gential direction,
or around the stick in direction of the annual rings of growth,
do the two forces coincide. Another factor to the same end is
that the denser bands of late wood are continuous in a
tangential direction, while radially they are separated by
alternate zones of less dense early wood. Consequently the
shrinkage along the rings (tangential) is fully twice as much as
toward the centre (radial). (See Table XIV.) This explains why
some cracks open more and more as drying advances. (See Fig.
27.)

Although actual shrinkage in length is small, nevertheless the
tendency of the rays to shorten a stick produces strains which
are responsible for some of the splitting open of ties, posts,
and sawed timbers with box heart. At the very centre of a tree
the wood is light and weak, while farther out it becomes denser
and stronger. Longitudinal shrinkage is accordingly least at the
centre and greater toward the outside, tending to become
greatest in the sapwood. When a round or a box-heart timber
dries fast it splits radially, and as drying continues the cleft
widens partly on account of the greater tangential shrinkage and
also because the greater contraction of the outer fibres warps
the sections apart. If a small hardwood stem is split while
green for a short distance at the end and placed where it can
dry out rapidly, the sections will become bow-shaped with the
concave sides out. These various facts, taken together, explain
why, for example, an oak tie, pole, or log may split open its
entire length if drying proceeds rapidly and far enough. Initial
stresses in the living trees produce a similar effect when the
log is sawn into boards. This is especially so in _Eucalyptus
globulus_ and to a less extent with any rapidly grown wood.

The use of S-shaped thin steel clamps to prevent large checks
and splits is now a common practice in this country with
crossties and poles as it has been for a long time in European
countries. These devices are driven into the butts of the
timbers so as to cross incipient checks and prevent their
widening. In place of the regular S-hook another of crimped iron
has been devised. (See Fig. 28.) Thin straps of iron with one
tapered edge are run between intermeshing cogs and crimped,
after which they may be cut off any length desired. The time for
driving S-irons of either form is when the cracks first appear.

[Illustration: FIG. 28.--Control of season checking by the use
of S-irons. _