108 



MECHANICS 



the beam. This line will also represent the base, or datum, 

 line of the shearing forces, positive values being laid off 

 above and negative values below the base line. The shear 

 at RI being positive and equal to 1,000 lb., draw from a 

 upwards a vertical line ac equal to 1,000 lb., according to 

 any convenient scale. From c draw to the same scale as ab 

 a horizontal line cd equal to 3 ft. Ordinates drawn from 



J -H e/t-J- 



FIG. 13 



any point on the line ab to cd will be of equal lengths, show- 

 ing that the shear retains the value of 1,000 lb. from the 

 reaction RI to the first load of 400 lb. At this point the 

 shear is reduced by the load of 400 lb.; therefore, from d, 

 draw downwards a vertical line de of a length corresponding 

 to 400 lb., as shown in the diagram. 



The shear is then uniform until the central load is reached , 

 and may be represented by the distance of the line ef from 



