156 TIMBEES AND THEIE USES 



practical importance of all forms of shake, as 

 they do not at first involve any decay, and con- 

 sequently do not much interfere with the em- 

 ployment of the logs in bulk. Heart shake, 

 however, is probably in itself an indication of 

 that incipient decay that comes when timber 

 has passed its maturity and the older layers 

 shrink more than the outer." 



Star shakes (Fig. 81), as their name implies, are 

 sphts which radiate from the centre of the tree 

 outwards, so as to form a star. Sometimes they 

 extend but a short distance from the centre, 

 again they may extend right across the tree. 



Ring shakes arise when the annual rings of 

 the wood fail to adhere to one another, with the 

 result that circular fissures, parallel to the bark, 

 occur in the wood. It is probable that all cases 

 of ring shake are due to external causes, and 

 various theories have been propounded to account 

 for them. The complete defoliation of the tree 

 by insects may cause such a check to growth 

 that ring shakes are induced. Sudden and 

 severe frosts are said to account for the defect, 

 and in the tropics the intense heat may be the 

 cause. Another theory is that the swaying of 

 the stems in all directions in the wind induces 

 ring shakes. 



Whatever form a shake may take it is likely 

 to affect the conversion into plank very seriously. 



