ill more when put into a hot, dry kiln. While most of
these smaller checks are only temporary, closing up again, some large
radial checks remain and even grow larger as drying progresses. Their
cause is a different one and will presently be explained. The
temporary checks not only appear at the ends, but are developed on
the sides also, only to a much smaller degree. They become especially
annoying on the surface of thick planks of hardwoods, and also on
peeled logs when exposed to the sun.

So far we have considered the wood as if made up only of parallel
fibres all placed longitudinally in the log. This, however, is not the
case. A large part of the wood is formed by the medullary or pith
rays. In pine over 15,000 of these occur on a square inch of a
tangential section, and even in oak the very large rays, which are
readily visible to the eye, represent scarcely a hundredth part of the
number which a microscope reveals, as the cells of these rays have
their length at right angles to the direction of the wood fibres.

If a large pith ray of white oak is whittled out and allowed to dry,
it is found to shrink greatly in its width, while, as we have stated,
the fibres to which the ray is firmly grown in the wood do not shrink
in the same direction. Therefore, in the wood, as the cells of the
pith ray dry they pull on the longitudinal fibres and try to shorten
them, and, being opposed by the rigidity of the fibres, the pith ray
is greatly strained. But this is not the only strain it has to bear.
Since the fibres shrink as much again as the pith ray, in this its
longitudinal direction, the fibres tend to shorten the ray, and the
latter in opposing this prevents the former from shrinking as much as
they otherwise would.

Thus the structure is subjected to two severe strains at right angles
to each other, and herein lies the greatest difficulty of wood
seasoning, for whenever the wood dries rapidly these fibres have not
the chance to "give" or accommodate themselves, and hence fibres and
pith rays separate and checking results, which, whether visible or
not, are detrimental in the use of the wood.

The contraction of the pith rays parallel to the length of the board
is probably one of the causes of the small amount of longitudinal
shrinkage which has been observed in boards. This smaller shrinkage of
the pith rays along the radius of the log (the length of the pith
ray), opposing the shrinkage of the fibres in this direct