32 



TIMBER. 



IV— SHRINKAGE OF WOOD. 



When a short piece of wood fiber, such as that shown in fig. 18, A, is 

 dried it shrinks, its wall grows thinner (as indicated by dotted lines), 



its width, a b, the thickness of the 



b 



r^\ a 



Fig. 18— Short pieces of wood fibers, one thick, 

 the other thin -walled ; magnified. 



y 



fiber, becomes smaller, and the 

 cavity or opening larger, but, 

 strange to say, the height or length, 

 b c, remains the same. In a similar 

 piece of fiber with a thinner wall 

 (fig. 18, B) the effect is the same, 

 but the wall, being only half as 

 thick the total change is only about 

 half as great. 1 



If sections or pieces of fibers are 

 dried and then placed on moist 

 blotting paper, they will 

 take up water and swell to 

 their original size, though 

 the water has been taken 

 up only by their walls and 

 none has entered into their 

 openings or lumina. This 

 indicates that the water in the cavity or lumen of a fiber has 

 nothing to do with its dimensions, and that if the cell walls 



are saturated it makes no difference in 

 the volume of a block of pine wood 

 whether the cell cavities are empty as 

 in the heartwood or three-fourths filled 

 as in the sap wood. 



If an entire fiber, as shown in fig. 19, is dried, 

 the wall at its ends a and 6, like those of the 

 sides, grow thinner, and thereby the length of 

 the entire cell grows shorter. Since this length 

 is often a hundred or more times as great as 

 the diameter, the effect of this shrinkage is 

 inappreciable; and if a long board shrinks 

 lengthwise, it is largely due, as we shall see, to 

 quite another cause. 



A thin cross section of several fibers (see fig. 

 20, A) like the piece of a single fiber shrinks 



Fig. 20.- harping of wood wheu ^^ ^ ^ Qf each fiber becomes fMn _ 



ner, and thus each piece smaller, and the piece on the whole necessarily 



1 Though generally true, it must not be supposed that the fibers of all species, or 

 even the fibers of the same tree, shrink exactly in proportion to the thickness of 

 their walls. 



Fig. 19.— 

 Isolated 

 cell. 



