78 SCIENCE OF SUCCESSFUL THRESHING 



end of the stroke is reached. The effective pressure is the 

 pressure remaining after subtracting the back pressure of 

 the exhaust, which is exerted on the opposite side of the 

 piston. 



Indicated Horse-Power. The pressure at the different 

 parts of the stroke can be actually measured only by means 

 of the steam-engine indicator. This instrument has a small 

 piston, connected to a pencil point in such a way that move- 

 ment of the piston is registered on a card. Since the 

 movement of the piston is resisted by a calibrated spring, 

 its position depends upon the amount of pressure it is sub- 

 jected to, and as the card moves, corresponding to the 

 movement of the engine piston, therefore, the amount 

 of pressure at all points may be known from the diagram 

 made by the pencil point. Knowing the pressures at the 

 various points of the stroke, it is easy to multiply the aver- 

 age by the travel of the piston in feet per minute and thus 

 determine the horse-power. The result so obtained is 

 called the "indicated" horse-power. The indicator meas- 

 ures the power developed in the cylinder and, of course, 

 it takes a part of this to run the engine itself. The 

 amount so consumed is, roughly, ten per cent, of the whole. 



Brake Horse-Power. The net horse-power delivered at 

 the fly-wheel may be actually measured by means of a de- 

 vice known as the "Prony brake." Sometimes the brake is 

 mounted on the engine shaft, but more often the engine is 

 belted to the brake, as shown in Fig. 22. A Prony brake 

 consists essentially of a band which may be tightened so 



