STEAM ENGINES 187 



direction will move the wheel in the wrong direction and give a 

 negative reading for the area. When measuring diagrams with 

 loops, like Fig. 3, move the tracing point so that it will follow 

 the outline of the loops in a direction opposite to the direction 

 of motion of the hands of a watch, as is indicated by the arrow- 

 heads on the diagram. This will cause the instrument auto- 

 matically to subtract the areas of the loops from the area of the 

 main part of the diagram. 



An excellent check on the work is to start with the recording 

 wheel at zero and pass the tracing wheel around the diagram 

 two or three times, noting the reading of the wheel each time 

 the tracing point returns to the point of starting. Each read- 

 ing of the wheel divided by the number of times the outline 



FIG. 3 



of the diagram has been traced should give, very nearly, the 

 value of the first reading; if there is considerable difference 

 between the first reading and the value obtained by dividing 

 the second reading by 2 or the third reading by 3, it is an indi- 

 cation that an error has been made, and the work should be 

 repeated. If the difference is small, the work may be assumed 

 to be satisfactory and the value to be used for the area or the 

 M. E. P. may be taken as the average found by dividing the 

 last reading by the number of times the tracing point passed 

 around the diagram. 



Finding M. E. P. by Ordinates. The M. E. P. may be 

 found from the diagram by the aid of two triangles, a scale, 

 and a hard lead pencil; if two triangles are not available, a 



