The following classification of timber is a modification by Professor Eankine and Mr. Hurst of that originally proposed by Tredgold: -
Pine Wood (natural order Coniferce).
Hard Wood or Leaf Wood (non-resinous and non-coniferous).
With distinct large medullary rays.
Subdiv. I. Annual rings distinct; one' side porous, the other compact.
Subdiv. II. Annual rings not distinct; texture nearly uniform.
No distinct large medullary -rays.
'subdiv. I. Annual rings distinct; one side porous, the other compact.
Chestnut, Ash, Elm.
Subdiv. II. Annual rings not distinct; texture nearly uniform,
With regard to the above Table Professor Rankine remarks: - "The chief practical bearings of this classification are as follows. "Fir wood, or coniferous timber, in most cases contains turpentine. It is distinguished by straightness in the fibre and regularity in the figure of the trees; qualities favourable to its use in carpentry, especially where long pieces are required to bear either a direct pull or a transverse load, or for purposes of planking. At the same time the lateral adhesion of the fibres is small, so that it is much more easily shorn and split along the grain than hardwood, and is therefore less fitted to resist thrust or shearing stress, or any kind of stress that does not act along the fibres. Even the toughest kinds of firwood are easily wrought.
"In hard wood, or non-coniferous timber, there is no turpentine. The degree of distinctness with which the structure is seen, whether as regards medullary rays or annual rings, depends upon the degree of difference of texture of different parts of the wood. Such difference tends to produce unequal shrinking in drying, and consequently those kinds of timber in which the medullary rays and the annual rings are distinctly marked are more liable to warp than those in which the texture is more uniform. At the same time, the former kinds of timber are, on the whole, the more flexible, and in many cases are very tough and strong, which qualities make them suitable for structures that have to bear shocks." 1
The classification shown above is that made by botanists and given by most writers on timber.
For many practical purposes, however, the timber used upon engineering and building works may be divided into two classes : -
Soft Wood, including firs, pines, spruce, larch, and all cone-bearing trees.
Hard Wood, including oak, beech, ash, elm, mahogany, etc.
The different trees included under the general head of "Fir Timber" are divided by botanists into the pines and firs, which produce timber of very different quality, and are distinguished in the growing tree by the leaves, the shape of the cones, and by other peculiarities.
The Pine (Pinus) has slender green needle-shaped leaves, growing in clusters of from two to six (according to the species) from the same stalk. It has one straight tap root, the trunk does not taper much, the wood is close grained, fibrous, very durable, full of resinous matter, and of a high bright colour. The cones have thick woody scales that do not fall away from the axis.
The Fir or Spruce (Abies) has straight short leaves, which come off singly from the stalks. The roots are ramified, the trunk tapers more than that of the pine, the shape of the tree is more pyramidal, the wood is of a much lighter colour, and not nearly so durable. The cones are long and pendulous, with thin woody scales that do not fall away from the axis.
1 Rankine, Civil Engineering, p. 440.
No attention is, however, paid to these botanical distinctions in the classification adopted on building or engineering works.