FORCE 31 



The lifted weight and the bent spring represent forces 

 ready to do work. 



It is assumed that energy is indestructible. The 

 quantity of energy in the universe is constant. 

 When one form of energy disappears it has become 

 one or more other forms of energy. 



Coal is burnt under a boiler. Chemical affinity 

 causes the atoms of oxygen of the air and the atoms 

 of the fuel to rush together with such enormous 

 velocity as to generate a large quantity of heat. This 

 heat increases the motion of the molecules of water 

 until they are forced wide apart in the form of steam, 

 and the expanding steam gives mechanical motion to 

 the engine. The engine runs a dynamo, thereby con- 

 verting some of its energy into electricity and mag- 

 netism, and the electricity is conveyed to a distant 

 motor and is converted into magnetism and mechan- 

 ical energy to run a car. Some of the electricity is 

 converted back into heat, and some into light; or, 

 again, it may be made to separate the atoms which 

 united to generate the original heat. 



Thus chemical" force, mechanical force, electricity, 

 magnetism flight, and heat are seen to be in succes- 

 sion manifestations of the same energy. I need 

 hardly say that the correlation of the different forms 

 of energy is one of the greatest discoveries of modern 

 science. All forms of energy seem to be essentially 

 one. Each may. be converted into any of the others. 

 And so the problem of energy is simplified; and yet 

 it should be remembered that the highest powers of 

 the human mind have been and are being taxed in 

 endeavoring to explain the methods by which the 

 forces of nature produce their results. Some of the 

 problems involved are so subtlo that it seems hardly 

 possible that they can ever be solved. 



How shall we explain magnetism, electricity, light, 



