APTITUDE FOR BEING WORKED. 113 



limits of arborescent vegetation the wood has annual zones 

 less than 3 mm. ; it is light and soft, but extremely regular 

 in growth, and yields timber that is best for carving or for 

 musical instruments. Tr.] 



It stands to reason that hardness is an obstacle to cutting, 

 and as hardness has already been shown to depend on specific 

 weight, the scale of specific weights (p. 64) affords a scale 

 of hardness. 



Moisture in wood facilitates the work of cutting hardwoods, 

 but in softwoods it renders it more difficult (cf. p. 48). 



Toughness heightens the labour, but softness assists it. 

 The soft, unelastic wood of Weymouth-pine is easy and smooth 

 to work; in this it surpasses all the other Abietuu-ac. Only 

 species of Chamaecyparis have similar soft and unelastic wood, 

 and therefore are esteemed highly by joiners and cabinet- 

 makers abroad. 



Regularity in the structure of the annual zones, straight- 

 ness of bole and vertical direction of the fibres are favour- 

 able conditions for the easy working of wood by cutting 

 implements. All knots, wavy or twisted fibres, or burrs 

 increase the labour of cutting and planing, often more than 

 does the extent of surface of the wood, for the chisel cuts 

 irregularly into such wood, and the piano has constantly to 

 be turned round while planing unevenly grained wood. 



The resistance wood offers to a saw differs considerably 

 from that offered to cutting implements. The edges of the 

 teeth are set in two parallel lines, each tooth with one or two 

 cutting edges according to the nature of the saw. The teeth first 

 scratch the fibres slightly, the second stroke penetrating more 

 deeply and tearing from its basis the wood left between the 

 two scratches made by the first stroke. Sawing is easiest 

 across the fibres of the wood, when the saw is applied first to 

 a radial section ; it is more difficult when a tangential section 

 is attacked, but most difficult of all in the direction of the 

 fibres, as when planks are sawn out of a log. The teeth must 

 then cut, more or less, through the whole length of each fibre 

 that it meets, instead of merely across the fibres (as with a 

 cross-cut saw), and must also tear adjacent fibres apart. For 

 such work, large teeth with a wide set are necessary. 



F.U. i 



