l purposes, may be
considered absolutely dry.

The general effect of the water content upon the wood substance
is to render it softer and more pliable. A similar effect of
common observation is in the softening action of water on
rawhide, paper, or cloth. Within certain limits the greater the
water content the greater its softening effect.

Drying produces a decided increase in the strength of wood,
particularly in small specimens. An extreme example is the case
of a completely dry spruce block two inches in section, which
will sustain a permanent load four times as great as that which
a green block of the same size will support.

The greatest increase due to drying is in the ultimate crushing
strength, and strength at elastic limit in endwise compression;
these are followed by the modulus of rupture, and stress at
elastic limit in cross-bending, while the modulus of elasticity
is least affected. These ratios are shown in Table XV, but it is
to be noted that they apply only to wood in a much drier
condition than is used in practice. For air-dry wood the ratios
are considerably lower, particularly in the case of the ultimate
strength and the elastic limit. Stiffness (within the elastic
limit), while following a similar law, is less affected. In the
case of shear parallel to the grain, the general effect of
drying is to increase the strength, but this is often offset by
small splits and checks caused by shrinkage.

|----------------------------------------------------------------------|
| TABLE XV |
|----------------------------------------------------------------------|
| EFFECT OF DRYING ON THE MECHANICAL PROPERTIES OF WOOD, SHOWN IN |
| RATIO OF INCREASE DUE TO REDUCING MOISTURE CONTENT FROM |
| THE GREEN CONDITION TO KILN-DRY (3.5 PER CENT) |
| (Forest Service Bul. 70, p. 89) |
|----------------------------------------------------------------------|
| KIND OF STRENGTH | Longleaf | Spruce | Chestnut |
| | pine | | |
|----------------------------+-------------+-------------+-------------|
| | (1) (2) | (1) (2) | (1) (2) |
| | | | |
| Crushing strength parallel | | | |