82 SCIENCE OF SUCCESSFUL THRESHING 



closely approximates the actual one for engines of the class 

 we are dealing with. Then, with a boiler pressure of one 

 hundred and thirty pounds, our average effective pressure 

 (or "mean effective pressure," as it is called) per square 

 inch will be fifty per cent, of one hundred and thirty 

 pounds, or sixty-five pounds. The formula commonly used 

 for determining the horse-power of an engine is as fol- 



lows: FLAN 



33,000 "~ ' 



in which P = mean effective pressure. 

 L = length of stroke in feet. 

 A = area of piston in square inches. 

 N number of strokes per minute, or twice the 

 number of revolutions. 



The area of a circle is equal to its diameter multiplied 

 by itself and the product by .7854. To show the applica- 

 tion of this formula, we will take, for example, an engine 

 with a Q-inch bore, a lo-inch stroke, a speed of 250 revo- 

 lutions per minute and a boiler pressure of 130 pounds 

 the size of the forty-five horse-power Case engine : 



P = 50% of 130 = 65 pounds. 



L = 10 -s- 12 or .833 feet. 



A = 9 x 9 x .78-54 = 63.617 sq. inches. 



N = 250x2 = 500 strokes per minute. 



Substituting these values for the letters in the formula ; 



we P"et * 



' 65 x .833 x 63.617 x 500 _ no , 



= 52.2 horse-power. 



Since this is the power developed in the cylinder, it 

 corresponds to the indicated horse-power, and it is greater 

 than the horse-power delivered at the fly-wheel or brake 

 horse-power by about ten per cent. Subtracting this ten 



