418 THE APPLICATIONS OF PHYSICAL FORCES. [BOOK iv. 



of steam in it, which the piston drives back and compresses before 

 the commencement of the new period of admission. 



Clapeyron's expansion system is chiefly employed in engines for 

 rapid motion, such as locomotives. 



In Meyer's expansion system the slide-valve is pierced by two 

 orifices, which are brought alternately into communication with the 

 entrance ports, and there are two plates, having a motion independent 

 of the slide, which come and close these orifices, so as to stop the 

 admission and start the expansion. 



Lastly, in Woolff' s system the expansion does not take place in 

 the cylinder itself, but in a cylinder of greater diameter placed close 



FIG. 294. Section of the two cylinders in Woolff' s expansion system. 



to the first (Fig. 295). It is for this reason that engines which 

 employ this method of expansion are called double-cylinder engines. 



Fig. 294 shows the distributing machinery in these engines. 



Each of the two cylinders A, B, is provided with a valve-chest in 

 which an ordinary slide-valve works, with entrance and exhaust ports 

 as usual. 



The steam comes from the boiler by the orifice v, which opens first 

 into the chest of the cylinder A, and thence passes, say, below the 

 piston P. This piston receives an upward motion, and drives back the 

 steam which was on the other side into the outlet pipe E, a pipe 



