CHAPTER VII. 

 TESTING WOOD. 



THE very general substitution of iron or steel for wood in per- 

 manent structures renders the exact investigation of the strength 

 of timber less important now than formerly. Nevertheless, in 

 merely temporary structures, such as scaffoldings or centerings, 

 its power of withstanding different strains is of very serious 

 concern. Practically, although not scientifically, every joist, 

 rafter, window- sash or door-frame, the chair we sit on, the floor 

 we walk on, the wood of the cart or boat we ride in, are all 

 tested as to their strength, their elasticity, their hardness and 

 their toughness. In the workshop it is recognized that the fitness 

 of a wood for a given purpose invariably depends upon a combina- 

 tion of several qualities. A spoke, for instance, must not only be 

 strong, it must be stiff to keep its shape, tough and hard; 

 and accordingly it must be made from wood split with the 

 grain, and not from sawn or cut material. The experienced 

 wood-worker judges the suitability of any particular piece 

 of wood for his purpose by rule-of-thumb. This rule-of- 

 thumb guess is largely based on the general rule that, in timber, 

 weight, hardness, and power of resistance to most strains, vary 

 together. To this rule, however, there are many important 

 exceptions, where testing would prove what no rule-of-thumb 

 is likely to perceive ; and it was in reference to this that Tredgold 



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