338 APPLIED SCIENCE 



is very liable to cause fracture. Therefore drainage should 

 not be attempted without first isolating the boilers so as to 

 minimize the danger. 



379. Turbines. We have already seen the uses of water 

 wheels or water turbines. Steam turbines (Fig. 171) consist 



of a wheel with blades. The 

 steam, in the form of jets, strikes 

 against the blades and moves 

 the wheel. This machine was 

 invented to overcome the back- 

 ward and forward (reciprocat- 

 ing) movement of the piston, 

 which jars and shakes the en- 

 gine. 



Steam turbines utilize the 

 kinetic energy of the steam. As 

 steam at the usual pressures em- 

 ployed has a very low density, 



a cubic inch of steam must have 

 FIG. 171. -Steam Turbine. & VQry high velocity if it ig to 



expel any considerable amount of kinetic energy. 



380. Action of Steam in a Turbine. In entering the 

 turbine, steam acts in two ways, and turbines are accordingly 

 constructed on two plans. The more important type and 

 the only one to be described here, is the impulse turbine, in 

 which the steam from the boiler is completely or almost 

 completely expanded into an expanding nozzle. As the 

 steam forcibly strikes the vanes of the wheel, the turbine 

 wheel rotates at a very high velocity. This is illustrated in 

 the De Laval turbine which is used in place of the ordinary 



