t. 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
_stiffness_ of the bow would be materially increased.

~Stiffness~ is the property by means of which a body acted upon
by external forces tends to retain its natural size and shape,
or resists deformation. Thus a material that is difficult to
bend or otherwise deform is stiff; one that is easily bent or
otherwise deformed is _flexible_. Flexibility is not the exact
counterpart of stiffness, as it also involves toughness and
pliability.

If successively larger loads are applied to a body and then
removed it will be found that at first the body completely
regains its original form upon release from the stress--in other
words, the body is ~elastic~. No substance known is perfectly
elastic, though many are practically so under small loads.
Eventually a point will be reached where the recovery of the
specimen is incomplete. This point is known as the ~elastic
limit~, which may be defined as