MECHANICAL ENERGY AND HEAT 187 



not at first look much as though it were made up of dead 

 plants (fig. 92). The story of the origin of coal is a very inter- 

 esting one, but we cannot take it up now. 



Of course, if there is energy in the coal, it must have been 

 in the plants from which the coal was made. That seems all 

 the more probable, since we know that we can burn wood to 

 make steam about as well as we can burn coal. Indeed, wood 

 is still sometimes used as fuel under boilers. Where, then, did 

 the plants get the energy ? Where do they get it now ? Do 

 you remember that when we were studying about how plants 

 make their food, we found that they could do so only when 

 they had light ? We called light the power that ran the fac- 

 tory. That may have seemed rather fanciful at the time, but 

 it is literally true. By means of the energy that comes to it 

 in the form of light, the plant is able to separate the carbon 

 from the oxygen of the carbon dioxide and to build tip those 

 complex compounds from which its body is constructed. 

 When -we burn the wood, oxygen again unites with the other 

 elements to form carbon dioxide, water, and other compounds, 

 and, most important to us, the energy that was put into the 

 compound is released in the form of heat. 



The sun, then, is the source of the energy which we find 

 stored up in wood and in coal. If we were to take any other 

 example and trace back the energy, we should in all cases 

 arrive at the sun as the last term in our series. The sun is 

 the great source of the world's energy. The sun is the engine 

 that runs the trolley car. 



212. Amount of energy from the sun. Some careful studies 

 have been made to find out how much energy the earth re- 

 ceives from the sun. It is believed that the amount is equiva- 

 lent to more than 200,000,000 horse power for each inhabitant 

 of the earth. It is this continual supply of energy from the 

 sun that maintains the temperature of the earth's surface, 

 evaporates water, causes convectional air currents, and provides 

 the energy for photosynthesis. Since the earth is continually 



