480 THE STEAM-ENGINE. 



These passages are formed in nozzles of iron or other hard metal cast upon 

 the side of the cylinder. These nozzles present a smooth face outward, upon 

 which the slides B B 7 , also formed with smooth faces, play. The slides B B 7 

 are attached by knuckle-joints to rods E E 7 , which move through stuffing-boxes, 

 and the connexion of these rods with the slides is such that the slides have 

 play so as to detach their surfaces easily from the smooth surfaces of the noz- 

 zles when not pressed against these surfaces. The steam in the steam-pipe 

 A A will press against the backs of the slides B B 7 , and keep their faces in 

 steam-tight contact with the smooth surfaces of the nozzles. These slides 

 may be opened or closed by proper mechanism at any point of the stroke. 

 When steam is to be admitted to the top of the cylinder, the upper slide is 

 raised and the passage^ S opened ; and when it is to be admitted to the bottom 

 of the cylinder, the lower slide is raised and the passage S 7 opened ; and its 

 communication to the top or bottom of the cylinder is stopped by the lowering 

 of these slides respectively. On the other side of the cylinder are provided 

 two passages C C 7 leading to a pipe G, which is continued to the condenser. 

 On this pipe are cast nozzles of iron or other metal, presenting smooth faces 

 toward the cylinder, and having passages D D' communicating between the 

 top and bottom of the cylinder respectively and the pipe G G leading to the 

 condenser. Two slides b V, having smooth faces turned from the cylinder, 

 and pressing upon the faces of the nozzles D D 7 , are governed by rods playing 

 through stuffing-boxes, in the same manner as already described. The faces 

 of these slides being turned from the cylinder, the steam in the cylinder having 

 free 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 con- 

 denser 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, BO as to allow the steam below the piston to 

 escape through G to the condenser, other two passages S 7 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 re- 

 maining unchanged ; and when the piston has reached the bottom of the cyl- 

 inder, the lower steam-slide opens the passage S 7 , 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 7 , 

 and at the same time the steam above it being drawn away to the condenser 

 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 y 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 con- 

 trivance will be very small. 



In the slide-valves commonly used, the packing of hemp at the back of the 

 slide, by which the pressure necessary to keep the slide in steam-tight contact 

 is obtained, requires constant attention from the engine-man while the engine 

 is at work. Any neglect of this will produce a corresponding loss in the power 

 of the engine ; and accordingly it is found that in many cases where engines 

 work inefficiently, the defect is owing either to ignorance or want of attention 

 on the part of the engine-man in the packing of the slides. In Seaward's 

 slides no hemp-packing is used, nor is any attention on the part of the engine- 



