274 THE UTILISATION OF WOODLAND PKODUCE. 



or less parallel to the grain so as to displace and separate the 

 fibres sideways (shearing). The breaking-strain is by far the 

 most important in timber used for construction, Of our British 

 woods, Oak, Ash, and Larch stand this pressure best ; then 

 other hardwoods ; and sapwoods and very resinous Conifer 

 timber least (though Spruce better than Silver Fir, and Silver 

 Fir better than Scots Pine) ; but any defect or unsoundness 

 weakens the strength. The coefficient of transverse strength 

 can be obtained from the formula 



WxL 

 ~B7D* 



where W is the weight in pounds placed on middle of the bar 

 which causes it to break, L the length of bar in feet, between 

 supports, B the breadth, and D the depth in inches. 



2. ELASTICITY, FLEXIBILITY, AND TOUGHNESS. Any sub- 

 stance changing shape under pressure is called pliable, and its 

 power -of completely resuming its original shape is its elasticity ; 

 if it breaks readily it is called brittle, and if it resists change 

 of form, tough. In timber, elasticity is proportionate to 

 the. strength; while flexibility and toughness usually increase 

 with the amount of water, and are greater in green than in 

 partly or wholly seasoned wood, and also much increased by 

 steaming. Hardwoods have generally the greatest elasticity, 

 and softwoods the greatest flexibility and toughness ; while in 

 Conifers a moderate amount of resin increases, and much resin 

 diminishes them ; but all these three properties depend on the 

 length and straightness of the woody fibres, and are diminished 

 by branch -knots and abnormal growth of any kind. The 

 modulus of elasticity is found from the formula 



L 3 W 



E = x , 



B x D 8 d 



where L, B, and D are as above, and W is the weight in pounds 

 supported at the centre of the bar and causing a deflection of 



