d of testing is
necessary and the values must be expressed in some defined
units. For these reasons laboratory experiments if properly
conducted have many advantages over any other method.

One object of such investigation is to find unit values for
strength and stiffness, etc. These, because of the complex
structure of wood, cannot have a constant value which will be
exactly repeated in each test, even though no error be made. The
most that can be accomplished is to find average values, the
amount of variation above and below, and the laws which govern
the variation. On account of the great variability in strength
of different specimens of wood even from the same stick and
appearing to be alike, it is important to eliminate as far as
possible all extraneous factors liable to influence the results
of the tests.

The mechanical properties of wood considered in this book are:
(1) stiffness and elasticity, (2) tensile strength, (3)
compressive or crushing strength, (4) shearing strength, (5)
transverse or bending strength, (6) toughness, (7) hardness, (8)
cleavability, (9) resilience. In connection with these,
associated properties of importance are briefly treated.

In making use of figures indicating the strength or other
mechanical properties of wood for the purpose of comparing the
relative merits of different species, the fact should be borne
in mind that there is a considerable range in variability of
each individual material and that small differences, such as a
few hundred pounds in values of 10,000 pounds, cannot be
considered as a criterion of the quality of the timber. In
testing material of the same kind and grade, differences of 25
per cent between individual specimens may be expected in
conifers and 50 per cent or even more in hardwoods. The figures
given in the tables should be taken as indications rather than
fixed values, and as applicable to a large number collectively
and not to individual pieces.

FUNDAMENTAL CONSIDERATIONS AND DEFINITIONS

Study of the mechanical properties of a material is concerned
mostly with its behavior in relation to stresses and strains,
and the factors affecting this behavior. A ~stress~ is a
distributed force and may be defined as the mutual action (1) of
one body upon another, or (2) of one part of a body upon another
part. In the first case the stress is _external_; in the other
_internal_. The same stress may be internal from one point of
view and external f