75. To know the resistance which a piece of timber offers to any force tending to change its form, is one of the most important species of knowledge that a carpenter has to acquire: and to be able to judge of the degree of resistance from observation only, even in common cases, requires nothing less than the practice of a life devoted wholly to carpentry.

Besides, it is a kind of knowledge that is confined to the person who has obtained it, and dies with him. It is a feeling of fitness that cannot be communicated, nor yet described; nevertheless it is a feeling that every thinking practical man is sensible he possesses. The author is far from having a wish to banish the nice observation that gives birth to this feeling; because it is more desirable that it should be encouraged than suppressed; but there are cases where it fails, - that is, when the magnitude of the object is beyond the range of ordinary practice, and where new combinations are attempted. In such cases the laws of the resistance of solids should be referred to, even by the expert practical man; and he will be better able to judge of their correctness if he finds them, in common cases, to give results that agree with the conclusions he has drawn from practice.

But there are many, besides practical carpenters, that ought to know something of the principles of building, and who have not an opportunity of becoming acquainted with those principles through practice; to such persons the rules and experiments detailed in this work will be found extremely useful.

In order to determine the dimensions or scantling of a piece of timber, that shall be capable of sustaining a given weight or pressure, the laws that regulate its resistance should be considered; and to accomplish this in a manner that is likely to be useful, we must consider what effect is produced when a piece of timber is overloaded. This effect, in general, is nothing more than a certain degree of flexure, or bending, as it seldom happens that timbers are absolutely broken; and generally, a small degree of bending renders a beam unfit for its intended purpose.

Much has been said on the irregular nature of timber, and that it is impossible to make rules or tables for scantlings on that account; but it must be observed that these remarks apply only to rules for the strength of timber to resist breaking; and even in that case timber is not so irregular as is generally imagined. The difference in good timber is still less perceptible when the bending only is considered, and the laws relating to flexure are founded on experiments of an unexceptionable character. It has been shown in the preceding section (Art. 18) that a change in the position of the resisting parts brings new forces into action, which is the cause of the irregularity observed by Buffon in his experiments;* but in a piece of carpentry these changes must never be so great as to produce a sensible effect; therefore it would be an useless refinement to attempt to form rules that would embrace all the circumstances these changes produce; besides, it would render them too complicated to be useful.

In all cases timbers that are exposed to considerable strains ought to be of a good quality; therefore the data should be drawn from experiments on good timber, and not from inferior specimens. For if inferior specimens were made the basis of calculation, at what point of inferiority should we begin? and how should it be described so as to enable us to compare it with any other timber? But when it is known what a piece of good timber will do, it will be easy to compare its description with that to be used. Good timber is that which is perfectly sound, straight grained, free from large knots or other defects, particularly near the strained points, and seasoned. Specimens of this kind are marked medium in the Tables of Experiments. In another Section the reader will find further information respecting the nature and qualities of different kinds of timber.

* 'Memoires de l'Academie des Sciences,' 1741, pp. 328-332.

But the qualities even of good timber vary in some degree according to the nature of the soil, and the dryness and exposure of the situation where it was grown. The age of trees at the time of cutting, the natural defects, such as knots, shakes, etc, also the mode of seasoning, or the comparative dryness, is the cause of some difference in the strength and stiffness of timber: all these things considered, it is impossible to calculate correctly its strength and stiffness. But, fortunately, that precision which is so essential to the philosopher, is not absolutely necessary to the architect and engineer. They content themselves with approximations that are simple, and more easily obtained; and, provided that the limits which cannot be passed with safety be pointed out, these approximations are sufficient to direct their practice.