CHAPTER XVI 



TIMBER 



233. Structure of timber. An examination of the cross section of 

 a tree usually shows that it is made up of a rather dark interior core, 

 or heartwood, and a lighter exterior portion, or sapwood, surrounded by 

 the bark. Ill some species, such as the oaks, radial lines, called 

 medullary rays, are seen running from the center toward the hark. 

 If the cross section happens to lr near a knot .,r other detect, this 

 normal structure may he changed. If. however. n<> knots are pr 



a closer examination shows that hoth the sapwood and heartwood are 

 made up of concentric rings, called annual rings, and that this appear- 

 ance is due to a difference in -nurture. 1'art of the ring is >een to 

 be denser than the rest, and, in fact, it is this difference in d : 

 which gives the section its characteristic appearance. 



The annual rings in one stick of a certain species may be n 

 widely separated than those in another stick of the same species, and 

 the relative thickness of heartwood and sapwood may differ in dill- 

 sticks. This indicates that the structure of timber varies considerably, 

 and that therefore the physical properties also vary. This wide varia- 

 tion is seen in all substances found in nature, one instance of which 

 has been shown in the case of natural stone. An investigation of the 

 physical properties of such substances, therefore, is more difficult than 

 that of a more homogeneous substance. However, the extensive u>e 

 of timber as a structural material makes a knowledge of its structure 

 and properties of the utmost importance. 



234. Annual rings. Kach of the concentric rings in timber r< 

 sents the growth of one year. The inner or less dense portion i 

 sents the more rapid spring growth, while the outer dense portion 

 represents the slower summer and fall growth. The number of rings 

 per inch indicates the rate of growth for that number <>f yean, If 

 the number of rings per inch be few, the growth has been rapid and 



