STRENGTH. 47 



The different kinds of strength in timber will now be con- 

 sidered first, then the factors which modify thern, as far as 

 these are known, and finally the comparative strength of the 

 different woods. 



The following kinds of strength will be discussed seriatim : — 

 tenacity, and resistance to crushing, torsion, shearing and trans- 

 verse straining actions. 



2. Tniadty. 

 The tenacity of all wood is greatest along its grain, attain- 

 ing 1,500 atmospheres and more, but may also sink as low as 

 one-fifth or one-sixth of this amount. Bauschinger's experiments 

 made in 1882, in the laboratory of the Engineering College at 

 Munich, appear to prove that this form of strength is proportional 

 to the specific gravity of the wood : it is, however, rare for 

 wood to be exposed to considerable tension, on account of the 

 difficulty of getting secure attachments to the ends of the 

 piece. In contrast with iron, fracture of wood by tension acts 

 suddenly, there being only a very small amount of extensibility 

 along its fibres. 



3. licsistancc to Crushing. 



This is opposite to the resistance oSered to tension, but also 

 along the grain of the wood, and comes into play when wood is 

 used for vertical piles, posts, wheel-spokes, and so on. Resist- 

 ance of coniferous wood to crushing is 150 — 300 atmospheres. 

 It also appears to be proportional to the specific gravity of 

 wood. Overweighted pillars bend and break transversely. 



4. licsistancc to Torsion. 



This is the resistance offered by the fibres of a fixed piece of 

 wood to a couple of forces tending to twist one end relatively 

 to the other. The windlass is about the only case in which 

 torsion has to be resisted, and the dimensions of wooden Avind- 

 lasses (as for wells) usually provide a large margin for safety. 



Resistance to Shearing. 

 This is a measure of the strength which resists the separation 

 of the fibres sidewavs from one another. 



