WATER-POWER AND STEAM-POWER 



The Mississippi River, by means of the great dam at Keo- 

 kuk, Iowa, is made a source of power for St. Louis and 

 many other cities. Do you know any other examples 

 of falling water that has been "harnessed" by man so that 

 it is the source of energy that does work many miles away ? 

 When you were studying the movements of water, 

 and its many changes from one form into another, you 

 learned what is meant by the indestructibility of matter. 

 Now, in thinking about the 

 various ways in which water- 

 power may be used, you can 

 see that a similar principle 

 is at work. Energy, which 

 means the power to do work, 

 is a thing that can be changed, 

 but it cannot be destroyed. 

 Matter and energy are the 

 two great things that make up 

 the world and neither of them 

 can be destroyed. They keep 

 changing all the time, but 

 they are not used up. Sup- 

 pose you carry a trunk up- 

 stairs. You may think you have used up a good deal of 

 energy in doing this. But the energy that you used in 

 carrying that trunk up-stairs still exists, as you would see 

 if you let the trunk slip and it fell down-stairs. In other 

 words, energy, like matter, may be easily transformed (as 

 water-power into electricity), but it cannot be destroyed. 

 You may not be able to understand this completely now, 

 but you will see more and more illustrations of it as you go 

 on with your work. 



FIG. 30. Diagram illustrating how 

 the water-wheel works. 



