DENSITY, FUEL VALUE AND STRENGTH 115 



or volatile substance of all woods is the same. Though resinous 

 woods give off more than 12 per cent, more heat on burning than do 

 non-resinous woods, at least this amount is lost in the case of the 

 former in the form of unconsumed carbon in the smoke. The 

 amount of heat obtained is, in fact, very nearly in direct proportion 

 to the specific gravity, %,€>, the heavier the wood the greater the 

 amount of heat obtained. Taking as the unit of fuel value an 

 imaginary wood with no ash and a specific gravity of 1, the relative 

 fuel value of 430 woods examined varied from 0*248 in Yucca to 

 1-194 in Black Ironwood {Conddlia ferrea). Taking as a unit of 

 heat the amount necessary to raise 1 cubic decimetre or 1 kilogram 

 of water 1"^ C, 4,000 units will be produced by burning a kilogram 

 of dry wood, i.e, the relative fuel value of any wood multiplied by 

 4,000 will give approximately the amount of heat obtained by 

 burning a cubic decimetre of it. 



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 compo- 

 sition : 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 continuous, or in parallel though opposite 

 directions, may be considered as identical, or rather as differing 

 only in mathematical sign (4-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 milHmetre, or per square centimetre.^ 



Broadly speaking, the strength of timber increases with its 

 heaviness. More accurately, the greater the density or weight the 

 greater the resistance to compressive strain. Density is no criterion 

 as to tenacity or tensile strength. The most valuable timbers for 

 structural purposes are those which have considerable strength 

 without excessive heaviness, as is the case with Pine. 



In 1676 Robert Hooke enunciated the law that (using modern 

 terminology) within the Hmits 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 



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

 that 1 ton, or 2,240 lbs. per sc[. inch =1 '511 kilos, per sq. mm., or 151 '1 kilos, per 

 sq centim. ; whilst 1 kilo, per sq. centim. = 14*22 lbs. per sq. inch. 



8—2 



