. Bulletin. Forests and forestry -- United States. 34 TIMBER. the several parts of the timber prevents such bending and the conse quent strain leads to their separation as shown at Z, the end surface of the timber being "; As the timber dries out, the line c d becomes shorter, the parts 1 to 6 are allowed to approach again, and the checks close up and are no longer visible. The faster the drying at the surface, the greater is the difference in the moisture of the different parts, and hence the greater the strains and consequently also the amount of checking. This becomes very
. Bulletin. Forests and forestry -- United States. 34 TIMBER. the several parts of the timber prevents such bending and the conse quent strain leads to their separation as shown at Z, the end surface of the timber being "; As the timber dries out, the line c d becomes shorter, the parts 1 to 6 are allowed to approach again, and the checks close up and are no longer visible. The faster the drying at the surface, the greater is the difference in the moisture of the different parts, and hence the greater the strains and consequently also the amount of checking. This becomes very evident when fresh wood is placed in the sun, and still more in a hot kiln. While most of these smaller checks are thus only temporary, closing up again, some large radial checks remain and even grow larger as drying progresses. Their cause is a different one and will presently be explained. The temporary checks not only occur at the ends, but are developed on the sides also, only to a much smaller degree. They become especially an- noying on the surface of thick planks of hard woods, and also on peeled logs when exposed to the sun. So far we have considered the wood as if made up only of parallel fibers all placed longitudinally in the log. This, however, is not the case. A large part of the wood is formed by the medul- lary or pith rays. In pine over 15,000 of these occur on a square inch of a tangential section, and even in oak the very large rays, which are readily visi- ble to the eye, represent scarcely a hundredth part of the number which the microscope reveals. As seen in fig. 22 the cells of these rays have their length at right angles to the direction of the wood fibers. If a large pith ray of white oak is whittled out and allowed to dry it is found to shrink greatly in the direction from c to d (fig. 22), while, as we have stated, the fibers to which the ray is firmly grown in the wood do not shrink in the same direction. Therefore, in the wood, as the cells of the
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