In reducing timber from the log or baulk to scantlings, the dimensions and form that the timber ought to possess when actually in use should be borne in mind, in order that proper allowance may be made for the alteration that will take place in consequence of the action of the atmosphere, which has an influence more or less even upon well-seasoned timber.
1 Faper read before the Philosophical Society of Glasgow, by Jas. Deas, Esq., M.I.C.E.
In straight-grained woods the changes in length caused by the effects of the atmosphere are very slight; but the variations in width and depth are very great, especially in new timber.
Rondelet found that the usual changes of weather produced the following expansion and contraction in wood of average dryness : -
In fir from 1/360 to 1/75 of width; mean 1/124. In oak from 1/412 to 1/80 of width; mean 1/140.
The first effect of atmospheric influence upon a log is that the external portions which are exposed to the air shrink; but the interior, which is protected from the air, remains at its original bulk. The consequence is that the exterior splits, as shown in Fig. 156.
The following extract, taken by permission from Dr. Anderson's lecture on applied mechanics given before the Society of Arts, explains very clearly the manner in which timber shrinks, when cut into scantling : -
"Notwithstanding the extent to which timber is used in the mechanical arts, it is singular that the natural law by which the contraction or shrinking of wood is governed is too much disregarded in practical operations. It is a subject which seems to have been entirely neglected by writers on the subject. . . .
"An examination of the end section of any exogenous tree, such as the beech or oak, will show the general arrangement of its structure. It consists of a mass of longitudinal fibrous tubes arranged in irregular circles that are bound together by means of radical strings or shoots which have been variously named. They are the "silver grains" of the carpenter, or the "medullary rays" of the botanist, and are in reality the same as end wood, and have to be considered as such, just as much so as the longitudinal woody fibre, in order to understand its action. From this it will be seen that the lateral contraction or collapsing of the longitudinal porous or tubular part of the structure cannot take place without first crushing the medullary rays; hence the effect of the shrinking finds relief by splitting in another direction, namely, in radial lines from the centre, parallel with the medullary rays, thereby enabling the tree to maintain its full diameter, as shown in Fig. 156.
"If the entire mass of the tubular fibre composing the tree were to contract bodily, then the medullary rays would of necessity have to be crushed in the radial direction, to enable it to take place, and the timber would thus be as much injured in proportion as would be the case in crushing the wood in the longitudinal direction. If such an oak or beech tree is cut into four quarters by passing the saw twice through the centre at right angles, before contracting and splitting has commenced, the lines a c and c b in Fig. 157 would be of the same length, and at right angles to each other, or in the technical language of the workshop they would be square; but after being stored in a dry place, say for a year, it would then be seen that a great change had taken place both in the form and in some of the dimensions; the lines c a and c b would be the same length as before, but it would have contracted from a to 6 very considerably, and the two lines c a and c b would not be at right angles to each other by the portion shown here in black in Fig. 158. The medullary rays are thus brought closer by the collapsing of the vertical figure.
"But, supposing that four parallel saw cuts are passed through the tree so as to form it into five planks, let us see what would be the behaviour of the several planks. Take the centre plank first. After due seasoning and contracting it would then be found that the middle of the board would still retain the original thickness from the resistance of the medullary rays, while it would be gradually reduced in thickness towards the edges for want of support, and the entire breadth of the plank would be the same as it was at first, for the foregoing reasons, and as shown in Fig. 159. Then taking the planks at each side of the centre, by the same law their change and behaviour would be quite different. They would still retain their original thickness at the centre, but would be a little reduced on each edge throughout, but the side next to the heart of the tree would be pulled round, or partly cylindrical, while the outside would be the reverse, or hollow, and the plank would be considerably narrower throughout its entire length, more especially on the face of the hollow side, all due to the want of support. Selecting the next two planks, they would be found to have lost none of their thickness at the centre, and very little of their thickness at the edges, but very much of their breadth as planks, and would be curved round on the heart side, and made hollow on the outside. Supposing some of these planks to be cut up into squares when in the green state, the shape that these squares would assume after a period of seasoning would entirely depend on the part of the tree to which they belonged; the greatest alteration would be parallel with the medullary rays. Thus, if the square were near the outside, as in Fig. 160, the effect would be that it would contract in the direction from a to b, and after a year or two it would be as in Fig. 161, the distance between c and a being nearly the same as it was before, but the other two angles a and b brought by the amount of their contraction closer together. By understanding this natural law, it is comparatively easy to know the future behaviour of a wood or plank by carefully examining the end wood in order to ascertain the part of the log from which it has been cut, as the angle of the ring growths and the medullary rays will show thus, as in Fig. 162. If a plank has this appearance it will evidently show to have been cut from the outside, and for many years it will gradually shrink all to the breadth, while the next plank, shown in Fig. 163, clearly points close to the centre or heart of the tree, where it will not shrink to the breadth, but to a varying thickness, with the full dimensions in the middle, but tapering to the edges, and the planks on the right and left will give a mean, but with the centre sides curved round, and the outside still more hollow.
"The foregoing remarks apply more especially to the stronger exogenous woods, such as beech, oak, and the stronger home firs. The softer woods, such as yellow pine, are governed by the same law, but in virtue of their softness another law comes into force, which to some degree affects their behaviour, 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 obeyed, the greater part of the shrinking, which we are all too familiar with, would be obviated." ....