658 



OIAURAM& 



points O, P, N. Q form a parallelogram whose angle POQ is constant and 

 a q Ul j to 9 -SOR, The product of the distances OP and OQ is constant. 



Th linkwork may be fixed at O. If any figure is traced by P, Q will 

 trace the inverse figure, but turned round O through the constant angle POQ. 



In the diagram forces Pp, Qq are balanced by the force Oo at the fixed 

 point The forces Pp and Q? are necessarily inversely as OP and OQ, and 

 make equal angles with those lines. 



Every closed area formed by the links or the external forces in the diagram 

 of configuration is marked by a letter which corresponds to a point of concourse 

 of lines in the diagram of stress. 



The stress in the link which is the common boundary of two areas is 

 represented in the diagram of stress by the line joining the points corresponding 

 to those areas. 



When a link is divided into two or more parts by lines crossing it, the 

 stress in each part is represented by a different line for each part, but as the 

 stress is the same throughout the link these lines are all equal and parallel. 

 Thus in the figure the stress in RV is represented by the four equal and 

 parallel lines ///, FG, DE, and AB. 



If two areas have no part of their boundary in common the letters cor- 

 responding to them in the diagram of stress are not joined by a straight 

 line. If, however, a straight line were drawn between them, it would represent 

 in direction and magnitude the resultant of all the stresses in the links which 

 are cut by any line, straight or curved, joining the two areas. 



For instance the areas F and C in fig. 1 have no common boundary, and 

 the points F and C in fig. 2 are not joined by a straight line. But every 

 path from the area F to the area C in fig. 1 passes through a series of other 

 areas, and each passage from one area into a contiguous area corresponds to 

 a line drawn in the diagram of stress. Hence the whole path from F to C 

 in fig. 1 corresponds to a path formed of lines in fig. 2 and extending from 

 F to C, and the resultant of all the stresses in the links cut by the path is 

 represented by FC in fig. 2. 



Automatic Description of Diagrams. 



There are many other kinds of diagrams in which the two co-ordinates of 

 a point in a plane are employed to indicate the simultaneous values of two 

 related quantities. 



