MECHANICAL ENERGY AND HEAT 193 



than a thousandth part of the whole. It has been esti- 

 mated 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 this energy 

 could be made useful. There hav.e been many attempts to 

 construct " solar 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 therefore 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. Although the sun must finally lose so much 

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

 light as it does now, it is estimated that a sufficient change 

 to affect life upon the earth will not occur for many thou- 

 sands of years. 



209. Energy and work in living things. Many of the 

 instances of work which have been given in the preceding 

 chapters 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. 

 Energy is supplied to living things through food somewhat as 

 energy is supplied to a steam engine through the agency of 

 the coal which is put into the furnace. Since the origin of 

 foods can be traced back to the manufacture of carbohydrates 



