If such a case-hardened stick of wood be resawed, the two halves will cup from the internal tension and external compression, with the concave surface inward.
[Illustration: FIG.

26 .-- Progress of drying throughout the length of a chestnut beam, the black spots indicating the presence of free water in the wood.

The first section at the left was cut one-fourth inch from the end, the next one-half inch, the next one inch, and all the others one inch apart.

The illustration shows case-hardening very clearly.

_Photo by U.S.
Forest Service._] For a given surface area the loss of water from wood is always greater from the ends than from the sides, due to the fact that the vessels and other water-carriers are cut across, allowing ready entrance of drying air and outlet for the water vapor.
Water does not flow out of boards and timbers of its own accord, but must be evaporated, though it may be forced out of very sappy specimens by heat.