34 



TIMBER. 



the several parts of the timber prevents such bending and the conse 

 quent strain leads to their separation as shown at Z, the end surface 

 of the timber being "checked." 



As the timber dries out, the line c d becomes shorter, the parts 1 to 6 

 are allowed to approach again, and the checks close up and are no 

 longer visible. 



The faster the drying at the surface, the greater is the difference in 

 the moisture of the different parts, and hence the greater the strains 

 and consequently also the amount of checking. This becomes very 

 evident when fresh wood is placed in the sun, and still more in a hot kiln. 

 While most of these smaller checks are thus only temporary, closing 

 up again, some large radial checks remain and even grow larger as 

 drying progresses. Their cause is a different one and will presently be 

 explained. 

 The temporary checks not only occur at the ends, but are developed 



on the sides also, only to a much smaller 

 degree. They become especially an- 

 noying on the surface of thick planks 

 of hard woods, and also on peeled logs 

 when exposed to the sun. 



So far we have considered the wood 

 as if made up only of parallel fibers all 

 placed longitudinally in the log. This, 

 however, is not the case. A large part 

 of the wood is formed by the medul- 

 lary or pith rays. In pine over 15,000 

 of these occur on a square inch of a 

 tangential section, and even in oak the 

 very large rays, which are readily visi- 

 ble to the eye, represent scarcely a 

 hundredth part of the number which 

 the microscope reveals. 



As seen in fig. 22 the cells of these 

 rays have their length at right angles 

 to the direction of the wood fibers. 

 If a large pith ray of white oak is whittled out and allowed to dry it 

 is found to shrink greatly in the direction from c to d (fig. 22), while, as 

 we have stated, the fibers to which the ray is firmly grown in the wood 

 do not shrink in the same direction. Therefore, in the wood, as the 

 cells of the pith ray dry, they pull on the longitudinal fibers and try 

 to shorten them, and, being opposed by the rigidity of the fibers, the 

 pitb ray is greatly strained. But this is not the only strain it has to 

 bear. Since the fibers from a to b (fig. 22) shrink as much again as the 

 pith ray in this, its longitudinal direction, the fibers tend to shorten 

 the ray, and the latter, in opposing this, prevents the former from 



-air 



Fig. 22.— Small pith ray in oak. 

 fibers ; c, d, cells of pith ray. 



a, 6, wood 



