TURBINES. 293 



wasted (Art 158). To avoid this, it is necessary to employ 

 a machine in which some rotatory velocity is given to the 

 water before entrance, in order that it may be possible to 

 discharge it with no velocity except that whk-h is absolutely 

 required to pass it through the machine. Such a machine 

 is called a Turbine, and it is described as "outward flow," 

 "inward flow," or "parallel flow," according as the water 

 during its passage through the machine diverges from, con- 

 verges to, or moves parallel to the axis of rotation.* 



Turbines are wheels, generally of small size compared 

 with water wheels, driven chiefly by the impulse of the 

 water. The water is allowed, before entering the moving 

 part of the turbine, to acquire a considerable velocity ; dur- 

 ing its action on the turbine this velocity is diminished, 

 and the impulse due to the change of momentum drives the 

 turbine. 



Koughly speaking, the fluid acts in a water-pressure 

 engine directly by its pressure ; in a water wheel chiefly by 

 its weight causing a pressure, but in part by its kinetic 

 energy, and in a turbine chiefly by its kinetic energy, which 

 again causes a pressure, f 



In the outward and inward flow turbines, the water en- 

 ters and leaves the turbine in directions normal to the axis 

 of rotation, and the paths of the molecules lie exactly or 

 nearly in planes normal to the axis of rotation. In outward- 

 flow turbines the general direction of flow is away from 

 the axis, and in inward-flow turbines towards the axis. In 

 parallel-flow turbines, the water enters and leaves the tur- 

 bine in a direction parallel to the axis of rotation, and the 

 paths of the molecules lie on cylindrical surfaces concentric 

 with that axis. 



There .are many forms of outward-flow turbines, of which 

 the best known was invented by Fourneyron, and is com- 

 monly known by his name. The inward-flow was invented 

 by Prof. Jas. Thomson. 



* CotterilPs App. Mechs., p. 506, t Encjr- Brit,, Vol. XII., p. 69Q. 



