more shearing planes. (See Fig. 14.)
In bending tests on wet beams, first failure occurs by
compression on top of the beam, gradually extending downward
toward the neutral axis. Finally the beam ruptures at the
bottom. In the case of very dry beams the failure is usually by
splitting or tension on the under side (see Fig. 17.), without
compression on the upper, and is often sudden and without
warning, and even while the load is still increasing. The effect
varies somewhat with different species, chestnut, for example,
becoming more brittle upon drying than do ash, hemlock, and
longleaf pine. The tensile strength of wood is least affected by
drying, as a rule.

In drying wood no increase in strength results until the free
water is evaporated and the cell walls begin to dry[49]. This
critical point has been called the _fibre-saturation point_.
(See Fig. 24.) Conversely, after the cell walls are saturated
with water, any increase in the amount of water absorbed merely
fills the cavities and intercellular spaces, and has no effect
on the mechanical properties. Hence, soaking green wood does not
lessen its strength unless the water is heated, whereupon a
decided weakening results.

[Footnote 49: The wood of _Eucalyptus globulus_ (blue gum)
appears to be an exception to this rule. Tiemann says: "The wood
of blue gum begins to shrink immediately from the green
condition, even at 70 to 90 per cent moisture content, instead
of from 30 or 25 per cent as in other species of hardwoods."
Proc. Soc. Am. For., Washington, Vol. VIII, No. 3, Oct., 1913,
p. 313.]

[Illustration: FIG. 24.--Relation of the moisture content to the
various strength values of spruce. FSP = fibre-saturation
point.]

The strengthening effects of drying, while very marked in the
case of small pieces, may be fully offset in structural timbers
by inherent weakening effects due to the splitting apart of the
wood elements as a result of irregular shrinkage, and in some
cases also to the slitting of the cell walls (see Fig. 25).
Consequently with large timbers in commercial use it is unsafe
to count upon any greater strength, even after seasoning, than
that of the green or fresh condition.

[Illustration: FIG. 25.--Cross section of the wood of western
larch showing fissures in the thick-walled cells of the late
wood. Highly magnified. _Photo by U. S. Forest Service._]

In green wood the cells are all intimately joined together and
are at their natural or