562 MACHINES, WORK, AND ENERGY 



Turbine wheels are generally used where a large flow of water 

 under a moderate "head" is available. They often give an 

 efficiency of 80 or 90 per cent. 



SOURCE OP ALL ENERGY 



669. The Sun is the Source of All Energy. The original 

 source of the energy in water power is the sun (see Art. 205). 

 We have seen that water is constantly evaporating when ex- 

 posed to the air. Evaporation is constantly taking place 

 from every body of water, from every moist surface, and 

 from the leaves of plant life (Arts. 21 and 241). We have 

 also seen that evaporation always means the absorption of 

 heat and the production of cold (Art. 244). It would seem, 

 therefore, that this constant evaporation ought to result in 

 the lowering of the temperature of the earth's surface. More- 

 over, the heat of the earth's crust is constantly radiating 

 through the atmosphere into space. Why, then, does not 

 the earth's surface become so chilled that all of the water 

 upon it is frozen and all life disappears? See Chap. IV, 

 Arts. 244 and 252, for your answer. Review the cause of 

 precipitation, i.e., of rain and snow. The source of energy in 

 running and falling water is the elevation of water by evapo- 

 ration and the distribution of the water vapor over the earth's 

 surface by winds, both of which are due to energy from the 

 sun. 



We have seen that the energy in all plant and animal life 

 is also originally obtained from the sun (Chap. VII, Art. 380). 

 Coal is known to be the product of the remains of plant life 

 long ago buried in the earth's crust. Geologists tell us that 

 petroleum also is the result of decomposed plant and animal 

 matter which was buried ages ago. The sun is the original 

 source of all stored-up energy which man may use to produce 

 power. 



670. Water Power, the Cheapest of All Power. Water 

 power is today the least expensive of all available power. 



