Defects of Timber. 



143 



73. Heart-Shake. 



If, on the contrary, a plank be laid flatwise, with the hollow of the 

 grain up, the pressure of whatever weight is applied, will be on the 

 inside of the arch, and if exposed to moisture, the water will tend 

 to settle between the layers of growth, and thus hasten its decay. 

 These principles find application in the laying of planks in side- 

 walks, bridges, platforms, the floors of warehouses, and the like. 



Defects of Timber from Various Causes. 



578. Timber is liable to acquire certain defects from old age, 

 shrinkage in seasoning, or other causes, 



that more or less injure its quality for 

 strength, and its value for use, when 

 sawn into lumber. One of these is the 

 " heart-shake," perhaps most frequently 

 seen in old timber, and caused by the 

 contraction of the inner portions of the 

 tree, as the first symptom of decay. 



579. The "star-shake" consists of a 

 similar contraction, but often wider to- 

 wards the outside, as seen in young tim- 

 ber when exposed to rapid drying after 

 being cut green. It, however, is often 

 seen in the tree before cutting, and some- 

 times a bulge on the outside of the tree 

 shows where these cracks come to the 

 surface. 



580. The "cup-shake" is a separation 

 between the rings of growth, more fre- 

 quently found near the roots of the tree. 

 It may be caused by a sudden change of 

 temperature, frosts, storms, or other 

 causes, and in some cases it extends 

 through the whole length of the tree, es- 

 pecially if the ring of separation is com- 

 plete at the base. If only a segment is 

 separate, it does not usually extend far 

 up. Again, there are various combina- 

 tions of these defects that may impair the 



value of timber, against which there is 76 ' CombinMRtw and Cup- 



74. Star-Shake. 



75. Cup-Shake. 



