DURABILITY. 79 



A uniform appearance of the stem, in its shape and bark, is a 

 good sign ; abrupt swellings in the lower part of the stem 

 are bad signs (fig. 21), and usually indicate root-rot in the 

 case of spruce. Local differences in the texture of the bark, 

 extensive splitting, blistering, or conspicuous smoothness of 

 bark, are bad signs. The presence of knots, fungi, frost- 

 cracks and cankers, the exudation of rotten sap from wounds, 

 entrance of ants, beetles, mice or weasels into holes among 

 the roots, the tapping of wood-peckers or of nut-hatches — all 

 these are signs of decay in a tree. 



iv. Market Value of Unsound TI^uoJ. 



Oar object must be to use only sound wood as timber, but 

 pieces which are only slightly affected by disease may still be 

 used for certain purposes. The oak and some other species are 

 seldom entirely free from some defect or other, and these may 

 be insufficient to prevent its use, but in all such cases the 

 wood must be at once throughly dried, and only used in dry 

 places under cover. If then the fungi which are causing the 

 decay should be killed by drying, and the wood has not become 

 too weak for the purpose in view, there need be no fear of any 

 increase in the decay, and the wood may still be serviceable. 



It is clear that w'henever the timber-market is well supplied, 

 there will be a much greater difficulty in disposing of doubtful 

 material than when the supply is small, and the demand 

 great. 



Section XII. — Dukability. 

 1. General Account. 



Durability is a measure of the duration of time during which 

 wood utilized in a certain way will last in a sound and useful con- 

 dition. This is the most important quality for timber, the first 

 question to be asked as regards any wood being whether, or not, 

 it is durable. 



"When a wood no longer forms part of a living tree, like all 

 organic bodies after a longer or shorter interval of time, it 

 becomes subject to gradual decomposition, as the substances of 

 which it is formed tend to revert directly, or indirectly, to the 

 air and soil from which they were originally taken. 



