[Illustration:  FIG. 2.—­Compression across the grain.]

The first effect of compression across the grain is to compact the fibres, the load gradually but irregularly increasing as the density of the material is increased.  If the specimen lies on a flat surface and the load is applied to only a portion of the upper area, the bearing plate indents the wood, crushing the upper fibres without affecting the lower part. (See Fig. 3.) As the load increases the projecting ends sometimes split horizontally. (See Fig. 4.) The irregularities in the load are due to the fact that the fibres collapse a few at a time, beginning with those with the thinnest walls.  The projection of the ends increases the strength of the material directly beneath the compressing weight by introducing a beam action which helps support the load.  This influence is exerted for a short distance only.

[Illustration:  FIG. 3.—­Side view of failures in compression across the grain, showing crushing of blocks under bearing plate.  Specimen at right shows splitting at ends.]

[Illustration:  FIG. 4.—­End view of failures in compression across the grain, showing splitting of the ends of the test specimens.]

When wood is used for columns, props, posts, and spokes, the weight of the load tends to shorten the material endwise.  This is endwise compression, or compression parallel to the grain.  In the case of long columns, that is, pieces in which the length is very great compared with their diameter, the failure is by sidewise bending or flexure, instead of by crushing or splitting. (See Fig. 5.) A familiar instance of this action is afforded by a flexible walking-stick.  If downward pressure is exerted with the hand on the upper end of the stick placed vertically on the floor, it will be noted that a definite amount of force must be applied in each instance before decided flexure takes place.  After this point is reached a very slight increase of pressure very largely increases the deflection, thus obtaining so great a leverage about the middle section as to cause rupture.

[Illustration:  FIG. 5.—­Testing a buggy spoke in endwise compression, illustrating the failure by sidewise bending of a long column fixed only at the lower end. Photo by U. S. Forest Service]

The lateral bending of a column produces a combination of bending with compressive stress over the section, the compressive stress being maximum at the section of greatest deflection on the concave side.  The convex surface is under tension, as in an ordinary beam test. (See Fig. 6.) If the same stick is braced in such a way that flexure is prevented, its supporting strength is increased enormously, since the compressive stress acts uniformly over the section, and failure is by crushing or splitting, as in small blocks.  In all columns free to bend in any direction the deflection will be seen in the direction in which the column is least stiff.  This sidewise bending can be overcome by making pillars and columns thicker in the middle than at the ends, and by bracing studding, props, and compression members of trusses.  The strength of a column also depends to a considerable extent upon whether the ends are free to turn or are fixed.