188 



STEAM ENGINES 



single triangle and a straightedge will suffice. Lines perpen- 

 dicular to the atmospheric line and tangent to the two ends of 

 the diagram must first be drawn. The perpendicular distance 

 between these tangents will be the length of the diagram, and 

 this length must be divided into some number of equal parts; 

 10 or 20 parts are the most convenient, but any other number 

 may be used. Midway between each pair of points of dndsion 

 draw a line parallel to the two tangents; the part of this line 

 included between the lines of the diagram is the middle ordinate 

 of its corresponding space. The sum of the lengths of all of 

 these middle ordinates divided by the number of ordinates is 

 the mean ordinate and gives, approximately, the average 

 height of the diagram. The length of the mean ordinate 



FIG. 4 



should agree very nearly with the value obtained by dividing 

 the area of the diagram, as measured by a planimeter, by the 

 length of the diagram. The M. E. P. is found by multiplying 

 the length of the mean ordinate by the scale of the spring 

 with which the diagram was taken. A diagram thus divided 

 into equal parts, with the lengths of the ordinates marked 

 thereon, is shown in Fig. 4. The sum of the lengths of the 

 ordinates is 9.06 in. As there are 14 ordinates the length of 



9 06 

 the mean ordinate is in., and if the diagram was taken 



9.06 



with an 80 spring, the M. E. P. is X80 



51.77 Ib. per sq. in. 

 If a scale graduated to correspond with the scale of the 





