INTRODUCTION. 3 



The character and the arrangement of cell-structures differ 

 with species. Wood is hard, soft, light, heavy, tough, porous, 

 elastic, or otherwise, because of these differences. Appearance 

 is affected, and woods may be distinguished from one another, 

 because of this fact.* 



Most wood is used in ' ' construction, ' ' that is, in mines, 

 railways, houses, and ships, where demand is for size or 

 quantity, and where finish and appearance amount to but little. 

 Much wood is used in decoration and furniture, where appear- 

 ance, appropriateness, and finish are called for; but these 

 woods, although much in evidence, are infinitely less in quantity 

 than those employed in construction. Some wood is required 

 for implements, turnery, carvings, and small-piece work, 

 where size is secondary and where qualities such as hardness, 

 fine grain, and uniformity, controllable in small pieces, are 

 primary. Some wood is used indirectly, as in the manufacture 

 of paper-pulp, gunpowder, and chemicals. There are also 

 by-products of trees, such as tanbark, turpentine, resin, nuts, 

 and sugar. 



The weight, strength, and other measurable properties of 

 Wood are variable. Weight varies from day to day as water 

 is absorbed and evaporated. Strength differs with grain, age, 

 moisture, specific gravity, and many other things. Two 

 pieces from different portions of the same tree differ from each 

 other. The proportions of sap and heart wood are seldom 

 constant. Results from small specimens may differ from those 

 obtained from larger ones.t The botanical accuracy of a 

 specimen is not always certain, therefore figures relating to the 

 physical properties of wood should be employed with greater 

 caution than those relating to the more homogeneous metals. 



Many of the experiments conducted to establish statements 

 regarding the physical properties of wood have been defective 



* Roth, U. S. Forestry Bui. No. lo, pp. 64-71. Also von Schrenk, U. S. Dept. 

 Agriculture, Bureau Plant Industry Bui. No. 14, pp.- 12-16. 



f Only because imperfections are more likely in larger pieces. Large and 

 small pieces of equally perfect wood are equally strong. (See Publications U. S. 

 Forestry Div. and J. B. Johnson's " Materials of Construction,'' p. 462.) 



