out.

It is for this reason that split billets and squares are
stronger than most sawed material. The presence of the spiral
grain has little, if any, effect on the timber when it is used
in the round, but in sawed material the greater the pitch of the
spiral the greater is the defect.

KNOTS

_Knots_ are portions of branches included in the wood of the
stem or larger branch. Branches originate as a rule from the
central axis of a stem, and while living increase in size by the
addition of annual woody layers which are a continuation of
those of the stem. The included portion is irregularly conical
in shape with the tip at the pith. The direction of the fibre is
at right angles or oblique to the grain of the stem, thus
producing local cross grain.

During the development of a tree most of the limbs, especially
the lower ones, die, but persist for a time--often for years.
Subsequent layers of growth of the stem are no longer intimately
joined with the dead limb, but are laid around it. Hence dead
branches produce knots which are nothing more than pegs in a
hole, and likely to drop out after the tree has been sawed into
lumber. In grading lumber and structural timber, knots are
classified according to their form, size, soundness, and the
firmness with which they are held in place.[32]

[Footnote 32: See Standard classification of structural timber.
Yearbook Am. Soc. for Testing Materials, 1913, pp. 300-303.
Contains three plates showing standard defects.]

Knots materially affect checking and warping, ease in working,
and cleavability of timber. They are defects which weaken timber
and depreciate its value for structural purposes where strength
is an important consideration. The weakening effect is much more
serious where timber is subjected to bending and tension than
where under compression. The extent to which knots affect the
strength of a beam depends upon their position, size, number,
direction of fibre, and condition. A knot on the upper side is
compressed, while one on the lower side is subjected to tension.
The knot, especially (as is often the case) if there is a season
check in it, offers little resistance to this tensile stress.
Small, knots, however, may be so located in a beam along the
neutral plane as actually to increase the strength by tending to
prevent longitudinal shearing. Knots in a board or plank are
least injurious when they extend through it at right angles to
its broadest surfac