88 HEAT AND ENERGY 



Various engines, run by steam, hot air, gas explosions, 

 and naphtha, which are now widely used for many pur- 

 poses, get their energy from heat. In all these engines 

 the force which finally causes the motion is the expansive 

 force of some gas ; but the energy which causes the gas 

 to expand is supplied by heat. 



100. The Steam Engine. It is of course well known 

 that if a certain amount of a liquid be changed to a gas, 

 the volume of the gas will be far greater than that of the 

 liquid. But if this change is made in a closed vessel, 



/ 



P 



FIG. 65 



the gas will exert great force in trying to expand to its 

 larger volume. A steam engine makes use of the force 

 exerted by steam when thus trying to expand. 



Heat is applied to water in a boiler, changing it to 

 steam. This steam is at once led to a cylinder where it 

 is allowed to expand, first on one side and then on the 

 other of a piston, p (Fig. 65). The figure (65) should be 

 carefully studied until the action of the engine is plain. 

 Steam comes from the boiler to the steam chest d through 

 a pipe t. A valve v moves to and fro in d, allowing 

 the steam to pass to the cylinder c, first to one end and 

 then the other through ports a and b. The arrows show 



