TIMBER, wood felled and seasoned for the purpose of building. Alany kinds of woods are useful for building pur poses, but those most frequently employed are oak, fir, and pine ; but before entering into the peculiar properties of these, it will be as well to give some idea of the structure and growth of trees in general.
Upon observing the transverse section of the stem or trunk of a tree, the wood will be found to be composed of numer ous concentric layers or rings, which are more or less defined in different trees. Where the rings are well defined, they will be found to consist of two parts, the outer being hard, compact, and of a dark colour, while the inner is of a lighter tint, more soft and porous. In the centre of the tree is the pith, and on the exterior the bark, and it will be observed, that the concentric rings become more soft, and contain more sap as they recede from the pith, the more compact layers nearest the pith being termed heart-wood.
The structure of a tree appears to be composed of minute vessels for conveying nutriment from the roots; the space between these vessels being occupied by cells, which are engaged in performing the function of secretion. The vessels in the growing tree convey the sap in a liquid state from the roots to the leaves, whence it descends in a less liquid state through the bark, and is at last deposited in an altered state between the bark and the last year's wood, forming a new layer of bark and sap-wood, the old bark being pushed out ward, and the inner layers being compressed, probably in an equal degree. The sap begins to ascend in the spring of the year, and flows principally through the annual rings next the bark, which contain most sap-wood. 111 its ascent, it would appear to dissolve some part of a substance which had accumulated in the vessels during the preceding winter, for the nourishment of the buds, leaves, and new wood ; and this accounts for the viscous state of the sap on its descent. As the leaves expand, the sap ceases to flow, and the bark again adheres to the wood, and, from the middle of June to the middle of August, there appears to be a pause in vegetation ; but after this period, the sap again begins to flow.
As this process goes on from year to year, the fluid parts of the interior of the wood, are absorbed by the new wood and leaves, and the vessels through which they flow, being pressed more closely together by the growth of new wood, become harder and harder, until at last the sap-wood is con verted into heart-wood ; for it would appear, that there is nothing of the character of solid fibres in wood, the inure compact parts being composed solely of the linings of the vessels and cells deprived of their moisture, and packed closely together. When trees arrive at this stage of exist
ence, that is to say, when the sap-wood has become heart wood, and the greater part of the moisture has been expelled, they are in a fit state to be felled for the purposes of building. The best time for felling is in mid-winter or midsummer; for in the former the sap has ceased to flow, and in the latter it is expended in the production of leaves. Besides the concentric rings, another series of lines may be observed with more or less facility in the sections of various trees; these lines radiate from the centre, and are termed medullary rays ; they produce that beautiful flowered appearance in the oak, to which the name of silver grain has been given.
It will not be necessary here to treat of the seasoning of timber, as that subject has been treated on in another place.
See SEASONING OF TIMBER.
If timber be properly seasoned, and kept in a dry situation, with a free circulation of air, it will last for several centuries, but even under the most favourable circumstances it gradually deteriorates, it loses its elastic and coherent properties, and becomes brittle at last. If it be kept immersed in water, it will also last a very considerable period; but it is not unin jured, for if it be taken out and dried, it becomes brittle, splits, and cracks in every direction. Alternate changes of dryness and moisture is very injurious to timber, and under such conditions it rapidly decays, as may be observed in the upper part of piles driven in a tidal river, viz., that part which is contained between high and low water. Moisture, combined with a certain degree of heat, about 45 degrees, will gradually decompose timber, especially when exposed to the air. This rot is usually divided into two kinds, the wet and dry rot ; these, however, are both produced by the same causes, the only difference being that the former takes place when there is a free evaporation, the latter when the evapora tion is imperfect ; the one takes place when there is a free circulation of air, the other when the air is confined.