THE ENGINE PROPER 77 



ing the engines of these countries, the difference in steam 

 pressure and speed must be taken into consideration, as 

 well as the difference in the size of cylinders. The methods 

 of obtaining the exact horse-power of an engine with the 

 indicator or the Prony brake are becoming better known, 

 but it is probable, however, that all engines will not be cor- 

 rectly rated for some time to come. 



Engine Horse-Power. The unit of power is a "horse- 

 power" which is defined as the amount of power necessary 

 to raise thirty-three thousand pounds one foot in one min- 

 ute. From this it will be seen that, if we know the amount 

 of force exerted in pounds and multiply by the number of 

 feet it travels in a minute and then divide the product so 

 obtained by 33,000, we will have the result in horse-power. 

 We have, then, as our unit of horse-power something that 

 means a definite amount and one that can be easily meas- 

 ured, with reasonable accuracy. 



The horse-power of an engine may be found by multi- 

 plying the average, total effective pressure on the piston, 

 by the number of feet it travels per minute, and dividing 

 by thirty-three thousand. The total effective pressure 

 on the piston is equal to its area in square inches, multi- 

 plied by the effective pressure per square inch, which is 

 not constant, but varies, being nearest boiler pressure dur- 

 ing the early part of the stroke and decreasing after the 

 point of cut-off is passed, as the steam expands, until the 



