STEAM ENGINES 



on the wheel, that line gives the exact area in tenths of a squ 

 inch. The zero of the vernier, however, will probably be 

 between two of the division lines on the wheel, in which case 

 write down the inches and tenths that are to the left of the 

 vernier zero, and from the vernier find the nearest hundredth 

 of a square inch as follows: Find the line of the vernier that is 

 exactly opposite one of the lines on the wheel. The number 

 of spaces on the vernier between the vernier zero and this line 

 is the number of hundredths of a square inch to be added to 

 the inches and tenths read from the wheel. An example is 

 presented in Fig. 2, where the of the vernier lies between 

 the lines on the wheel representing 4.7 and 4.8 sq. in., respect- 

 ively, showing that the area is something more than 4.7 sq. in. 

 Looking along the vernier it is seen that there are three spaces 

 between the vernier zero and the line of 

 the vernier that coincides with one of the 

 lines on the wheel; this shows that .03 

 sq. in. is to be added to the 4.7 sq. in. 

 read from the wheel, making the area 

 4.73 sq. in., to the nearest hundredth of a 

 square inch. The reading thus taken is 

 the area of the diagram, in square inches. 

 The M. E. P. is found by dividing this 

 area by the length of the diagram on a line parallel with the 

 atmo spheric line, and multiplying by the scale of the spring. 



EXAMPLE. The area of the diagram is 4.73 sq. in., the length 

 is 3.5 in., and a 40 spring is used; find the M. E. P. 



4.73 



SOLUTION. M. E. P. = - - X40 = 54 1 Ib. per sq. in. 

 3.o 



Hints for Use of Planimeter. It is well to place the fixed 

 point p. Fig. 1, so that, as the tracing point moves around the 

 diagram, the arms will swing about equally on each side of a 

 position at right angles with each other. A slight dot is gen- 

 erally made with the tracing point to mark the point a' 

 its motion around the diagram begins; when the tracing point 

 reaches this dot in the paper, the operator knows that the 

 motion around the diagram has been completed. The Hirer - 

 tion of motion of the tracing point must always be the 

 same as that of the hands of a watch; motion in the opposite 





