TIMBER AND TIMBER TREES. 



different species by applying the same tests to similarly 

 cut samples of each. 



He thus obtains empirical numbers expressing how- 

 many units of weight are needed to slowly extend or 

 compress a bar of the wood, longitudinally, by so much 

 of its original length ; or how many units are necessary 

 to bend it transversely up to such a point that it can, or 

 can not, recover its original shape ; or, again, to such a 

 point that it snaps, and so on. 



Since numerous trials convince him that different 

 species differ in these respects, and that wood of the 

 same species differs considerably according to its age, 

 dryness, and the manner of its growth, he obtains long 

 lists of somewhat rough data for his calculations. 



By these means he arrives at numerical expressions 

 for the average rigidity, elasticity, resistance to com^ 

 pression or shearing stress, to tearing or abrasion, and 

 so forth. He also tests the resistance to torsion, to 

 splitting, etc., and to the cutting power of various 

 instruments. 



Additional information is obtained by experience — 

 another word for experiment if properly controlled and 

 recorded— as to the durability of various specimens of 

 wood after long years of exposure in dry air, or soil, or 

 in damp media, or, finally, in an environment subject ta 

 changes in the degree of moisture. Some kinds of 

 wood last a fairly long time if kept dry, which decay 

 rapidly if exposed to damp ; others, again, will endure 

 for years if always wet, but soon rot if alternately ex- 

 posed to moisture and to a dry air. Moreover, different 

 woods offer different degrees of resistance to the attacks 

 of predatory animals and plants ; such as white ants 

 and teredo on the one hand, and the fungus of dry-rot 

 on the other. 



