FOREST UTILISATION 99 



the middle parts, but above that again, in the crown, the 

 weight increases. 



As a rule, the better the soil and situation, the heavier is 

 the timber of trees of like species and of similar age. 



Distinction is made in the condition of timber, in respect 

 of the moisture which it contains. It is said to be " green " 

 when, as in the case of freshly felled trees, half of its weight 

 is water ; " forest-dried " timber, which has long remained in 

 the wood, but still contains from twenty to thirty per cent, of 

 moisture ; while " air-dried " timber is that which has been 

 long stored under cover in a dry, airy place, and contains, on 

 an average, ten per cent, of water. 



Timbers are subject to SHRINKING, SWELLING, and WARPING. 

 Shrinkage takes place during the process of drying. As a 

 piece of timber parts with the water it contains, its volume 

 decreases ; the change is dependent on the species hard 

 woods shrinking more than soft woods. It does not occur to 

 the same extent in all directions, but is most marked along 

 the annual rings, and least along the grain, i.e. lengthwise 

 with the vessels and wood-fibres. When the reduction in 

 volume proceeds unevenly cracks are produced in the timber. 



If laid in water or in a damp atmosphere, wood absorbs 

 the moisture lost in the drying process and swells, gradually 

 regaining its original volume. Irregular shrinkage and swell- 

 ing, together with the not absolutely straight course of the 

 wood fibres, account for the warping of manufactured timber. 

 The more quickly a wood is dried, the faster does it shrink, 

 and the greater is the tendency to crack. 



Strength. Timber has to resist strain put upon it in 

 different ways, but it is most important practically that it 

 withstand the transverse straining action, as in beams and 

 rafters ; and also the crushing action, as when weight is put 

 upon upright posts or pillars. The heavier a wood is, the 

 greater, as a rule, is its strength. Unsoundness and the 

 presence of knots or irregularity of grain affect timber most 

 prejudicially as regards its strength. 



The order which the species take in respect to resistance 



