TEE CONSERVATION OF ENERGY. 95 



the moving parts of the machine, and have all parts of .the 

 engine at the end of the experiment exactly at the same 

 temperature as at its beginning, the quantity of heat thus 

 obtained would be exactly equal to the difference between 

 that amount of heat originally generated in the furnace 

 of the engine, and the quantity which had been carried off 

 from it to tiie air since its fire was lighted. Having 

 turned some of the heat into mechanical work we could 

 thus turn the work back into heat again, and find it yield 

 exactly the amount which seemed lost. 



Or we might use the engine to drive an electro-magnetic 

 machine and so turn part of the heat liberated in its fur- 

 nace, first into mechanical work, and this afterwards into 

 electricity; and if we chose to use the latter with the proper 

 apparatus, as now used for electric lighting, we could turn 

 more or less of it into light; and so have a great part of the 

 energy which first became conspicuous as heat in the engine 

 furnace, now manifested in the form of light at some dis- 

 tant point. In fact, starting with a given quantity of one 

 kind of energy, we may by proper contrivances turn all 

 or some of it into one or more other forms ; but if we col- 

 lected all the final forms and retransformed them into the 

 first, we should have exactly the amount of it which had 

 disappeared when the other kinds appeared. 



Why we need food. Energy, as we have seen, cannot be 

 created from nothing ; since the body constantly expends 

 energy, it must have a steady supply. This supply comes 



Give an example of the transformation of heat into electrical 

 force. Of electrical force into light. Given a supply of one kind of 

 energy what can we do with it? What would we find if we collected 

 all the final manifestations of energy and turned them back into the 

 original form ? 



Why mast the body have a steady supply of energy ? Where 

 ioes the supply come from ? 



