ENERGY 21 



bing against the air that they shine. . From the fact that 

 whenever one form of energy disappears another form appears 

 in its place, we conclude that energy, like matter, cannot be 

 destroyed. 



26. Kinetic and Potential Energy. Upon examination we 

 find that all kinds of energy fall into one of two classes. When 

 energy is actually doing work, as in water falling over a dam, 

 or the uncoiling of a spring, we speak of it as kinetic energy. 

 When it is capable of work, but not actually working, as is the 

 water above the dam, or the coiled spring, it is called potential 

 energy. Energy, as we have noted, may be changed from one 

 form to another, and it may also be transferred from one body 

 to another. In winding up a clock, we transfer some energy 

 from our own body to the clock spring. If the clock is run 

 by weights, we transfer energy when we lift them. The kinetic 

 energy expended in this work becomes potential energy until 

 the clock starts to run, and then it becomes kinetic energy 

 again. 



27. The Source of Energy. The sun is the earth's one great 

 source of energy. Engines have been constructed that derive 

 their energy directly from the sun's rays, but generally we 

 make use of the energy in sunlight after it has been trans- 

 formed in various ways. Windmills, for instance, are turned 

 by currents of air which owe their direction and force to the 

 unequal distribution of heat over the earth's surface. Water- 

 wheels secure their energy from falling water which has pre- 

 viously been evaporated by the sun's heat, blown over ele- 

 vated regions, in the form of clouds, and condensed again as 

 rain. Peat, coal, oil, and gas have been formed from the re- 

 mains of plants and animals of past ages, which built up their 

 tissues from energy derived from the sun's light and heat, just 

 as they are being built up at present. 



28. Heat and Light Compared. Heat and light are closely 

 related in that they come from the sun in the form of radiant 



