THE STEAM-ENGINE. 



483 



The four-way cock, as above described, admits the steam to one end of the 

 piston at the same moment that it stops it at the other end. It would therefore 

 be inapplicable where steam is worked expansively. A slight modification, 

 however, analogous to that already described in the slides, will adapt it to 

 expansive action. This will be accomplished by giving to one of the pas- 

 sages through the cock one aperture larger than the other, and working the 

 cock so that this passage shall always be used to conduct steam to the 

 cylinder ; also by enlarging both apertures of the other passage, and using 

 it always to conduct steam from the cylinder. The effect of such an arrange- 

 ment w'ill be readily understood. 



Let the position of the cock at the commencement of the descending stroke 

 be represented in fig. 42. Steam flows from S through T to the top of the 



Fig. 42. 



Pig. 43. 



cylinder, while it escapes from B through C from the bottom of the cylinder. 

 When the piston has arrived at that point at which the steam is to be cut off, 

 let the cock ke shifted to the position represented in fig. 43. The passage of 

 steam from the boiler is now stopped, but the escape of steam from the bottom 

 of the cylinder through C continues, and the cock is maintained in this position 

 until the piston approaches the bottom of the cylinder, when it is further shifted 

 to the position represented in fig. 44. Steam now flows from S through B 



to the bottom of the cylinder, while the steam from the top of the cylinder es- 

 capes through C to the condenser. When the piston has arrived at that point 

 where the steam is to be cut off, the cock is shifted to the position represented 

 in fig. 45. The communication between the steam and the bottom of the pis- 

 ton is now stopped, while the communication between the top of the cylinder 

 and the condenser is still open. During the next double stroke of the piston, 

 the position of the cock is similarly changed, but in the contrary direction, and 

 in the same way the motion is continued. Under these circumstances the 



