TIMBER 



13 



the long diameter is in the direction of the longitudinal 

 axis of the tree, it will be understood why the shrinkage of 

 boards and timber generally is inappreciable in a long 

 length. Some of these cells have thick walls, others have 

 thin walls, and as both kinds are generally mixed the 

 shrinkage is greater in one case than in the other, causing 

 different strains to occur. Again, the cells forming the 

 medullary rays, which 

 constitute a consider- 

 able proportion of all 

 wood, have their length 

 at right angles to the 

 others (see Fig. 5) and 

 like the others shrink 

 most in the direction 

 of their breadth, so 

 that two severe strains 

 at right angles to each 

 other are set up, and, 

 with those previously 

 referred to, cause the 

 splitting and cracking 

 which occur when 

 timber is being sea- 

 soned. If done too 

 rapidly the cells have not time to " give " or adapt them- 

 selves to the altered circumstances ; they became distorted 

 or ruptured, and the wood is damaged. 



Shrinkage of wood then is caused by the walls of the 

 cells or pores becoming thinner, and as the thicker walled 

 cells are found in the harder woods, these as a rule shrink 

 and split the most. Wood shrinks or twists to an extent 

 dependent upon the quality and size of the timber ; this is 

 more evident if it be dried too rapidly, is more noticeable in 



FlG. 5. a, 1>, longitudinal cells or 

 wood fibres ; c, d, cells of medul- 

 lary ray. 



