DENSITY, FUEL VALUE AND STRENGTH. 133 



Strength. All measurements of the strengths of timbers are 

 determinations of their powers of resisting certain stresses, or 

 forces tending to produce strains, or changes of shape. It must 

 always be remembered that, unlike metals or many artificial 

 products, wood is not, and cannot be, considered as uniform in 

 structure and composition : it is not homogeneous or isotropic. 

 Stresses applied to it, and the resultant strains must, therefore, 

 be considered separately. Those stresses which are exerted in a 

 direction normal, or at right angles, to a cross-section or imaginary 

 surface of division are termed pushes or pulls, and being contin- 

 uous, or in parallel though opposite directions, may be considered 

 as identical, or rather as differing only in mathematical sign 

 ( + or - ). Those which are exerted at a tangent to such a cross- 

 section are termed shearing stresses. The intensity of a stress is 

 its amount per unit of surface, and, may, therefore, be expressed 

 in pounds or tons per square inch, or in kilograms per square 

 millimetre, or per square centimetre. 1 



In 1676 Robert Hooke enunciated the law that (using modern 

 terminology) within the limits of elasticity, or recovery from 

 strain when the stress is removed, strain is proportional to stress. 

 In accordance with Hooke's law, Thomas Young postulated the 

 modulus of longitudinal extensibility that bears his name. This 

 is generally called the modulus of elasticity, but incorrectly, since 

 there are others. It is constant for any material, being repre- 

 sented by the letter E, and is, in fact, the ratio of the intensity 

 of push or pull to longitudinal strain. Thus, if 1 = length and 

 81 = change of length under a stress p, then 81 : 1 : : p : E, or 



Obviously, a stress applied to a transverse section of wood will 

 have to break the fibres across, while one applied to a longitudinal 

 section tends to separate the fibres from each other. Thus the 



1 To facilitate the conversion of results thus variously stated, it may be 

 mentioned that 1 ton, or 2240 Ibs. per sq. inch = 1 '511 kilos, per sq. mm., 

 or 151 '1 kilos, per sq. centim.; whilst 1 kilo, per sq. centim. 14*22 Ibs. per 

 sq. inch. 



