CHAP. V.] 



THE STEAM-ENGINE. 



395 



the cylinder ; when the upper tap is closed and the other opened, 

 the steam will pass into A, where it will work on the lower surface of 

 the piston and tend to make it rise. 



But here a difficulty presents itself if the steam is present at the 

 same time in A and B, since its elastic force is the same on both sides, 

 its action on the lower face will exactly compensate its action on the 

 upper face and no motion will be produced. 



Some means then must be found to destroy its elastic force as 

 soon as it has acted, and this alternately in the two chambers of the 

 cylinder. This is accomplished by opening successively the taps R',R' J 

 by which the steam is permitted, after forcing the piston to the 

 opposite end of the cylinder, to escape freely into the open air, or 

 to pass into a vessel which 

 contains cold water, the 

 sides of the chamber being 

 also kept at a low tempera- 

 ture. As soon as the steam 

 reaches this chamber, which 

 is called the condenser, it is 

 almost entirely precipitated 

 in the form of liquid, and 

 what remains is at a very 

 low pressure, far inferior to 

 that of the steam either in 

 the boiler or the cylinder. 



This arrangement is necessary in engines in which the steam acts 

 with a tension not much greater than that of the atmosphere ; when 

 the tension of the steam is equal to several atmospheres, a condenser 

 is no longer required, the condensation may take place in the open air. 



It is easy to see then that in either of these cases the difficulty is 

 overcome ; for if we imagine the upper tap R open and the lower one 

 closed while the upper tap R' is closed and the lower one opened, the 

 steam enters B where it exerts its force, while that which is in A 

 condenses, and a vacuum is formed below the piston which descends to 

 the bottom of the cylinder. At this moment the taps are reversed ; 

 the steam in the boiler enters A, that in B condenses, and the piston 

 is lifted from the bottom to the top. And so on indefinitely. 



This then in its principle and fundamental arrangements is the 



FIG. 273. The essential parts of the steam-engine. 



