 is raised, so as to open the passage s, and admit steam above the 

 piston ; and the lower exhausting slide b' is also raised, so as to 

 allow the steam below the piston to escape through G, the other 

 two passages s' and c being closed by their respective slides. The 

 slide which governs s is lowered at that part of the stroke at which 

 the steam is intended to be cut off, the other slides remaining 

 unchanged ; and when the piston has reached the bottom of the 

 cylinder, the lower steam slide opens the passage s', and the upper 

 exhausting slide opens the passage c, and at the same time the 

 lower exhausting slide closes the passage c'. Steam being admitted 

 below the piston through s', and at the same time the steam above 

 it being drawn away through the open passage c and the tube G, 

 the piston ascends. "When it has reached that point at which the 

 steam is intended to be cut off, the slide which governs s' ( is 

 lowered, the other slides remaining unaltered, and the upward 

 stroke is completed in the same manner as the downward. 



These four slides may be governed by a single lever, or they 



may be moved by separate means. From the small spaces between 



the several slides and the body of the cylinder, it will be evident 



that the waste of steam by this contrivance will be very small. 



32. The admission and escape of the steam is sometimes 



governed by cocks, more espe- 

 cially in engines constructed on a 

 small scale. The most common 

 form for cocks is that of a cylin- 

 drical or slightly conical plug 

 (fig. 20), inserted in an aperture 

 of corresponding magnitude pass- 

 ing 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 obstructed. The simple cock is designed to 

 open or close the passage through a single tube. When the cock 

 is turned, as in fig. 21, so that the passage through the cock shall 

 be at right angles to the length of the tube, then the passage 



27 



