In the strain applied to a post supporting a weight imposed upon it (Fig. 36), we have an instance of an essay to shorten the fibres of which the timber is composed. The strength of the timber in this case is termed the resistance to compression. In the strain on a piece of timber like a king-post or suspending piece (A, Fig. 37), we have an instance of an essay to extend or lengthen the fibres of the material. The strength here exhibited is termed the resistance to tension. When a piece of timber is strained like a floor-beam or any horizontal piece carrying a load (Fig. 38). we have an instance in which the two strains of compression and tension are both brought into action; the fibres of the upper portion of the beam being compressed, and those of the under part being stretched.
This kind of strength of timber is termed resistance to cross-strains. In each of these three kinds of strain to which timber is subjected, the power of resistance is in a measure due to the lateral adhesion of the fibres, not so much perhaps in the simple tensile strain, yet to a considerable degree in the compressive and cross strains. But the power of timber, by which it resists a pressure acting compressively in the direction of the length of the fibres, tending to separate the timber by splitting off a part, as in the case of the end of a tie-beam, against which the foot of the rafter presses, is wholly due to the lateral adhesion of the fibres.