The tension or pulling strength of wood is much affected by the direction of the grain, a cross-grained piece being only 1/10th to 1/20th as strong as a straight-grained piece. But under compression there is not much difference; so that if a timber is to be subjected to cross strain, that is the lower half under tension and the upper half under compression, a knot or other cross-grained portion should be in the upper half. [Illustration: Fig. 43. Shearing Strength is Measured by the Adhesion of the Portion A, B, C, D or to the Wood on both sides of it.] Strength also includes the ability to resist shear. This is called "_shearing strength_." It is a measure of the adhesion of one part of the wood to an adjoining part. Shearing is what takes place when the portion of wood beyond a mortise near the end of a timber, A B C D, Fig. 43, is forced out by the tenon. In this case it would be shearing along the grain, sometimes called detrusion. The resistance of the portion A B C D, _i.e._, its power of adhesion to the wood adjacent to it on both sides, is its shearing strength. If the mortised piece were forced downward until it broke off the tenon at the shoulder, that would be shearing across the grain. The shearing resistance either with or across the grain is small compared with tension and compression. Green wood shears much more easily than dry, because moisture softens the wood and this reduces the adhesion of the fibers to each other.[7] CLEAVABILITY OF WOOD. Closely connected with shearing strength is cohesion, a property usually considered under the name of its opposite, cleavability, _i.e._, the ease of splitting. When an ax is stuck into the end of a piece of wood, the wood splits in advance of the ax edge. See _Handwork in Wood_, Fig. 59, p. 52. The wood is not cut but pulled across the grain just as truly as if one edge were held and a weight were attached to the other edge and it were torn apart by tension. The length of the cleft ahead of the blade is determined by the elasticity of the wood. The longer the cleft, the easier to split. Elasticity helps splitting, and shearing strength and hardness hinder it. A normal piece of wood splits easily along two surfaces, (1) along any radial plane, principally because of the presence of the pith rays, and, in regular grained wood like pine, because the cells are radially regular; and (2) along the annual rings, because the spring-wood separates easily from the next ring of summer-wood. Of the two, radial cleavage is 50 to 100 per cent. easier. Straight-grained wood is much easier to split than cross-grained wood in which the fibers are interlaced, and soft wood, provided it is elastic, splits easier than hard. Woods with sharp contrast between spring and summer wood, like yellow pine and chestnut, split very easily tangentially.