Some of the defects found in lumber, as wind-checks, cross-grain, warping, and improper seasoning, have already been alluded to. Wood may be shaky (b. Fig. 14), which is a separation of the annual rings, showing checks or splits, sometimes including nearly all of the central portion and extending throughout the length of the stem. No wood furnishes a better example of this than hemlock. This shaky condition is caused by the swaying from the force of wind, acting upon trees in open places, along the borders of forests, and especially those adjoining cleared tracts.

Knots in the wood are imperfections arising from the deflection of the fibers which form branches. Near the center of the stem the fibers are few and the knot small, but as the stem enlarges in size the number of fibers in the branch increases so that at the circumference of the stem the knots are largest. The great strength required at the union of branch and stem is shown by the superior hardness and density of the wood composing the knot. Dead branches give rise to loose and dark-colored knots (Fig. 16, b), and the fibers of the stem that form afterward bend around the branch, continue up the stem, and produce cross-grained wood in the vicinity of the knot. Fast knots are the result of living branches, and boards containing them may be used wherever strength or finish is not required.

Sap-wood. - The edges of boards frequently retain a portion of the sap-wood, which must not be placed in any permanent structure, because of its softness and tendency to induce decay.

Resin-pockets are spaces between the annual rings of pine timber, filled completely or in part with resin. These slightly weaken the board, and if used in any portion of a building exposed to the warmth of the sun, will exude drops of turpentine, even if the surface has been painted or varnished.

Fig. 16. - Knots, a, fast; 6, loose.

Decay. - Of all the defects in wood, decay or rot is at once the most prevalent and disastrous to the strength and usefulness of the material, and, when begun, will continue until the whole of the wood is consumed.

Defects in milling are frequent. Lumber may be uneven in width or thickness. The saw may have torn out fibers in places, or have cut irregularly, so that, in planing the boards, marks of the saw remain. When the edges of boards are not squared, they are termed wany.