Fig. 199.
Fig. 200.
Since the cells of the pith or medullary rays extend at right angles to the main body, [Fig. 200], their smaller shrinkage along the radius of the log opposes the shrinkage of the longitudinal fibers. This is one reason why a log shrinks more circumferentially, that is along the rings, than it does radially or along the radii. A second cause lies in the fact that greatly shrinking bands of summer wood are interrupted, along the radii, by as many bands of slower shrinking spring wood, while they are continuous along the rings.
Fig. 201.
This tendency of the log to shrink tangentially or along the radii leads to permanent checks. [Fig. 201] A. It causes logs sawed into boards to take forms as shown in [Fig. 201] B.
Warping is caused by uneven shrinkage. Sapwood, as a rule, shrinks more than heartwood of the same weight. The wood of pine, spruce, cypress, etc., shrinks less than the wood of trees such as the oak.
118. Weight.
—Wood substance is 1.6 times as heavy as water; it matters not whether it be wood of oak, pine or poplar. Wood placed in water floats because of the air enclosed in its cells; when the cells become filled with water it sinks.