62 SCIENCE OF SUCCESSFUL THRESHING. 



the indicator, we can, for calculation, assume a value which 

 approximates the correct one. This we will take to be 

 fifty per cent, of the boiler pressure. 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. This, multiplied by 

 the area of the piston, will give the total mean effective 

 pressure on the piston in pounds. The area of a circle is 

 equal to its diameter multiplied by itself and the product by 

 .7854. The travel of the piston is equal to twice the stroke 

 (there being two strokes to each revolution), multiplied by 

 the number of revolutions per minute. As the length of 

 the stroke is usually given in inches, this product must be 

 divided by twelve to reduce the result to feet per minute. 



The following example is of an engine with a Nine by 

 Ten cylinder, a speed of two hundred and fifty revolutions 

 per minute and a boiler pressure of one-hundred and thirty 

 pounds the size of the Case engine, rated at Fifteen horse- 

 power. 



250 revolutions per minute. 

 20 travel of piston in inches per revolution. 



12)5,000 travel of piston in inches per minute. 



416.6 travel of piston in feet per minute. 



.7854 



8i=(9X9), square of diameter of piston. 



7854 

 62832 



63,6174 area of piston in square inches. 



