25 



SCIENCE OF HOME AND COMMUNITY 



air is drawn in through the pipes and fire box, thus making a 

 draft. Some engines are so built that they use liquid oil 

 instead of coal. This is forced in the form of a fine spray 

 into the fire box, where it burns quickly. 



Cylinder and valves. The parts so far described are used 

 to convert water into steam. The rest of the engine con- 

 verts the energy into the motion of the wheels. The steam 

 is led from the boiler into the steam chest S (figure 93). 

 From here two openings p and p f , called ports, lead to the 



cylinder C. Over these 

 openings is the slide valve 

 V, with its under surface 

 hollowed out. This valve 

 is connected with an eccen- 

 tric. In the cylinder is a 

 piston connected on the out- 

 side by means of a connect- 

 ing rod with the drive wheel 

 of the locomotive. 



When the steam enters 

 the chest 5, it passes 

 through p or p f and enters 

 the cylinder C, pushing the 

 piston to the other end of the cylinder. This turns the wheel, 

 which moves the eccentric, and this moves the valve V in 

 such a way that the port p is closed and the port p' is 

 opened. Thus the steam enters through this opening and 

 pushes the piston back, -and the steam in the other end of 

 the cylinder is forced out through the opening p, and then 

 through the exhaust e to the outside. 



Eccentric. The eccentric consists of a circular plate 

 mounted on the crank shaft. The opening in this plate is 

 not at the center, but at one side, so that as the shaft re- 

 volves, it gives a slight motion to the eccentric rod, similar 

 to that given by a small crank, and this rod in turn moves 



FIG. 93. Cylinder and steam chest of 

 steam engine. 



