62 TECHNICAL PROPERTIES OF WOOD. 



frequently, however, and especially iu the case of simple heart- 

 shake, extend through the whole stem and into the boughs, as 

 frequently with aspen, and other poplars, elm, horse-chestnut. 



Heart-shake is commoner in the case of large trees than with 

 younger ones. In many species, as for instance, oaks and 

 sweet chestnut, heart-shake is present before the tree has 

 been felled, or sawn ; in other cases it is due to felling, or 

 happens after it has taken place, as in Scotch pine, beech, 

 hornbeam, silver-fir and spruce (being commoner with silver- 

 fir than spruce). It is then caused by a shock from another 

 tree, wind, or the action of felling with the saw, for experience 

 has shown that this defect is more frequent with sawn trees than 

 ■tthen they are felled with the axe. 



The cause of this defect is often shrinkage of the wood, and 

 it is more frequent the thicker the stem and the drier the heart- 

 wood when compared with the sapwood, comparative dryness 

 of the heart of the tree causing radial cracks. Injected round 

 timber, which has been exposed to strong pressure at the trans- 

 verse section, is also very subject to heurt-shake. 



The only means of protecting heartshaken wood from splitting 

 further, consist in slowly drying the afiected wood ; hence 

 trees felled in Avinter are less subject to this defect than 

 summer-felled wood. "Wood intended for water-pipes is protected 

 from heart-shake by boring it out as soon as it has been felled. 



Simple heart-shake does not render timber unfit to be sawn 

 into planks and scantling, provided the axis of the tree is fairly 

 straight and the cracked part can be removed by two cuts of 

 the saw ; timber with star-shake can scarcely be used in this 

 Avay, even when only a few large cracks occur. 



(b) Frost-cracks. — These arc long radial clefts in the wood 

 running up the stem, which originate in the bark and penetrate 

 more or less deeply into the supwood and heartwood ; they often 

 split the stem from a good height, down to the roots, or the 

 reverse. They are caused by circumferential shrinkage, generally 

 due to sudden extremes of cold, as explained in detail under 

 Forest Protection.* 



The crack closes again, as the temperature rises and becomes 

 occluded by a new growth of wood, especially if it is not deep ; 

 * Vide Muiiual df Forestry, vol. iv. p. 435. 



