THOROUGH knowledge of the nature and properties of different kinds of timber is very important to the engineer or architect.
Before entering upon a description of the different varieties of timber under the forms in which they generally come into the market, it will be advisable to make a few remarks on the growth of trees. A very slight knowledge of this branch of the subject is necessary in order that other points more intimately connected with the practical use of timber may be clearly understood.
The timber used in building and engineering work is obtained from trees of the class known by botanists as "E.vogens," or outward growers.
In trees of this class the stem grows by the deposit of successive layers of wood on the outside under the bark, while at the same time the bark becomes thicker by the deposit of layers on its under side.
Upon examining the cross section of such trees (see Fig. 147) we find that the wood is made up of several concentric layers or rings, each ring consisting in general of two parts - the outer part being generally darker in colour, denser, and more solid than the inner part, the difference between the parts varying in different kinds of trees.
In the centre of the wood is a column of pith, p, from which planes, seen in section as thin lines, m m (in many woods not discernible), radiate toward the bark, and in some cases similar lines, m m, converge from the bark toward the centre, but do not reach the pith.
These radiating lines are known as "medullary rays" or "transverse septa." When they are of large size and strongly marked, as in some kinds of oak, they present, if cut obliquely, a beautiful figured appearance, called "silver grain" or "felt."
The wood is composed of bundles of cellular tubes, which serve to convey the required nourishment from the earth to the leaves.
The process of growth in a temperate climate is as follows : - In the spring the root absorbs juices from the soil, which are converted into sap, and ascend through the cellular tubes to form the leaves.
At the upper surface of the leaves the sap gives off moisture, absorbs carbon from the air, and becomes denser; after the leaves are full grown, vegetation is suspended until the autumn, when the sap in its altered state descends by the under side of the leaves, chiefly between the wood and the bark, where it deposits a layer of new wood (the annual ring for that year), a portion at the same time being absorbed by the bark. During this time the leaves drop off, the flow of sap then almost stops, and vegetation is at a standstill for the winter.
With the next spring the operation recommences, so that year after year a distinct layer of wood is added to the tree.
The above description refers to temperate climates, in which the circulation of sap stops during winter; in tropical climates it stops during the dry season.
Thus, as a rule, the age of the tree can be ascertained from the number of annual rings, but this is not always the case. Sometimes a recurrence of exceptionally warm or moist weather will produce a second ring in the same year.
As the tree increases in age, the inner layers are filled up and hardened, becoming what is called "duramen" or "heartwood."1 The remainder is called "alburnum" or "sapwood." The sapwood is softer and lighter in colour than the heartwood, and can generally be easily distinguished from it.
In addition to the strengthening of the wood caused by the drying up of the sap, and consequent hardening of the rings, there is another means by which it is strengthened - that is, by the compressive action of the bark. Each layer, as it solidifies, expands, exerting a force upon the bark, which eventually yields, but in the meantime offers a slight resistance, compressing the tree throughout its bulk.2
The sapwood is generally distinctly bounded by one of the annual rings, and can thus be sometimes distinguished from stains of a similar colour which are caused by dirty water soaking into the timber while it is lying in the ponds (see p. 390). These stains do not generally stop abruptly upon a ring, but penetrate to different depths, colouring portions of the various rings.
The heartwood is stronger and more lasting than the sapwood, and should alone be used in good work.
The annual rings are generally thicker on the side of the tree that has had most sun and air, and the heart is therefore seldom in the centre.
While the tree is growing the heartwood is the strongest, but after the growth has stopped the heart is the first part to decay. It is important, therefore, that the tree should be felled at the right age.
The proper age varies with different trees, and even in the same tree under different circumstances. The induration of the sapwood should have reached its extreme limits before the tree is felled, but the period required for this varies with the soil and climate.
Trees cut too soon are full of sapwood, and the heartwood is not fully hardened.
1 Sometimes called the "Spine."
The ages at which the under-mentioned trees should be felled are state by Tredgold to be as follows :-
60 to 200 years; 100 years the befit
Ash . .
From 50 to 100 years.
Larch . .
Elm . .
Spruce Scotch Fir ... .
From 70 to 100 years.
Oak bark, which is very valuable, is sometimes stripped in the spring, when it is loosened by the rising sap. The tree is felled in the winter, at which time the sapwood is found to be hardened like the heart. This practice is said by Tredgold to improve the timber.
Mr. Laslett says that "to select a healthy tree for felling we must seek for one with an abundance of young shoots, and the topmost branches of which look strong, pointed, and vigorous, this being the most certain evidence that it has not yet passed maturity."
The best season for felling timber is at midsummer or midwinter in temperate, or during the dry season in tropical climates, when the sap is at rest.