MECHANICAL ENEKGY AND HEAT 183 



heat to the water in the boiler. The coal is therefore only a 

 means of supplying heat, and heat from any other source 

 would be just as effective ; that is, if only heat is supplied 

 to the boiler in sufficient quantity, we shall be able to get 

 work out of the engine. Work may produce heat, as when 

 we hammer a nail, and heat may produce work, as in the case 

 of the steam engine. 



We have shown that heat has the ability to do work ; but 

 ability to do work is the definition of energy; therefore heat is 

 a form of energy, and work may be transformed into heat as 

 well as into other forms of energy. 



208. The steam engine. The machine which is commonly 

 called a steam engine really consists of two principal parts 

 the steam engine proper and the boiler. The engine and 

 boiler may be very closely connected, as in the railway loco- 

 motive, or they may be located at a considerable distance from 

 each other, as in some manufacturing establishments or on 

 board ship, where they are usually in different rooms. In the 

 boiler, water is changed into steam by means of the heat sup- 

 plied by the fire in the furnace. This steam is conveyed 

 through a pipe to the parts of the engine, where the energy is 

 changed from heat into the mechanical energy of moving parts. 



The more important facts regarding the structure and oper- 

 ation of a simple engine may be understood by referring to 

 the accompanying diagram (fig. 91). The essentials of a 

 steam engine are a cylinder (7, containing a tightly fitting 

 piston p, together with some means to control the admission 

 of steam into the cylinder. The piston is free to move from 

 end to end of the cylinder, and as it moves, the connected 

 piston rod r moves with it. With the parts in the position in- 

 dicated in the diagram, if steam from the boiler is admitted 

 through the pipe st into the steam chest S, it will pass through 

 the channel ch into the cylinder. Here it will exert pressure 

 upon the piston and drive it to the other end of the cylinder. 

 Steam which may have remained in the opposite end of the 



