182 ELEMENTS OF GENERAL SCIENCE 



bending it back and forth has had occasion to know that it 

 becomes very hot at the place of bending, and the harder and 

 longer one works, the hotter the wire becomes. The same 

 thing results from hammering a nail, and a saw becomes much 

 heated when in use, particularly if there is much friction. If 

 a wheel is allowed to run for some time without oil on the 

 axle, the latter becomes very much heated. A match is lighted 

 by the heat which is produced by friction when it is rubbed 

 on a smooth surface. In all of these cases work is done, and a 

 part of its result appears only in the form of heat. 



206. Energy transformed into heat. Careful studies have 

 shown that in cases like the above the amount of heat de- 

 veloped is in proportion to the amount of energy lost in over- 

 coming friction ; that is, it is proportional to the amount of 

 wasted work. In lifting a weight by means of pulleys a part 

 of the energy which is exerted at the end of the rope goes to 

 the direct raising of the weight and makes its appearance in 

 the form of potential energy in the weight which has been 

 lifted. But not all of the energy which was applied to the 

 machine can be found in the weight. You may have noticed 

 in the calculation of work done upon a pulley system that the 

 work out is less than the work in. The difference between 

 the work put in and the work taken out measures the work 

 which was necessary to overcome friction and which has re- 

 sulted in the formation of heat. Careful studies have shown 

 that the amount of energy lost and the amount of heat 

 produced are proportional to each other. 



207. Work done by heat. Most of us have noted that a steam 

 engine, such as a railroad locomotive, must be supplied with 

 water and coal. There are some kinds of engines which are so 

 equipped that the escaping steam is condensed and returned 

 to the boiler, and in this case water need not be supplied ex- 

 cepting 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 



