DEFECTS AND UNSOirKDNBBe. 



33 



course of the shake. In extreme cases the rupture in the formation 

 of the shake may be so violent as to extend to the centre of the 

 trunk. Strong .-winds may also cause the shake to extend thus. 

 JTrom what precedes we are able to understand why radial- shakes 

 affect trees of large rather than small girth, solitary trees rather 

 than those standing in the midst of a leaf-canopy, portions of a tree 

 where the wood is not of uniform structure (the foot, vicinity of a 

 large knot, &c.) rather than other parts ; also why a wet soil and 

 the possession of easily fissile wood and thick medullary plates 

 favour the occurrence of such shakes. The utility of a log affected 

 with radial-shakes will depend on the number and continuity of 

 the shakes, on whether most of them have healed over, and on 

 whether decay has made any progress along the sides of the cracks.. 

 In some cases the log may be completely ruined for timber pur- 

 poses, in others beams and even smaller scantlings may be sawn 

 out of them. 



C. Cup-shakes. — In a cup-shake the crack follows the Hn^ be- 

 tween two adjacent concentric zones of growth, and it may do go 



Fig. 3. 



Cfiip-Shalte. 



a. Complete. i. Partial. e. Partial, accompanied 



with rot. d. Combined with a radial-shake. 



for any distance, from a few inches to the entire length of the 

 circumference. The cause of separation may be (a) excessive 

 expansion by frost of one or more of the outer zones, so that they 

 can no longer fit tight over the enclosed solid cylinder of wood, or 

 (b) violent swaying or bending of the tree, so that the limit up to 



