ACCIDENTS DURING GROWTH 157 



especially as aU shakes become more pronounced 

 during drying, owing to the fact that the outer 

 parts of the wood dry more rapidly than the 

 inner. In certain cases, especially in heart 

 shake, the defect may be so pronounced that it 

 may be detected by an examination of the bark 

 before the tree is felled. 



Certain coniferous timbers are very subject 

 to Resin or Pitch Pockets (Fig. 77). These 

 pockets are merely accumulations of resin of 

 such an extent as to form cavities of various 

 sizes. As a rule they are more or less linear in 

 shape and, unless of large dimensions, do not 

 materially weaken the timber, though they spoil 

 its appearance. It is impossible to indicate the 

 position of a pitch pocket till it is exposed in 

 working. 



One of the commonest timber defects is caused 

 by knots, which produce "local cross grain." 

 A knot is really a portion of a branch which 

 passes through the tissues of the stem. When 

 the branch dies the knot becomes merely " a 

 peg in a hole, and hkely to drop out after the 

 tree has been sawed into lumber." Knots of 

 large size or number seriously weaken wood, 

 moreover they render it exceedingly difficult to 

 work. Small knots are not nearly so injurious, 

 in fact they may, in certain cases, strengthen the 

 wood. 



