THE ENGINE PROPER 



79 



as to apply friction to a revolving shaft, pulley or drum, 

 in such a manner that the tendency of the band to revolve 

 with the shaft is resisted by weighing scales, which will 

 show the amount of pull. The result so obtained is called 

 the "brake" horse-power. It is evident that the difference 

 between the indicated and brake horse-power is the power 

 required to run the engine, that is, to overcome the friction 

 in the engine itself and sometimes in the brake also. 



Since we can know the speed of the revolving shaft, 



FIG. 22. ENGINE BELTED TO PRONY BRAKE. 



the scale reading and the distance in feet that the point 

 at which the scales are applied would travel in one revo- 

 lution if allowed to turn with the shaft, we can determine 

 the horse-power by multiplying these three amounts to- 

 gether and dividing by 33,000. Sometimes the radius of 

 the circle that the scales act on, or "brake-arm" as it is 

 called, is made 63 inches and when this is the case the 

 calculating is simplified because it is then only necessary 

 to multiply the number of revolutions per minute by the 

 number of pounds shown by the scales and divide the prod- 

 uct by 1,000 in order to determine the horse-power. 



A good example of a Prony brake is shown in Fig. 23. 



