296 TIMBERS AND THEIR USES 



air, a fact which imparts buoyancy to the wood. 

 The density of a sample of wood is merely its 

 weight per unit volume, whether expressed in 

 lbs. per cubic foot or grammes per cubic centi- 

 metre. Relative density, more frequently known 

 as specific gravity, is the ratio of the density to 

 that of distilled water at 4° C. Seeing that a 

 cubic foot of distilled water at 4° C. weighs 

 62-43 lbs.," the specific gravity of any substance = 



— — Where D = the density per cubic foot. 

 62-43. ^ ^ 



Or conversely, knowing the specific gravity, it 



is easy to find the weight per cubic foot, thus 



S X 62-43, where S =the specific gravity. To find 



the dry weight per cubic foot, use the formula 



W(l — M) lbs., where W = total weight per cubic 



foot and M = moisture per cent of total weight. 



It has been stated that the weight and density 



of a seasoned timber is to a certain extent a 



measure of its strength. ... It is more nearly 



correct, however, to state that the greater the 



density and therefore the weight, the greater 



is the strength to resist compressive strain, 



whether apphed edgewise or crosswise. It has 



been found that the density is no criterion as 



to the " Tenacity " or tensile strength of the 



material, and hence, therefore, affords no guide 



as to the relative strength of beams which largely 



involves the tensile strength of the timbers. 



