188 ELEMENTS OF GENERAL SCIENCE 



giving off heat to the surrounding space, it is continually 

 losing energy. The part of the sun's energy which is stored up 

 in plants and animals, or used in evaporating water, supplies 

 most of the energy which is used by man in accomplish- 

 ing his work. This is probably not more than the one thou- 

 sandth part of the whole. It has been estimated that enough 

 energy falls upon the deck of a steamship in tropical regions 

 to propel it at a rate of several miles per hour if there were 

 some way in which all of this energy could be made useful. 

 There have been many attempts to construct "sojar engines" 

 which should use this supply of energy directly, but so far 

 none of them has been very successful. 



Of course, only a small part of the energy given off by the 

 sun comes to the earth, 1 part in 2,000,000,000, and there- 

 fore the sun must be giving off energy very rapidly. Perhaps 

 its supply of energy is growing less, but it is so big that the 

 loss has not been great enough to be noticed during the short 

 time that men have been making careful observations. It 

 seems certain, however, that the sun must finally lose so much 

 energy that it will not be able to continue to give off as much 

 heat and light as it does now. 



213. Energy and work in living things. Many of the in- 

 stances of work which have been given in the preceding chap- 

 ters are examples of work accomplished by living beings. If 

 you climb a flight of steps, push a lawn mower, or saw a 

 board, you are doing work ; you are expending energy. Even 

 when you are sitting still or when you are sleeping, there is 

 some expenditure of energy, for the heart continues to beat, 

 you continue to breathe, and heat is radiated from your body. 

 The lower animals expend energy in similar ways. Plants too 

 use energy, for they open and close their flowers, move their 

 leaves, lift large quantities of water from the soil to the leaves, 

 and engage in many other activities. Everything is expending 

 energy as long as it lives. Energy is supplied to living things 

 through food somewhat as energy is supplied to a steam engine 



