160-166.

Kempfer, Wm. H.: The air-seasoning of timber. In Bul. 161, Am.
Ry. Eng. Assn., 1913, p. 214.]

The important consideration in regard to this question is the
series of circumstances attending the handling of the timber
after it is felled. Wood dries more rapidly in summer than in
winter, not because there is less moisture at one time than
another, but because of the higher temperature in summer. This
greater heat is often accompanied by low humidity, and
conditions are favorable for the rapid removal of moisture from
the exposed portions of wood. Wood dries by evaporation, and
other things being equal, this will proceed much faster in hot
weather than in cold.

It is a matter of common observation that when wood dries it
shrinks, and if shrinkage is not uniform in all directions the
material pulls apart, causing season checks. (See Fig. 27.) If
evaporation proceeds more rapidly on the outside than inside,
the greater shrinkage of the outer portions is bound to result
in many checks, the number and size increasing with the degree
of inequality of drying.

In cold weather, drying proceeds slowly but uniformly, thus
allowing the wood elements to adjust themselves with the least
amount of rupturing. In summer, drying proceeds rapidly and
irregularly, so that material seasoned at that time is more
likely to split and check.

There is less danger of sap rot when trees are felled in winter
because the fungus does not grow in the very cold weather, and
the lumber has a chance to season to below the danger point
before the fungus gets a chance to attack it. If the logs in
each case could be cut into lumber immediately after felling and
given exactly the same treatment, for example, kiln-dried, no
difference due to the season of cutting would be noted.

WATER CONTENT[48]

[Footnote 48: See Tiemann, H.D.: Effect of moisture upon the
strength and stiffness of wood. Bul. 70, U.S. Forest Service,
Washington, D.C., 1906; also Cir. 108, 1907.]

Water occurs in living wood in three conditions, namely: (1) in
the cell walls, (2) in the protoplasmic contents of the cells,
and (3) as free water in the cell cavities and spaces. In
heartwood it occurs only in the first and last forms. Wood that
is thoroughly air-dried retains from 8 to 16 per cent of water
in the cell walls, and none, or practically none, in the other
forms. Even oven-dried wood retains a small percentage of
moisture, but for all except chemica