90 HEAT AND ENERGY 



3. State examples of transformations of energy. What form 

 of energy is now commonly used in producing motion ? 



4. State the principle upon which heat engines are based. 



5. Describe the steam engine, explaining its manner of 

 working. 



6. Name other heat engines, briefly telling how they apply 

 heat in causing motion. 



7. What fuels are commonly used in these engines as sources 

 of energy ? How were these fuels made ? Where did the neces- 

 sary energy come from ? What, then, is the great original source 

 of the energy now commonly used on earth to cause motion? 



