THE MEASUREMENT OF POWER 19 



the cross-head, rotates the drum on which the record sheet is 

 placed in exact accord with, and in proportion to, the movement 

 of the piston of the engine. The pencil is raised and lowered 

 vertically by the movement of the indicator piston. The result- 

 ing diagram on the recording sheet is what is known as an 

 indicator card, the area of which in inches, divided by the 

 length, and multiplied by the resistance of the spring, gives 

 the average pressure per square inch during the working 

 stroke. 



Since the pressure is known and the area of the piston can 

 be calculated from the diameter, it is necessary only to know 

 the length of the piston stroke in feet to get the foot-pounds of 

 work produced at each stroke. By applying a revolution 

 counter or speed indicator to the fly wheel of the engine we may 

 determine the number of revolutions per minute. In the steam 

 engine two power strokes are made to each revolution of the 

 flywheel. In the ordinary four-cycle, throttling-governed, 

 single-acting type of gas engine, there is but one power stroke 

 to two revolutions of the flywheel. In some types, the number 

 of explosions is still less, and must be counted in some manner, 

 in order to determine the number per minute. The foot-pounds 

 of work at each stroke, multiplied by the number of power 

 strokes per minute, gives us accurately the amount of power 

 developed in the cylinder. The formula for i.h.p. is: 



gPXLXAXN 

 1 **** 33^00- 



in the case of the steam engine, and 



PXLXAXN 



(33.0000X2) 



in the case of the four-cycle, single-acting, throttling-governed 

 gas engine, in which P = mean effective pressure, (m.e.p.) in 

 pounds per square inch during the working stroke; L = length 

 of piston stroke, in feet; A = area of piston in square inches, 

 and N = number of revolutions of flywheel per minute. 



In determining the power at the flywheel of an engine, 



