558 MACHINES, WORK, AND ENERGY 



665. Breast Wheels. BREAST WHEELS are generally used 

 when a larger flow of water is available but less fall can be 

 secured (Fig. 342). How does this wheel differ from the 

 overshot wheel? The efficiency of the breast wheel is usu- 

 ally less than that of the overshot wheel. Can you see why 

 this should be so? 



666. Undershot Wheels. UNDERSHOT WHEELS are used 

 when only a slight fall of water is obtainable (Fig. 343) . While 

 the weight of the falling water is the principal source of power 

 in the overshot and breast wheels, in the case of the under- 

 shot wheel, the force of the impact of the water against the 

 blades or paddles is the chief source of power. Undershot 

 wheels frequently have low efficiency. Why is this so? 



FIG. 343. The undershot FIG. 344. The Pelton wheel or 



wheel. impulse wheel. 



667. Impulse Wheels. IMPULSE WHEELS are used when 

 there is a small supply of water but available under a great 

 "head" (Art. 574) or pressure. Frequently small streams or 

 lakes located high up in a mountain may be utilized for power 

 purposes. In such cases the water is often conveyed down the 

 mountain side in strong iron pipes. At the foot of the moun- 

 tain, the water under high pressure is permitted to escape 

 through a nozzle at high velocity. This stream is directed 

 against cup-shaped buckets on the rim of the wheel (Fig. 

 344). After striking the buckets, the water falls to the ground 

 robbed of its energy. 



These wheels have some advantages over other kinds of 



