GAS ENGINES 



317 



low, steam is formed slowly and the piston moves less 

 rapidly. 



The steam engine as seen on our railroad trains is very 

 complex and cannot be discussed here ; in principle, however, 

 it is identical with that just described. Figure 190 shows a 

 steam harvester at work on a modern farm. 



: 



FIG. 190. Steam harvester at work. 



In both engine and turbine the real source of power is not 

 the steam, but the fuel, such as coal or oil, which converts the 

 water into steam. 



Gas engines. Automobiles have been largely responsible 

 for the gas engine. To carry coal for fuel and water for steam 

 would be impracticable for most motor cars. Electricity is 

 used in some cars, but the batteries are heavy, expensive, and 

 short-lived, and are not always easily replaceable. For this 

 reason gasoline is extensively used, and in the average auto- 

 mobile the source of power is the force generated by exploding 

 gases. 



It was discovered some years ago that if the vapor of gaso- 

 line or naphtha was mixed with a definite quantity of air, and 



