Transportation on Land 309 



as determined by the slide valve and its controlling mechanism. 

 This involves the second point stated above. 



The compressibility of steam together with its volume as 

 compared with the volume of the water from which it is pro- 

 duced, makes it wonderfully useful for power purposes. Pres- 

 sure may be exerted to compress steam by mechanical means. 

 Like air as in the case of the bicycle pump, it may be reduced 

 in volume and when released resume its former volume with 

 much force. The greater the pressure, the more the volume of 

 a unit weight of steam is decreased. And as the pressure 

 decreases, the more the volume of a unit weight 6f steam in- 

 creases. In the boiler, steam is produced at increasing pressure 

 and is consequently not able to expand to its full capacity. 

 When it passes from the steam chest, where it is under the high 

 pressure of the boiler, into the cylinder, a space of low pressure, 

 it expands, rapidly occupying all available space. It drives 

 the piston forward or backward as it enters from one port or the 

 other (pages 293-4). In expanding it uses its own heat energy 

 and becomes cool. The same fact may be observed when air is 

 escaping from an inflated tire or basketball. In using up its 

 heat energy by expanding, the steam may be made to do useful 

 work, that is, to drive the piston back and forth, its heat energy 

 being converted into mechanical energy. Thus the steam engine 

 is really a heat engine. 



136. Superheated steam. For the purpose of power to do 

 work, steam is generated in boilers under high pressure and has 

 thus a correspondingly high temperature. In many types of 

 the modern engine steam is utilized only after it has been 

 removed from the boilers and subjected to additional heat for 

 the purpose of raising its temperature and thereby increasing 

 its energy. Steam thus treated is known as superheated. A 

 superheater is shown in Fig. 97. The superheating of steam 

 has another advantage in the fact that it effects an economy of 

 fuel, a matter of serious consideration to-day in view of the 

 rapidly increasing cost of fuel. 



