STRENGTH. 89 



section, while the force that can stretch such a rod to double 

 its length, if this were possible, within the limits of its elas- 

 ticity, is termed the modulus of tension. [We may however 

 say, with a greater regard to possibility, the modulus of 

 tension is one million times the force, which would stretch 

 the bar to a million times its length. Tr.] These and other 

 strength coefficients are given in kilograms per square centi- 

 meter, and as the atmospheric pressure on a square centi- 

 meter is very nearly a kilogram, the force is frequently 

 represented in atmospheric pressures (at). 



In the case of resistance to crushing, the force acts in the 

 opposite direction to a tensile strain, and the coefficient and 

 modulus are determined analogously. Resistance to torsion 

 is that offered by the fibres of a rod, the axis of which is fixed, 

 to a couple of forces tending to turn it on its axis. Resistance 

 to sheering is that offered by wood to a force which tends to 

 make sections of the wood slide on one another. 



[None of the above strains (forces) or stresses (effects on 

 the wood) are of much practical importance. In contrast 

 with iron, fracture of wood by tension acts suddenly, there 

 being little extensibility along its fibres. If a piece of wood 

 be fastened at both its ends and then subject to a load in the 

 middle, it may bo bent and the fibres on the convex side are 

 stretched, but the consideration of such a strain practically 

 comes under the heading of transvt ngth. Crushing 



strains come into play, when wood is used for vertical piles, or 

 posts, mining props, wheel-spokes, etc. Over-weighted wooden 

 pillars bend and break transversely. 



AYheel-spnkes are subject to crushing strains and the best 

 woods for the purpose are robinia and oak ; the former wood is 

 now coming into use for the spokes of motor-cars, where the 

 pressure on the outside; wheels in going rapidly round curves 

 is very great. In using peeled oak coppice-shoots for pit- 

 props it is essential that the horizontal cut made at the base 

 of the shoot in order to strip off the bark should sever none of 

 the wood-fibres, as any such cut very greatly diminishes the 

 strength of the prop. 



Mathey (op. cit., p. 7) goes into detail on the question of 

 the strength of pit-props, which were tested in 1891 18 ( J5 by 



