e. Knots which occur near the ends of a beam
do not weaken it. Sound knots which occur in the central portion
one-fourth the height of the beam from either edge are not
serious defects.

Extensive experiments by the U.S. Forest Service[33] indicate
the following effects of knots on structural timbers:

[Footnote 33: Bul. 108, pp. 52 _et seq._]

(1) Knots do not materially influence the stiffness of
structural timber.

(2) Only defects of the most serious character affect the
elastic limit of beams. Stiffness and elastic strength are more
dependent upon the quality of the wood fibre than upon defects
in the beam.

(3) The effect of knots is to reduce the difference between the
fibre stress at elastic limit and the modulus of rupture of
beams. The breaking strength is very susceptible to defects.

(4) Sound knots do not weaken wood when subject to compression
parallel to the grain.[34]

[Footnote 34: Bul. 115, U.S. Forest Service: Mechanical
properties of western hemlock, p. 20.]

FROST SPLITS

A common defect in standing timber results from radial splits
which extend inward from the periphery of the tree, and almost,
if not always, near the base. It is most common in trees which
split readily, and those with large rays and thin bark. The
primary cause of the splitting is frost, and various theories
have been advanced to explain the action.

R. Hartig[35] believes that freezing forces out a part of the
imbibition water of the cell walls, thereby causing the wood to
shrink, and if the interior layers have not yet been cooled,
tangential strains arise which finally produce radial clefts.

[Footnote 35: Hartig, R.: The diseases of trees (trans. by
Somerville and Ward), London and New York, 1894, pp. 282-294.]

Another theory holds that the water is not driven out of the
cell walls, but that difference in temperature conditions of
inner and outer layers is itself sufficient to set up the
strains, resulting in splitting. An air temperature of 14 deg.F. or
less is considered necessary to produce frost splits.

A still more recent theory is that of Busse[36] who considers
the mechanical action of the wind a very important factor. He
observed: (_a_) Frost splits sometimes occur at higher
temperatures than 14 deg.F. (_b_) Most splits take place shortly
before sunrise, _i.e._, at the time of lowest air and soil
temperature; they are never heard to take place at noon,
afternoon, or evening. (_c_) They alway