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 in- 

 variably depends upon a combination 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 spht 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 excep- 

 tions, where testing would prove what no rule-of-thumb is likely 

 to perceive ; and it was in reference to this that Tredgold remarked 

 that actual testing may take the place of a life-time of practical 

 experience in carpentry. 



In the scientific testing of timber each property is examined 

 separately. A beam resists bending, and is accordingly termed 

 stiff ; wicker bends readily, or is flexible ; while the rod or beam 

 that straightens itself again on the removal of a load that has been 

 applied to it is termed elastic. Resistance to a pull in the direction 

 of the grain is known as tensile strength ; whilst a force appHed in 

 an opposite but parallel direction is a crushing force. The pressure 

 of a hammer-head across the grain of the handle tends to shear the 

 fibres, and a nail entering a board tests its cleavability or tendency 

 to split. 



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