308 HOW MACHINES LIGHTEN LABOR 



of work per second is said to be one horse power (H.P.). This 

 unit was chosen by James Watt, the inventor of a steam 

 engine, when he was in need of a unit with which to compare 

 the new source of power, the engine, with his old source of 

 power, the horse. Although called a horse power, it is greater 

 than the power of an average horse. 



An ordinary man can do one sixth of a horse power. The 

 average locomotive of a railroad has more than 500 H.P., 

 while the combined engines of an ocean liner may have as high 

 as 70,000 H.P. 





FIG. 1 80. A farm engine putting in a crop. A very complex machine. 



Waste work and efficient work. In our study of ma- 

 chines we omitted a factor which in practical cases cannot be 

 ignored, namely, friction. No surface can be made perfectly 

 smooth, and when a barrel rolls over an incline, or a rope 

 passes over a pulley, or a cogwheel turns its neighbor, there is 

 rubbing and slipping and sliding. Motion is thus hindered, 

 and the effective value of the acting force is lessened. In order 

 to secure the desired result, it is necessary to apply a force in 

 excess of that calculated. This extra force, which must be sup- 

 plied if friction is to be counteracted, is in reality waste work. 



If die force required by a machine is 150 pounds, while that 



