[Illustration: FIG. 1.--Stress-strain diagrams of two longleaf
pine beams. E.L. = elastic limit. The areas of the triangles
0(EL)A and 0(EL)B represent the elastic resilience of the dry
and green beams, respectively.]

There are three kinds of internal stresses, namely, (1)
~tensile~, (2) ~compressive~, and (3) ~shearing~. When external
forces act upon a bar in a direction away from its ends or a
direct pull, the stress is a tensile stress; when toward the
ends or a direct push, compressive stress. In the first instance
the strain is an _elongation_; in the second a _shortening_.
Whenever the forces tend to cause one portion of the material to
slide upon another adjacent to it the action is called a
_shear_. The action is that of an ordinary pair of shears. When
riveted plates slide on each other the rivets are sheared off.

These three simple stresses may act together, producing compound
stresses, as in flexure. When a bow is bent there is a
compression of the fibres on the inner or concave side and an
elongation of the fibres on the outer or convex side. There is
also a tendency of the various fibres to slide past one another
in a longitudinal direction. If the bow were made of two or more
separate pieces of equal length it would be noted on bending
that slipping occurred along the surfaces of contact, and that
the ends would no longer be even. If these pieces were securely
glued together they would no longer slip, but the tendency to do
so would exist just the same. Moreover, it would be found in the
latter case that the bow would be much harder to bend than where
the pieces were not glued together--in other words, the