194 FARM FORESTRY 



Within the same kind of wood the strength will vary with the 

 density. With woods that form large pores in the spring, 

 like oak, chestnut and hickory, the wood from fast-growing 

 trees is stronger than from slow-growing trees. This is be- 

 cause of the greater percentage of hard, strong summer wood. 

 The amount of spring wood will remain the same in these 

 woods whether the wood grows fast or slowly. In the fast- 

 growing wood the increase will almost entirely be made up 

 of the summer wood. Second growth hickory is preferred to 

 the forest grown wood for wagon stock because of this fact. 

 It is stronger because of the faster growth. With conifers, on 

 the other hand, the faster the growth the weaker the wood. 

 In a fast-growing conifer there is a greater percentage of 

 spring wood which is much softer and weaker than the sum- 

 mer wood. In conifers fast growth means a large growth 

 in the spring wood. Since summer wood is darker than spring 

 wood, the darker the wood in the same species, leaving out 

 of account resin content, the stronger it will be. In selecting 

 a strong piece of pine, then, a dark-colored piece that has 

 narrow annual rings should be chosen. 



With hard woods that do not produce large spring pores 

 like tulip, poplar, maple and birch rapidity of growth has little 

 effect on the strength or durability of wood. 



Defects in boards or timbers decrease the strength, such 

 as knots, checks, splits, etc. Clear lumber is stronger than 

 knotty material. With timbers the position of the defect 

 has some influence on the strength. A timber laid so that the 

 defect is near the top when set will be stronger than the same 

 timber set with the defect below. This is because in a timber 

 subject to a stress the upper half of the timber will be subject 

 to compression, while in the lower half of the timber the 

 fibers will have a tendency to be pulled apart, being subjected 

 to a tension. The manner of sawing lumber from logs and 

 the direction of the annual rings passing through the wood 

 will also influence the strength. 



