. Bulletin. Forests and forestry -- United States. 32 TIMBER. IV—SHRINKAGE OF WOOD. When a short piece of wood fiber, such as that shown in fig. 18, A, is dried it shrinks, its wall grows thinner (as indicated by dotted lines), its width, a b, the thickness of the <4:L jlt-T:ifej ]"l*~--^r_r—"- | ,11 b. r^\a Fig. 18— Short pieces of wood fibers, one thick, the other thin-walled; magnified. y fiber, becomes smaller, and the cavity or opening larger, but, strange to say, the height or length, b c, remains the same. In a similar piece of fiber with a thinner wall (fig. 18, B) the eff
. Bulletin. Forests and forestry -- United States. 32 TIMBER. IV—SHRINKAGE OF WOOD. When a short piece of wood fiber, such as that shown in fig. 18, A, is dried it shrinks, its wall grows thinner (as indicated by dotted lines), its width, a b, the thickness of the <4:L jlt-T:ifej ]"l*~--^r_r—"- | ,11 b. r^\a Fig. 18— Short pieces of wood fibers, one thick, the other thin-walled; magnified. y fiber, becomes smaller, and the cavity or opening larger, but, strange to say, the height or length, b c, remains the same. In a similar piece of fiber with a thinner wall (fig. 18, B) the effect is the same, but the wall, being only half as thick the total change is only about half as If sections or pieces of fibers are dried and then placed on moist blotting paper, they will take up water and swell to their original size, though the water has been taken up only by their walls and none has entered into their openings or lumina. This indicates that the water in the cavity or lumen of a fiber has nothing to do with its dimensions, and that if the cell walls are saturated it makes no difference in the volume of a block of pine wood whether the cell cavities are empty as in the heartwood or three-fourths filled as in the sap wood. If an entire fiber, as shown in fig. 19, is dried, the wall at its ends a and 6, like those of the sides, grow thinner, and thereby the length of the entire cell grows shorter. Since this length is often a hundred or more times as great as the diameter, the effect of this shrinkage is inappreciable; and if a long board shrinks lengthwise, it is largely due, as we shall see, to quite another cause. A thin cross section of several fibers (see fig. 20, A) like the piece of a single fiber shrinks Fig. harping of wood wheu ^^ ^ ^ Qf each fiber becomes fMn_ ner, and thus each piece smaller, and the piece on the whole necessarily 1 Though generally true, it must not be supposed that the fibers of all species, or even the fibers of
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