The same ingenious author has laid down as a practical rule, that a cylinder of one inch diameter will carry, when loaded to one-fourth of its absolute strength, as follows :—iron, 135 cwt ; good rope, 22 cwt.; oak, 14 cwt ; and fir, 9 cwt. Parent has shown that the force required to crush a body is nearly equal to that which will tear it asunder. This may be an approximation to the truth as respects woods, but it will not apply to any other bodies. Glass, for instance, will carry a hundred times more on it than oak in this way, but will not bear suspended above four or five times as much. Oak will suspend a great deal more than fir, but fir will carry twice as much as a pillar. Some woods which are very soft, and consequently yield to pressure, possess very strong fibres, and will resist a longitudinal strain. The softness of texture is chiefly owing to the crooked nature of their fibres, and to the existence of considerable vacuities between each fibre, so that they are more easily bent in a lateral direction and crushed. In all cases where the fibres lie oblique to the strain,
the strength is considerably diminished, which may be ascribed to the circum stance, that the parts in such case slide on each other, and the connecting force of the cementing matter is for that reason easier overcome. The strain which most commonly acts on materials of any nature, is that which tends to break them in a transverse direction. For the results derived from experiment in strains of this nature, we must refer the reader to the article STRINOTH OF MATERIALS; but the complete investigation of the resistance of materials, according to the direction and situation of the forces applied, would require a volume. The reader who wishes for full information on this subject, cannot do better than consult Mr. Barlow's valuable Essay on the Strength and Stress of ?other. Some excellent practical rules on this important subject will, however, be found in The New Practical Builder, by Nicholson, and in the works of Banks, Emerson, and Roberson.