Art. X.—The Building Materials of Otago. By William N. Blair, C.E. [Read before the Otago Institute, 31st October, 1876.] Timbers. Properties of Timbers. Although the properties of timbers generally are better known than those of the other building materials that have already been discussed, it is necessary for the proper investigation of our subject to consider the leading characteristics that bear on their economic value, and in doing so I shall trace the timber through the various stages of its existence. Structure.—As you are probably aware, the structure of ordinary timber is, to all intents and purposes, identical with that of a brick wall: it is composed of vertical and horizontal layers, breaking joints, and cemented together in much the same way. The vertical joints, consisting of the annual rings and medullary rays, are quite clear and distinct; but the horizontal ones, made from the interlacing of bundles of woody fibre of irregular lengths, are only visible to the microscopist. It is this difference in the length of the scarf, or joint, that makes splitting timber so much easier than cutting it across the grain. The concentric rings represent the growth in a year or season; they are generally very distinct in timber grown in a cold climate, where there is a decided period of repose in the the vegetation; but in many tropical trees the rings are scarcely discernable, and some botanists allege that occasionally so many as four rings are formed in one year. The medullary rays are thin plates of woody matter that radiate from the pith to the bark, and form the weft which interlaces with the warp of the annual rings. Although believed to exist in all timbers, these rays cannot be traced in the firs and pines of the old country, but are very conspicuous in oak, beech, and other hard woods; this rule does not hold good in Otago, for there are few timbers hard or soft in which they do not appear. These medullary rays are what give the peculiar watered figure called silver grain, which is so much prized by cabinet-makers and other manufacturers of fancy wood-work. Growth.—The principal agent in the formation and development of woody fibre and tissue is the sap, which performs the same functions in plants that blood does in animals. After being extracted by the roots from the soil, it rises through the trunk to the leaves, and is there subjected to certain chemical changes that fit it for the formation of timber. In saplings, the fluid permeates and rises through the whole trunk; but in old trees with solid heart-wood, it is confined to the sap-wood and the bark. At this stage the heart-wood contributes nothing to the other parts of the tree