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.