THE MEASUREMENT OF POWER 23 



will seldom show a thermal efficiency higher than from 4 to 6 

 per cent. 



In the case of the steam engine there is still another efficiency 

 which is frequently cited i. e., boiler efficiency. Under 

 ordinary conditions the traction engine boiler, which usually 

 works in an exposed place with none too perfect insulation, has 

 an efficiency of 50 to 55 per cent. In stationary engine practice 

 the boiler efficiency often ranges from 65 to 75 per cent. 

 This means that of the heat which is supposed to be generated 

 by burning fuel in the fire box, from 50 to 75 per cent, is ab- 

 sorbed by the water. The efficiency, of course, will vary widely 

 with the type of boiler, the protection, the quality of water 

 used, and whether or not salts from the water have been allowed 

 to form a scale upon the flues and other surfaces exposed to the 

 heat of the fire. 



