482 



THE STEAM-ENGINE. 



It is sometimes required to put one tube or passage alternately in communica- 

 tion with two others. This is accomplished by a two-way cock. In this cock the 

 passage is curved, opening usually at points on the surface of the cock, at right 

 angles to each other. 



When it is required to put four passages alternately in communication by 

 pairs, a four-way cock is used. Such a cock has two curved passages (fig. 40), 



Fig. 40. 



Fig. 41. 



each similar to the curved passage in the two-way cock. Let S C B T be the 

 four tubes which it. is required to throw alternately into communication by pairs. 

 When the cock is in the position fig. 40, the tube S communicates with T, and 

 the tube C with B. By turning the cock through a quarter of a revolution, as 

 in fig. 41, the tube S is made to communicate with B, and the tube C with T ; 

 and if the cock continue to be turned at intervals through a quarter of a revo- 

 lution, these changes of communication will continue to be alternately made. 

 It is evident that this may be accomplished by turning the cock continually in 

 the same direction. 



The four-way cock is sometimes used as a substitute for the valves or slides 

 in a double-acting steam-engine to conduct the steam to and from the cylinder. 

 If S represent a pipe conducting steam from the boiler, C that which leads to 

 the condenser, T the tube which leads to the top of the cylinder, and B that 

 which leads to the bottom, then when the cock is in the position fig. 40, steam 

 would flow from the boiler to the top of the piston, while the steam below it 

 would be drawn off to the condenser ; and in the position fig. 41, steam would 

 flow from the boiler to the bottom of the piston, while the steam above it would 

 be drawn off to the condenser. Thus, by turning the cock through a quarter of 

 a revolution toward the termination of each stroke, the operation of the machine 

 would be continued. 



One of the disadvantages which is inseparable from the use of a four-way 

 cock for this purpose is the loss of the steam at each stroke, which fills the 

 tubes between the cock and the ends of the cylinder. This disadvantage could 

 only be avoided by the substitution of two two-way cocks instead of a four-way 

 cock. A two-way cock at the top of the cylinder would open an alternate com- 

 munication between the cylinder and steam-pipe, and the cylinder and con- 

 denser, while a similar office would be performed by another two-way cock at 

 the other end. 



The friction on cocks of this description is more than on other valves ; but 

 this is in some degree compensated by the great simplicity of the instrument. 

 When the cock is truly ground into its seat, being slightly conical in its form, 

 the pressure of the steam has a tendency to keep the surfaces in contact ; but 

 this pressure also increases the friction, and has a tendency to wear the seat 

 of the cock into an elliptical shape. Consequently, such cocks require to be 

 occasionally ground and refitted. 



