STEAM AS A WORKING POWER 



183 



When water is heated in an open vessel, the pressure of its 

 steam is too low to be of practical value, but if on the contrary 



FIG. 126. Steam as a source of 

 power. 



FIG. 127. Steam turbine. 



water is heated in an almost closed vessel, its steam pressure 

 is considerable. If steam at high pressure is directed by noz- 

 zles against the blades of a wheel, rapid rotation of the wheel 

 ensues just as it did in Figure 121, although in this case steam 

 pressure replaces water pressure. After the steam has spent 

 itself in turning the turbine, it condenses into water and makes 

 its escape through openings in an inclosing case. In Figure 

 127 the protecting case is removed, in order that the form of 

 the turbine and the positions of the nozzles may be visible. 



The steam turbine is very much more efficient than its 

 forerunner, the steam engine. The installation of turbines 

 on ocean liners has been accompanied by great increase 

 in speed, and by an almost corresponding decrease in the 

 cost of maintenance. 



