FORCE OF EXPANDING GASES 



23 



great pressure, because it is crowded into so small a 

 space. As it rushes out of the cartridge it expands, and 

 drives the bullet rapidly before it. 



The steam engine, like the gun and cannon, is a device for producing 

 motion by means of the expansion of a compressed gas. Steam is pro- 

 duced, under high pressure, in a boiler (Fig. 19), and is allowed to ex- 

 pand in a cylinder, first on one side and then on the other, of a 



FIG. 19. 

 Principle of the Steam Engine. 



piston. The piston is thus moved rapidly to and fro. This forward and 

 backward motion is then changed to motion in a circle by means of a 

 shaft. The eccentric turns with the shaft and moves the slide valve. 

 The cause of motion in a gasoline engine is also the force exerted by 

 expanding gases. A mixture of gasoline vapor and air, which is ex- 

 ploded by an electric spark or a hot wire, is used in place of steam. 

 The explosion produces a large volume of hot gases, and the expansion 

 of the gases sets a piston in motion. 



The force exerted in the examples given may be put in 

 one of the following classes : 



