130 OF WOOD IN GENERAL. 



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. Eesistance to a pull in the 

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

 applied 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. 



The results of the many tests that have been published are 

 often widely discordant. This arises from various causes, e.g. 

 incorrect identification of the species, nature of the locality where 

 the tree was grown, the age of the tree when felled, the part of 

 the tree from which the test specimen was taken, the extent to 

 which it was seasoned, the size of the piece tested, and the method 

 of stating the experimental results. The use of popular names, 

 such as Ironbark or Blue Gum, each applicable to half-a dozen 

 different species, is an obvious source of error. As we have 

 already seen, the same species grown under different conditions of 

 heat, moisture, etc., varies widely in rate of growth, and accord- 

 ingly in strength also. Timber is at its best when the 

 tree is at its maturity, an age which depends upon the species, 

 the climate and the soil. Before that age not only does the less 

 durable sapwood predominate, but the heartwood has not yet 

 reached its full strength ; whilst after maturity the heartwood is 

 the first to show symptoms of weakness. As we have already 

 seen, the centre, with its many knots, is generally the weakest 

 part of the heartwood, and a scantling will have greater transverse 

 strength, or resistance to bending stress, and tensile strength in 

 proportion to the number of rings that occur both at its butt and 

 its top. Seasoning, as we have seen, may double the strength of 

 timber. Early experiments on the strength of timber were 

 generally made with very small pieces owing to the difficulty of 



