48 \MJOD AND FOREST. 



the ijith along the pith rays and widen outward; lirari sluihes. Fig. 

 41, h, splits crossing the centi'al rings and widening toward the cen- 

 ter; and cup or r'uuj .shakes. Fig. 41, c, splits between the annual 

 rings. Honetjcomhing, Fig. 41, (/, is splitting along the pitli rays 

 and is due largely to case hardening. 



These are not all due to shrinkage in drying, but may occur in 

 the growing tree from various harmful causes. See p. 232. 



Wood that has once been dried may again be swelled to nearly if 

 not fully its original size, l)y being soaked in water or subjected to 

 wet steam. This fact is taken advantage of in wetting wooden 

 wedges to split some kinds of soft stone. The processes of shrinking 

 and swelling can be repeated indefinitely, and no temperature short 

 of burning, completely prevents wood from shrinking and swelling. 



Eajiid drying of wood tends to "case harden" it. i. c, to dry and 

 shrink the outer part before the inside has had a chance to do the 

 same. This results in checking separately both the outside and the 

 inside, hence special jtrecautions need to be taken in the seasoning 

 of wood to prevent this. When wood is once thoroly l.ient out of 

 shape in shrinking, it is very difficult to straighten it again. 



Woods vary considerably in flie amounts of their shrinkage. The 

 conifers with their regular structure shrink less and shrink more 

 evenly than the broad-leaved woods.' Wood, even after it has been 

 well seasoned, is subject to frequent changes in volume due to the 

 varying amount of moisture in the atmosphere. This involves con- 

 stant care in handling it and wisdom in its use. These matters are 

 considered in Handwork in Wood, Chapter III, on the Seasoning of 

 Wood. 



'The followiiiK tal)le from Kotli, p. 37, nives tlie aiiproximate shrinkage 

 of a board, or set of boards, 100 iiiclies wide, drying in the open air: 



Shrinlvage 

 Inches. 



1. All light conifers (soft pine, spruce, cedar, cypress) 3 



2. Heaxf>' conifers (hard pine, tamarack, yew) honey locust, box eider', 



wood of old oaks ) ' ' ' , 



3. Ash, elm, walnut, poplar, maple, beech, sycamore, cherrv , black locust! .3 



4. Bas,swood, liirch, chestnut, horse chestnut, blue beech, young locust.... G 



5. Hickory, young oak, especially red oak '. . . .'i^ . . . . Up to 10 



Tlie figures are the average of radial and tangential shrinkaoes. 



