60 SCIENCE OF SUCCESSFUL THRESHING. 



rather than a "Fifteen Horse" engine. However, besides 

 the cylinder size, the steam pressure carried and the speed 

 are also important factors in determining the amount of 

 horse-power .an engine will develop. English engines are 

 more under-rated than those built in the United States, but 

 in comparing the engines of these countries, the difference in 

 steam pressure arid speed must be taken into consideration, 

 as well as the difference in the size of cylinders. The meth- 

 ods 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 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 minute. 

 The horse-power of an engine is equal to the average, total, 

 effective pressure on the piston multiplied by the number of 

 feet it travels per minute, and divided by thirty-three thou- 

 sand. The total effective pressure on the piston is equal to 

 its area in square inches, multiplied by the effective pressure 

 per square inch, which is not constant, but varies, being 

 nearest boiler pressure during the early part of the stroke 

 and decreasing after the point of cut-off is passed, as the 

 steam expands, until the end of the stroke is reached. The 

 effective pressure is the pressure remaining after subtracting 

 the back pressure of the exhaust. 



Indicated Horse-Power. The pressure at the different 

 parts of the stroke can be measured only by means of the 



