426 POPULAR LECTURES AND ADDRESSES. 



among different pieces of matter, in virtue of 

 their relative positions, has been called potential 

 energy by Rankine. To show the use of these 

 terms, and explain the ideas of a store of energy, 

 and of conversions and transformations of energy, 

 various illustrations were adduced. A stone at a 

 height, or an elevated reservoir of water, has 

 potential energy. If the stone be let fall, its 

 potential energy is converted into kinetic 

 energy during its descent, exists entirely as the 

 energy of its own motion at the instant be- 

 fore it strikes, and is transformed into heat at 

 the moment of coming to rest on the ground. 

 If the water flow down by a gradual channel, its 

 potential energy is gradually converted into 

 heat by fluid friction, and the fluid becomes 

 warmer by a degree Fahr. for every 772 feet of 

 the descent. There is potential energy, and there 

 is kinetic energy, between the earth and the 

 sun. There is most potential energy and least 

 kinetic energy in July, when they arc at their 

 greatest distance asunder, and when their rela- 

 tive motion is slowest. There is least potential 



