414 THE APPLICATIONS OF PHYSICAL FORCES. [BOOK iv. 



as it does, of two partial operation s, the admission of the steam 

 and its escape, which must be repeated twice to obtain a complete 

 phase of the to-and-fro movement of the slide-valve ? There are 

 various methods employed according to the different engines we will 

 describe first that which is represented by the figure. 



In the valve chest BB, is seen a prismatic box, open on one side, 

 called the slide-valve. The slide-valve is applied by its open face to 



the well-polished plane on which, 

 as we mentioned before, the three 

 ports open. The space BB is called 

 the valve-chest ; the steam coming 

 from the boiler by the pipe v spreads 

 out freely in it, but the inside 

 of the slide-valve, on the con- 

 trary, is always closed to the en- 

 tering steam, but is constantly in 

 communication with the escape-pipe, 

 and also with first one and then 

 the other of the entrances to the 

 cylinder. Lastly, the movement 

 of the slide-valve is produced by 

 the engine itself, by the aid of a 

 rod and an excentric fixed to the 

 shaft of the fly-wheel. 



By following the successive and alternating motions of the 

 slide-valve as represented in Fig. 291 we can easily comprehend the 

 different phases of the distribution of the steam. 



This is the machinery for the distribution of steam in engines 

 where the three-port slide-valve is adopted. But, as already said^ 

 there are other arrangements employed. There is first Watt's system 

 of distributing valves, then there are the piston slide-valves of the 

 same inventor, and lastly the I) valves, a name due to the re- 

 semblance that the principal part bears to the letter D (Figs. 290, 291, 

 and 292). 



In the first of these three systems two valve boxes are fitted to 

 the two ends of the body of the cylinder. Each of these is divided 

 by two valves moved by a system of rods into three compartments, 

 of which the middle is in direct communication with each port, 



r r 



FIG. 290. Longitudinal section of a cylinder. 



