. Bulletin. Forests and forestry -- United States. SHRINKAGE AND CHECKS. 33. x shares this diminution of size, the distances, a b and c r7, each becoming shorter. Where the cells are very similar in size and in the thickness of their walls, as in the case of piece A, fig. 20, a b and c d become shorter by about the same amount; but if the piece is made up of fibers, some of which have thin and others thick walls, as piece i>, fig. 20, then the row of thick-walled cells shrinking much more than the row of thin-walled cells, the piece becomes unevenly shrunk or warped as shown in fig. 20, G.
. Bulletin. Forests and forestry -- United States. SHRINKAGE AND CHECKS. 33. x shares this diminution of size, the distances, a b and c r7, each becoming shorter. Where the cells are very similar in size and in the thickness of their walls, as in the case of piece A, fig. 20, a b and c d become shorter by about the same amount; but if the piece is made up of fibers, some of which have thin and others thick walls, as piece i>, fig. 20, then the row of thick-walled cells shrinking much more than the row of thin-walled cells, the piece becomes unevenly shrunk or warped as shown in fig. 20, G. Not only is the piece warped, but the force which led to this warping continues to strain the interior parts of the piece in different directions. Since in all our woods cells with thick walls and cells with thin walls are more or less intermixed, and especially as the spring wood and summer wood nearly always differ from each other in this respect, strains and tendencies to warp are always active when wood dries out, because the summer wood shrinks more than the spring wood, heavier wood in gen- eral more than light wood of the same kind. If the piece A, fig. 20, after drying, is placed edgewise on moist blotting paper, the cells on the underside, at c d, take up moisture from the paper and swell before the upper cells at a b receive any moisture. This causes the underside of the piece to be- come longer than the upper side and, as in the case of piece G, warping occurs. Soon, however, the moisture penetrates to all the cells and the piece straightens out. A thin board behaves exactly like this minute piece, only the process is slower and more easily ob- served. But while a thin board of pine curves laterally, it remains quite straight length- wise, since in this direction both shrinkage and swelling are small. A thin disk or cross section swells, and when moistened on one side warps as readily in one direction as in another. If a green board is exposed to the sun with one sid
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