f 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 HI, FG, DE
and AB. If two areas have no part of their boundary in common the
letters corresponding 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.

Many examples of stress diagrams are given in the article on BRIDGES
(q.v.).

_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. If a sheet of paper is made to move, say
horizontally, with a constant known velocity, while a tracing point is
made to move in a vertical straight line, the height varying as the
value of any given physical quantity, the point will trace out a curve
on the paper from which the value of that quantity at any given time
may be determined. This principle is applied to the automatic
registration of phenomena of all kinds, from those of meteorology and
terrestrial magnetism to the velocity of cannon-shot, the vibrations
of sounding bodies, the motions of animals, voluntary and involuntary,
and the currents in electri