ually shrink in the breadth; while the next plank, shown in Fig. 6, must have been derived from near the centre or heart of the tree, and it will not shrink in the breadth but in thickness, with the full dimension in the middle, but tapering to the edges.
The foregoing remarks apply more especially to the stronger exogenous woods, such as beech, oak, and the stronger firs. The softer woods, such as yellow Canadian pine, are governed by the same law; but, in virtue of their softness, another law comes into force, which to some degree affects their behavior, as the contracting power of the tubular wood has sufficient strength to crush the softer medullary rays to some extent, and hence the primary law is so far modified. But even with the softer woods, such as are commonly used in the construction of houses, if the law is carefully observed, the greater part of the evils of shrinking would be obviated. Hence, also, it is that when a round block, as a mast, is formed out of a tree, it retains its roundness because it contracts uniformly or nearly so, whereas, if a round spar is formed out of a quartering of the same tree it will become an oval, or otherwise contorted toward that shape.
It would, not be in accordance with the object to enumerate all the woods that are employed in the arts, therefore a few only are selected, or such as are employed for purposes where strength is the primary object, viz., ash, beech, elm, fir, hornbeam, mahogany, oak, and teak.
Ash is a coarse wood, but possessed of considerable strength, and is distinguished for its great toughness and elasticity, and is usually employed where severe shocks and wrenches have to be encountered, such as for agricultural implements, the felloes and spokes of wheels, and the shafts of carriages, for hammer-shafts, and for spring purposes generally wherever wood is employed for that purpose.
From its great flexibility it is seldom employed where rigidity is a desideratum. The combination of strength with flexibility is the characteristic of ash, and when the wood is from a young tree, or a tree not too old, it is an invaluable wood in many respects; but as the tree becomes older, the change to brittleness sets in and soon renders it less valuable. It is also remarkable for its endurance when kept dry, but when exposed to damp or to wet it rapidly decays. The numerical value of its properties varies considerably, but in general terms it may be stated that, as compared with oak, good ash has frequently a still greater tenacity and likewise a greater degree of toughness, but, from its flexibility, especially when young, it has considerably less stiffness, which unfits it for many purposes.
Beech has frequently considerable strength, and is chiefly distinguished for its uniformity, its smoothness of surface, and closeness of grain. It likewise possesses no little beauty, and takes a good polish, more especially when its silver grain is skilfully exposed. When well seasoned and not too old, it is frequently used for the cogs of mill-gear
