CHAP, vi.] THE STEAM-ENGINE. 411 



CHAPTER VI. 



THE STEAM-ENGINE. THE DRIVING MACHINERY. 



I. THE CYLINDER. 



THE steam being produced we will now see how its elastic force is 

 used. The steam leaves the steam-space of the boiler by a pipe 

 which conducts it to the inside of a cylinder, and it acts alternately on 

 one side and the other of a piston which is movable in this cylinder, 

 and this alternate action results in a to-and-fro movement of the 

 piston and its rod. 



The steam, coming from the boiler to the cylinder, acts first on one 

 face of the piston, which is pushed towards the opposite extremity. 

 At this moment the steam should enter on the other side of the cylin- 

 der, and exercise its force on the opposite face of the piston. To 

 enable this force to act effectually we must get rid of the steam that 

 has just acted in the contrary direction, because the elastic force 

 which it still possesses is opposed to the motion. This object is 

 attained by giving to the steam that has played its part an exit to 

 the outside of the cylinder at alternate ends. The space into which 

 it passes is either open to the air, or to a vessel exhausted of air 

 and kept at a low temperature by a continuous flow of cold water. 



In the first case, which is that of engines worked with high- 

 pressure steam, that is, having an elastic force of several atmospheres, 

 the steam that has done its work escapes, and its tension becomes 

 rapidly reduced to that of the ordinary air, and thus it allows the 

 steam to work on the opposite surface of the piston. 



In the second case, the steam is quickly condensed by its introduc- 

 tion into an empty and cool space, which for this reason is called 

 the condenser. Its elastic force, which is not much greater than 



