shrinkage.

A substance that can undergo little change in shape without
breaking or rupturing is ~brittle~. Chalk and glass are common
examples of brittle materials. Sometimes the word _brash_ is
used to describe this condition in wood. A brittle wood breaks
suddenly with a clean instead of a splintery fracture and
without warning. Such woods are unfitted to resist shock or
sudden application of load.

The measure of the stiffness of wood is termed the ~modulus of
elasticity~ (or _coefficient of elasticity_). It is the ratio of
stress per unit of area to the deformation per unit of
{ unit stress }
length. { E = ------------- } It is a number indicative of
{ unit strain }
stiffness, not of strength, and only applies to conditions
within the elastic limit. It is nearly the same whether derived
from compression tests or from tension tests.

A large modulus indicates a stiff material. Thus in green wood
tested in static bending it varies from 643,000 pounds per
square inch for arborvitæ to 1,662,000 pounds for longleaf pine,
and 1,769,000 pounds for pignut hickory. (See Table IX.) The
values derived from tests of small beams of dry material are
much greater, approaching 3,000,000 for some of our woods. These
values are small when compared with steel which has a modulus of
elasticity of about 30,000,000 pounds per square inch. (See
Table I.)