John Anderson

“ The natural law that governs the shrinking or contraction of timber is most important to practical men, but it is too often overlooked.
The amount of the shrinkage of timber in length, when seasoning, is so inconsiderable that it may in practice be disregarded. But the shrinkage in transverse directions is much greater, and presents some peculiarities which can only be explained by examining the structure of the wood, as resulting from its mode of growth. An examination of the end section of any exogenous tree, such as the beech or oak, will show the general arrangement of its structure. ”

“ The worst part of a tree is the sap-wood, which is next the bark. It is softer than the other parts of the wood, and is liable to premature decay. The deleterious component of the sap-wood is absorbed, if the tree is allowed to grow for a longer period, and in time the old sap-wood becomes proper timber-fibre similar to heart-wood. Hence, the goodness of a tree, for timber purposes, depends on the age at which the tree was cut down. When young, the heart-wood is the best; at maturity, with the exception of the sap-wood, the trunk is equally good throughout ”

“ Some varieties of wood, as, for instance, lignumvitæ, are so dense that they sink in water, while some of the softer woods have not half the density of that fluid. The presence of gum or resin in any wood adds both to its strength and durability. Many woods will last a long time if kept constantly under water, but scarcely any wood is very durable when allowed to become wet and dry alternately.
The strength of a piece of timber depends upon the part of the tree from which it is taken. Up to a certain age, the heart of the tree is the best ”

“ The strength of many woods is nearly doubled by the process of seasoning, hence it is very thriftless to use timber in a green state, as it is not only weak, but is exposed to continual change of bulk, form, and stability. After timber is cut, and before it is properly seasoned, the outside is found to crack and to split more than the inside of the mass, because it is more exposed to the desiccating effect of the surrounding atmosphere, but, as the outside dries, the air gradually finds its way to the interior. ”

“ 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. ”

“ Its cohesive strength and other properties depend much upon its age, as a plank, and still more on the age of the tree from which the plank was taken. When in the most favorable condition, it is fully equal to the average of oak (even when considered merely as a wood) , but when cut from older trees, and when over-seasoned, it is frequently found worthless, and has soon to be renewed. When of proper age and quality, it has no equal for its own special purposes. ”

“ Wood will always warp after a fresh surface has been exposed, and will likewise change its form by the presence of any moisture, either from that contained in the atmosphere or from wetting the surface. The effect of moisture on dry wood is to cause the tubular fibres to swell; hence it is that, if a plank or board is wetted upon one side, the fibres there will be distended, and the plank, in consequence, must bend. ”

“ ALTHOUGH it is of less importance to investigate the strength of timber at the present time than it was formerly, in consequence of the diminished use of that material in permanent structures, and the more general employment of iron, still it will always be a very valuable material for certain purposes, and ought not to be neglected. ”

“ If timber is cut up by the saw when green, and allowed to season or dry in a gradual manner, it is found to be the most durable. In the arts, however, artificial drying is often resorted to, as in the case of gun-stocks. These are put into a desiccating chamber, where a current of air at 90° or 100° is passed over them, at such a rate as to change the whole volume of air in the chamber every three minutes, and it is found that a year of seasoning may thus be saved. ”

“ 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. ”

“ 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 ”

“ Then, taking the planks at each edge of the centre, by the same law their change and behavior will be quite different: they will still retain their original thickness at the centre, but will be a little reduced on each edge throughout, but the side next to the heart of the tree will be pulled round or bent convex, while the outside will be the reverse, or hollow, and the plank will be considerably narrower throughout its entire length, more especially on the surface of the hollow side. ”

“ It is a wood which is little affected by constant immersion in water, but decays rapidly when alternately wet and dry, and consequently is not very durable for purposes involving exposure to a wet climate. ”

“ 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. ”

“ 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-gearing gearing, and is usually considered by millwrights as next to hornbeam, both in strength, toughness, and general suitability for that purpose. ”