62 THE SUN'S GIFT OF HEAT 



The ability of the earth to support plant or animal life or 

 to furnish man the power necessary to carry on his industries 

 is due to the energy furnished by the sun. Plants cannot 

 grow without the energy furnished by the sunlight, and 

 animals could not live were it not for the energy furnished^ 

 them by the plants. 



We often think that there are many different sources of 

 energy such as waterpower, wood, coal, oil, and others ; but 

 when these are traced back, their energy is found to have 

 come from one source, the sun. The water which the sun 

 has evaporated and carried by cloud and shower to the 

 mountain lake is stored there and has potential energy. It 

 is ready to run down the valleys changing its potential 

 energy into kinetic and doing work. Without the heat of 

 the sun there would be no life upon the earth, no flowing 

 streams, no changing winds, none of the restless energy 

 which makes the world as we know it. 



For untold ages plants utilized the sun's energy and stored 

 it up. It was preserved in the remains of plants in the form 

 of coal. This coal is now being burned to furnish power to 

 carry on man's industries. Thus nature has run a savings 

 bank. The sun's kinetic energy was transformed and stored 

 for ages in the earth's vaults as potential energy, and now 

 issues from the burning coal as kinetic energy to do our 

 bidding. 



The motion of the falling brick was a manifestation of 

 energy due to gravitation. The explosion of the gunpowder 

 was due to chemical energy. The ordinary 'street car runs 

 by virtue of electrical energy. Thus we see that there are 

 other forms of energy besides heat and light. But one form 

 of energy may be readily changed into another form, as 

 when the steam engine transforms the energy in coal into 



