THE STEAM ENGINE. 



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

 that position through a quarter of a revolution, as in fig. 22, then 

 the passage through the cock takes the direction of the passage 



Fig. 21. 



Fig. 22. 



Fig. 



through the tube, and the cock is opened, and the passage through 

 the tube unobstructed. In such a cock the passage may be mo:-e 

 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.. 



33. It is sometimes required to put one tube or passage alter- 

 nately in communication 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 com- 

 munication by pairs, a four-way cock is used. Such a cock has 

 two curved passages (fig. 23), each similar to the curved passage 

 in the two-way cock. Let s c B T be 

 the four tube? which it is required to 

 throw alternately into communication 

 by pairs. When the cock is in the 

 position (fig. 23), the tube s communi- 

 cates with T, and the tube c with B. 

 By turning the cock through a quarter 

 of a revolution, as in fig. 24, 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 revolution, 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 to conduct the steam to and from the cylinder. 

 If s represent a pipe conducting steam from the boiler, c the 

 exhausting pipe, 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. 23), steam would flow from the boiler 

 to the top of the piston, while the steam below it would be drawn 

 off: and in the position (fig. 24), steam would flow from the boiler 

 to the bottom of the piston, while the steam above it would be 

 28 



