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 backward and forward (reciprocating) movement of the piston, which jars and shakes the engine.

Steam turbines utilize the kinetic energy of the steam. As steam at the usual pressures employed has a very low density, a cubic inch of steam must have a very high velocity if it is to expel any considerable amount of kinetic energy.