WOOD 



107 



Fig. 



Pith 



Rings 

 Bark 



108. Section of log showing 

 growth of tree 



the dark outer part is dense and strong and is the summer 

 growth. The rings decrease in width as they approach the 

 outside of the tree. The weakest, softest zone is the last- 

 growing part, found immediate- 

 ly inside the bark and known as 

 sapwood. The densest, strong- 

 est part is found in the interior 

 and is known as heartwood. 

 The change from, sapwood to 

 heartwood is gradual from the 

 bark toward the heart. 



Grain. A close- or fine-grain 

 wood is one in which the rings are relatively narrow; an 

 open- or coarse-grain is one in which they are wide. A 

 straight-grained timber is one in which the grain runs parallel 

 to the length, and a cross-grained timber is one in which the 

 grain is twisted, distorted, or runs at an angle to its long side. 

 A straight-grained timber is stronger and will split in more 

 uniform sections than one that is cross-grained. In Fig. 109 

 A is sawed parallel to the grain, and B across the grain. 



Lumber production. The pio- 

 neer selected the most suitable 

 trees near his building site, cut 

 them into lengths with an axe, 

 notched the ends, and laid them 

 up round, or, by the aid of a broad 

 axe, flattened one or more sides by 

 hewing. If a few flat light pieces 

 were needed for finish, they were 

 made by whip sawing, which con- 

 sisted in cutting a log lengthwise 

 with a long saw operated by 

 hand. This method of building 

 was laborious, wasteful of timber, and fell into disuse as 

 soon as power machinery became available for the working 

 of wood. 



Fig. 109. Grain of wood 



A — Straight, slash, or flat grain 

 B — Quarter-sawed or vertical grain 



