1 84 



THE POWER BEHIND THE ENGINE 



A single large turbine wheel may have as many as 800,000 

 sails or blades, and steam pours out upon these from many 

 nozzles. 



179. Steam Engines. A very simple illustration of the 

 working of a steam engine is given in Figure 128. Steam 



. under pressure enters 

 through the opening F, 

 passes through N, and 

 presses upon the piston M. 

 L As a result M moves 

 downward, and thereby in- 

 duces rotation in the large 

 wheel L. 



As M falls it drives the 

 air in D out through O and 

 P (the opening P is not 

 visible in the diagram). 



As soon as this is accom- 

 plished, a mechanical de- 

 vice draws up the rod E t 

 which in turn closes the 

 opening N, and thus pre- 

 vents the steam from pass- 

 ing into the part of D 

 above M. 



But when the rod E is 



FiG. 128. The principle of the steam engine. 



in such a position that N is closed, O on the other hand is 

 open, and steam rushes through it into D and forces up the 

 piston. This up-and-down motion of the piston causes con- 

 tinuous rotation of the wheel L. If the fire is hot, steam is 

 formed quickly, and the piston moves rapidly ; if the fire is 

 low, steam is formed slowly, and the piston moves less 

 rapidly. 



