MECHANICAL ENERGY AND HEAT 191 



boiler and engine into the air, and some passes off in the 

 exhaust steam. In these and other ways the steam engine 

 wastes possibly 85 per cent or more of the energy produced 

 by the coal. Of the energy actually utilized by the engine 

 some is lost by friction in the engine and dynamo, showing 

 itself in the form of heated bearings, and some is lost through 

 the heating of the wires by the currents of electricity. Notice 

 that in all these cases of lost energy the energy is not de- 

 stroyed. It changes into some form in which it is not useful 

 to us (usually into the form of heat which may warm the 

 surrounding air), but it has not gone out of existence. 



There is no way known by which we can destroy energy. 



There is no. way known by which we can create energy. 



207. The source of energy. Since there is no way in which 

 we can create energy, where does the energy come from 

 which we are all the time using and allowing to escape from 

 us? If a trolley car is used as an illustration, we find that 

 it is operated by electricity. The current is generated by the 

 dynamo, which is run by a steam engine. The engine secures 

 its energy from coal. Plainly the coal contains energy, and 

 we must find out where this energy came from. 



Those who have studied the origin of coal tell us that it 

 is formed of the remains of plants of former ages. The plants 

 grew, and their dead bodies accumulated in great abundance 

 in some places, somewhat as plant materials are accumulat- 

 ing now in our peat swamps. These swamps were buried by 

 sediments which later became rocks. The plant material was 

 buried far enough to be affected by the heat of the interior 

 of the earth, and it was under great pressure from 'the rocks 

 lying over it. All these things caused it to condense into a 

 solid, stonelike mass, which does not at first look much as 

 though it were made up of dead plants. A full account of 

 the origin of coal cannot be given in this connection, but 

 the subject will prove most interesting as an accompanying 

 study. 



