THE ENGINE PROPER. 6l 



steam-engine indicator. This instrument has a small piston, 

 connected to a pencil point in such a way that movement of 

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

 the piston is resisted by a calibrated spring, its position de- 

 pends upon the amount of pressure it is subjected to, and 

 therefore, the amount of pressure at all points may be 

 known from the diagram made by the pencil point. Know 

 ing the pressures at the various points of the stroke, it is 

 easy to multiply the average 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 measures 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 device 

 known as the "Prony brake." This consists of a brake band 

 for applying friction to the rim of a pulley. The brake band 

 is prevented from turning by an arm which rests on a weigh- 

 ing scale. From the scale reading, the speed of the pulley 

 and the length of the brake arm, the horse-power can be 

 figured, and 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. 



Calculating the Horse-Power. Although, as already 

 stated, the mean effective pressure can be measured only by 



