-36 TECHNICAL PROPERTIES OF WOOD. 



an instrument, without being torn, and close-in towards the 

 neighbouring fibres, thus rendering the wood locally denser for 

 the time being. Porous woods, such as black poplar, aspen, 

 willows, &:c. chiefly share in this peculiarity, for their wood 

 afl'ords the largest spaces into which the fibres can be com- 

 pressed. This pliability of the fibres is chiefly noticed at right 

 angles to their greatest length, or transversely. 



As a rule, in case there be no great difference in density 

 between heartwood and sapwood, the heartwood, imperfect 

 heartwood and older parts of trees, owing to their comparative 

 dryness, are harder than the corresponding sapwood and the 

 younger parts of the tree. This is the case only when the 

 wood is sound, for even a beginning of decay in old trees will 

 render the heartwood soft. 



5. Action of Instruments. 



It is chiefly iron or steel instruments which have to penetrate 

 the substance of wood, and their shape and action on wood is 

 very variable, as may be seen from the following list : — auger, 

 file, plane, saw, chisel, knife, graving-tool, polishing-stone, 

 &c. No proof is needed to show that the resistance wood offers 

 to a tool varies considerably according to the kind of tool used. 

 It is often, for instance, difficult to drive a nail into a narrow- 

 zoned larch post which has stood long exposed to wind and rain, 

 but whicli can be easily sawn in two. In order then to know 

 the hardness of wood in all directions, it must be considered 

 with reference to its resistance to the instrument used. It is 

 therefore impossible to distinguish wood by absolute degrees 

 of hardness. The forester is chiefly concerned with its re- 

 sistance to the axe, saw, pruning knife and bill-hook. 



(a) The resistance a wood offers to the axe varies with the 

 direction in which the wood is attacked, and is greatest perpen- 

 dicular to its fibres and least in the plane of the medullary rays. 

 By hardness in this case is understood the resistance the fibres 

 offer to a cut with the axe more or less at right angles to them. 

 The density of the wood, the pliability of its fibres and its 

 degree of moisture will influence matters. It is evident, however, 

 that softwoods with pliable fibres require heavier axes than 



