Materials used in construction are constituted in their structure either of fibres (threads) or of grains, and are termed, the former fibrous, the latter granular. All woods and wrought metals are fibrous, while cast iron, stone, glass, etc., are granular. The strength of a granular material lies in the power of attraction acting among the grains of matter of which the material is composed, by which it resists any attempt to separate its grains or particles of matter. A fibre of wood or of wrought metal has a strength by which it resists being compressed or shortened, and finally crushed; also a strength by which it resists being extended or made longer, and finally sundered. There is another kind of strength in a fibrous material: it is the adhesion of one fibre to another along their sides, or the lateral adhesion of the fibres.