MECHANICAL ENERGY AND HEAT 187 



steam is condensed and returned to the boiler, and in this 

 case water need not be supplied excepting to replace leakage. 

 Thus the water is a permanent part of the machine and not 

 a source of energy. The coal is quite useless unless it is 

 burned under the boiler and supplies heat to the water in the 

 boiler. The coal or gasoline are 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. 



204. The engine. The machine 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 locomotive, 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 supplied 

 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 

 operation of a simple engine may be understood by refer- 

 ring to the diagram on page 189. The essentials of a steam 

 engine are a cylinder C, containing a tightly fitting piston 

 jt?, 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 



