STEAM ENGINES 195 



existing and that calculated from diagrams taken at different 

 times. 



Referred Mean Effective Pressure. The indicated horse- 

 power of compound engines is sometimes found by referring 

 the mean effective pressure of the high-pressure cylinder to 

 the low-pressure cylinder and calculating the horsepower of 

 the engine on the assumption that all the work is done in the 

 low-pressure cylinder. To do this, the mean effective pressures 

 of the two cylinders are found from indicator diagrams; the 

 mean effective pressure of the high -pressure cylinder is then 

 divided by the ratio of the volume of the low-pressure cylinder 

 to that of the high -pressure cylinder; and the quotient is 

 added to the mean effective pressure of the low-pressure cyl- 

 inder, the sum being the referred mean effective pressure. This 

 sum is then taken as the mean effective pressure of the engine, 

 and the area of the low-pressure piston as the piston area; 

 with these data, the length of stroke and the number of strokes, 

 the horsepower is computed as for any simple engine. In the 

 case of a triple-expansion engine, the mean effective pressures 

 of the high-pressure and intermediate cylinders are referred 

 to the low-pressure cylinder and added to its mean effective 

 pressure. Thus, suppose that in a 12", 20", -and 34"X30" 

 engine the mean effective pressures are 83.2 lb., 27.8 lb., and 



83 2 

 10.6 lb., respectively. Then, the referred M. E. P. is 



8.03 



27 8 10.6 



I = 10.4 + 10+10.6 = 31 lb., and this value must be 



2.78 1 



substituted for P on finding the horsepower of the engine. 

 While this method shortens the labor of computing the horse- 

 power, it obviously does not show the distribution of work 

 between the cylinders. 



Dynamometers. Dynamometers are instruments for meas- 

 uring power. They are divided into two main classes: absorp- 

 tion dynamometers and transmission dynamometers. The most 

 common form of absorption dynamometer is the Prony brake, 

 which consists simply of a friction brake designed to absorb 

 in friction and measure the work done by a motor, or the power 

 given out by a shaft. A transmission dynamometer is used 



