THE ENGINE PROPER 81 



There are several other forms of Prony brake in com- 

 mon use. A very satisfactory one is similar to the one in the 

 cut except that the band is tightened by a lever and weights 

 instead of the screw. The influence of these tension 

 weights on the scale reading must be taken into account, 

 but otherwise this arrangement has an advantage since 

 the tension on the band is not changed by contraction and 

 expansion due to changes in temperature. In another 

 form of the Prony brake, sometimes called a "rope brake," 

 several strands of rope are used in place of the friction 

 band with wood blocks. The principle, however, is the 

 same in all. 



Draw-Bar Horse-Power. Since we know that horse- 

 power is simply force in pounds multiplied by the dis- 

 tance in feet travelled in one minute and divided by 33,0x30, 

 it is a very simple matter to determine the horse-power 

 being delivered at the draw-bar of a traction engine. For 

 this purpose a draw-bar dynamometer is used. This in- 

 strument is simply a form of spring scales of suitable 

 proportions and is sometimes provided with a recording 

 device. 



Calculating the Horse-Power. Although, as already 

 stated, the average pressure on the piston can be meas- 

 ured only by means of the steam-engine indicator, we 

 can, for calculation, assume a value for it that is ac- 

 curate enough for ordinary purposes. This we will take 

 to be fifty per cent, of the boiler pressure, a value which 



