英文科學讀本 第四冊·Lesson 16 Timber—Its Shrinkage and Preservation

Lesson 16 Timber—Its Shrinkage and Preservation

Our last lesson on timber dealt with the methods of seasoning the wood to render it fit for use. During the seasoning process the wood shrinks, but always according to a natural law. The amount of shrinkage in length is so small that it may be altogether disregarded. The shrinkage takes place in the breadth of the plank.

If we think of the structure of the exogenous stem we shall see at once the reason for this. The woody fibers are arranged lengthwise down the stem in irregular circles, and these fibers are bound together by the radiating plates, known as the medullary rays or the silver grain. This silver grain is not of the same nature as the woody fibres. It resembles the pith in structure. As the drying process of the seasoning goes on, the woody fibers contract or shrink in bulk. But they can only shrink by tearing away, at the same time, the substance of the medullary rays. That is to say, the shrinking of the woody bundles finds relief by splitting the timber in the direction of the medullary rays. This explains why timber, after it is cut, and before it is properly seasoned, always cracks and splits on the outside more than the inside of the mass.

It is interesting to note the behavior of a trunk of one of the stronger exogenous woods, such as oak or beech, when it is cut up into planks. Imagine a trunk cut lengthwise by the saw into seven planks. After the planks had been properly seasoned, the middle one would be found to retain its original thickness in the center, but the edges of the board would be thinner. The breadth of the plank would remain the same as at first. The medullary rays, being closer together towards the center of the trunk, offer greater resistance there to the tearing than at the edges. The planks on either side of the middle one would, from the same cause, become bent out of shape, or pulled round into a convex from the center of the trunk, and each board would be narrower.

Timber, even after it has been well seasoned, is liable to decay from many causes. Moisture and changes in temperature are its worst enemies. They not only lead to the decay of the wood, but they encourage the attacks of insects and worms, and the wood gradually crumbles away. Where timber structures have to be exposed to changes of air, light, heat, and moisture, something must be done to protect them from the effects of all these.

All woodwork is covered with a coating of paint or tar for this very purpose. Both these substances are impervious to water, and will not allow the wet to penetrate into the wood. Tar itself, moreover, is a powerful preventer of decay. This explains why, in fixing a wooden post into the ground, we soak the lower end in tar, or stand it in pitch, and why ships, boats, and barges are always kept well painted and tarred.

Another enemy to timber structures is dry rot The simplest and best way of preventing this is to saturate the wood in oil.