EFFECT OF TANGENTIAL SHRINKAGE. 



35 



shrinking as much as they otherwise would. Thus the structure is 

 subjected to two severe strains at right angles to each other, and herein 

 lies the greatest difficulty of wood seasoning, for whenever the wood 

 dries rapidly these fibers have not the chance to "give" or accommo- 

 date themselves, and hence fibers and pith rays separate and checks 

 result which, whether visible or not, are detrimental in the use of the 

 wood. 



The contraction of the pith rays parallel to the length of the board is 

 probably one of the causes of the small amount of longitudinal shrink- 

 age which has been observed in boards. 1 The smaller shrinkage of the 

 pith rays along the radius of the log (the length of the pith ray) oppos- 

 ing the shrinkage of the fibers in this direction becomes one of the 

 causes of the second great trouble in wood seasoning, namely, the dif- 

 ference in the amount of the shrinkage along the radius 

 and that along the rings or tangent. 



This greater tangential shrinkage appears to be due, 

 in part, to the cause just mentioned, but also to the 

 fact that the greatly shrinking bands of summer 

 wood are interrupted, along the radius, by as many 

 bands of porous spring wood, while they are contin- 

 uous in the tangential direction. In this direction, 

 therefore, each such band tends to shrink, as if the 

 entire piece were composed of summer wood, and since 

 the summer wood represents the greater part of the 

 wood substance, this tendency of greater tangential 

 shrinkage prevails. 



The effect of this greater tangential shrinkage 

 affects every phase of woodworking. It leads to per- 

 manent checks, and causes the log to split open on 

 drying. 



Sawed in two, the flat sides of the log become con- 

 vex, as in fig. 23; sawed into a timber, it checks along 

 the median line of the four faces, and if converted 

 into boards, the latter take on the forms shown in fig. 23, all owing to 

 the greater tangential shrinkage of the wood. 



Briefly, then, shrinkage of wood is due to the fact that the cell walls 

 grow thinner on drying. The thicker cell walls and therefore the 

 heavier wood shrinks most, while the water in the cell cavities does not 

 influence the volume of the wood. Owing to the great difference of 

 cells in shape, size, and thickness of walls, and still more in their 

 arrangement, shrinkage is not uniform in any kind of wood. This 

 irregularity produces strains, which grow with the difference between 



Fig. 23.— E fleets of 

 shrinkage. 



'In addition to this all fibers having an oblique position, as those at pith rays and 

 knots, also the oblique, tapering ends of all fibers contribute to this longitudinal 

 shrinkage, since one component of their normal shrinkage is longitudinal. 



