TESTING WOOD 113 



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 locahty 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 apphcable 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 accordingly in strength also. 

 Timber is at its best when the tree is at its maturity, an age which 

 depends upon the species, the chmate and the soil. Before that age 

 not only does the less durable sapwood predominate, but the heart- 

 wood 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 scanthng 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 difl&culty 

 of holding and bringing strains to bear upon large scantHngs. 

 Pieces less than a quarter of an inch square were often used. Such 

 pieces might give an unduly unfavourable result from the cutting 

 across of individual fibres ; or, on the other hand, being freer from 

 knots or other defects, more readily seasoned throughout and more 

 homogeneous, they are rather picked than average samples, and 

 may give an unduly favourable result. 



In an excellent series of tests carried out for the Forest Depart- 

 ment of the United States Board of Agriculture by Professor 

 Johnson in 1891-92, an accurate record was kept, when each tree 

 was felled, as to the condition of the soil, the chmate, the size, age 

 and growth of the tree and the date of felling. 



Density. — ^We can only give here a bare outHne of the principles, 

 methods, and results of testing. Much, as we have seen, depends 

 upon density ; and, admitting that, owing to air or moisture in the 

 wood, the results are not as satisfactory as could be wished, we 

 have two simple methods, described by Professor Unwin, for 

 determining this character, viz. (i) by measuring and weighing 

 planed rectangular blocks, and (ii) by weighing the block and the 

 water it displaces. In the former method, if & — the breadth, 

 ^=the thickness, and A=the height of the block in inches, its 



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