These branches of the subject do not fall within the province of the engineer or builder, and will not here be entered upon; it will be sufficient to point out some of the characteristics by which good timber may be known.
Good timber should be from the heart of a sound tree - the sap being entirely removed, the wood uniform in substance, straight in fibre, free from large or dead knots, flaws, shakes, or blemishes of any kind.
If freshly cut it should smell sweet; "the surface should not be woolly, or clog the teeth of the saw," but should be firm and bright, with a silky lustre when planed; a disagreeable smell betokens decay, and "a dull chalky appearance is a sign of bad timber."
The annual rings should be regular in form; sudden swells are caused by rind-galls; closeness and narrowness of the layers indicate slowness of growth, and are generally signs of strength. When the rings are porous and open, the wood is weak, and often decayed.
The colour of good timber should be uniform throughout; when it is blotchy, or varies much in colour from the heart outwards or becomes pale suddenly towards the limit of the sapwood, it is probably diseased.
Good timber is sonorous when struck. A dull heavy sound betokens decay within (see p. 393). Among specimens of the same timber, the heavier are generally the stronger.
Timber intended for use in important work should of course be free from the defects mentioned in page 388. The knots should not be large or numerous, and on no account should they be loose.
The worst position for large knots is when they are near the centre of the balk required, and more especially when they are so situated as to form a ring round the balk at one or more points.
The sap should be entirely removed. According to Mr. Laslett. however, the heart of trees having the most sapwood is generally stronger and better in quality than the heart of trees of the same species that have but little sapwood.
The strongest part of the tree is generally that which contains the last-formed rings of heartwood, so that the strongest scantlings are obtained by removing no more rings than those containing the sap.
Timber that is thoroughly dry weighs less than when it was green (see p. 388); it is also harder, and consequently more difficult to work.
There are several defects in timber caused by the nature of the soil upon which the tree was grown, and by the vicissitudes to which it has been subjected while growing.
Heartshakes are splits or clefts occurring in the centre of the tree. They are common in nearly every kind of timber. The splits are in some cases hardly visible; in others they extend almost across the tree, dividing it into segments.
When there is one cleft right across the tree it does not occasion much waste, as it divides the squared trunk into two substantial balks. Two clefts crossing one another at right angles, as in Fig. 148, make it impossible to obtain scantlings larger than one-fourth the area of the tree.
The worst form of heartshake, however, is one in which the splits twist in the length of the tree, thus making it impossible to convert the tree into small scantlings or planks.
Starshakes are those in which several splits radiate from the centre of the timber, as in Fig. 149.
Cupshakes are curved splits separating the whole or part of one annual ring from another (see Fig. 150). When they occupy only a small portion of a ring they do no great harm.
Rind-Galls are peculiar curved swellings, caused generally by the growth of layers over the wound remaining after a branch has been imperfectly lopped off.
Upsets are portions of the timber in which the fibres have been injured by crushing.
Twisted Fibres are caused by the action of a prevalent wind, turning the tree constantly in one direction. Timber thus injured is not fit for squaring, as so many of the fibres would be cut through.