238 Introduction to the Study of Science 



one horse power of steam. It is therefore readily computed 

 that the amount of energy produced by the supply of water 

 with this fall is equivalent to that delivered by approximately 

 7,800,000 tons of coal a year. 



XI. WATER WHEELS 



99. Modern water wheels. It is just such conditions and 

 possibilities as these which have brought about a wide-spread 

 revival of water power. The quaint, old-fashioned water 

 wheels, known as the overshot and undershot wheels, while 

 chiefly of historical interest, revealed a principle that has been 

 worked out more fully in modern wheels and turbines of the 

 highest efficiency and adaptability to different conditions. The 

 modern water wheels and turbines are adaptable to the volume 

 of stream flow, to the fall or head of water, to different rates of 

 speed and to almost unlimited demands for power. They have 

 made it possible to harness and utilize streams that have 

 plunged riotously to waste or those that have rolled sluggishly 

 to the sea. 



The Pelton wheel. Falling water is made to yield its energy 

 for useful work by two chief kinds of machines the Pelton water 

 wheel, designed to utilize a high fall and limited quantity of 

 water, and the turbine, adapted to a low head and large 

 quantities of water. 



The Pelton wheel (Fig. 70) invented by a carpenter 

 whose name the wheel bears, is made to generate from one 

 fifth of a horse power to 20,000 horse power. At Rjukan, 

 Norway, ten sets of Pelton wheels are installed in the first power 

 plant, each set developing between 14,000- and 19,000 horse 

 power with a fall of 970 feet. A second power plant, three miles 

 distant, and 909 feet lower, uses the same water. The electric- 

 current thus generated is chiefly used in manufacturing nitrates 

 for fertilizers. 



The Pelton wheel is extensively used on the Pacific coast, 

 where it was invented. It is especially adapted to the kind of 



