38 TIMBER. 



shape, it must be tough to avoid shattering to pieces, and it must also 

 be hard or else its tenons will become loose in their mortises. 



Selecting wood in this way, the woodworker has learned almost all 

 that is at present known about his material, but in many cases the 

 great difficulty which always attends the judgment of complex phe- 

 nomena has led to erroneous conclusions, and not a few well-established 

 beliefs have their origin more in accidental error of observation than 

 in fact. 



The experimenter endeavors to avoid this complexity by testing the 

 wood for each kind of resistance separately; when tested as to their 

 stiffness, the pieces are all shaped, placed, and loaded alike. The wood 

 is selected with a definite object in view; it is green or dry, clear or 

 knotty, straight or crossgrained, according as he wishes to find out the 

 influence of each of these conditions. If pine and oak are to be com- 

 pared, the pieces are from the same position in the tree and are tried 

 under exactly the same conditions, and thus the case is simplified. 



But even results thus arrived at can not be used indiscriminately, and 

 the figures on the strength of oak given in any book must not be sup- 

 posed to apply to all oak, if tested in the given manner. This is due 

 to the fact that a piece of wood is not simply a material but a structure, 

 just as much as a railroad bridge or a balloon frame, and as such varies 

 greatly even in the wood of the same tree, nay, more than that, even in 

 the same year's growth of the same cross section of a log. 



A scantling resists bending; it is stiff". On removal of the load it 

 straightens; it is elastic. A column, a prop, or the spoke of a wagon 

 wheel resists being crushed endwise. So does the upper side of a joist 

 or beam when loaded, while the underside of the beam or of an ax 

 handle suffers in tension. The tenons of a window sash or door tend to 

 break out their mortises, the wood has to resist shearing along the fibers ; 

 the steel edge of the eye tends to cut into the hammer handle, it tries 

 to shear it across the grain, and every nail, screw, bore hole, or mortise 

 tends to split the board and tries the wood as to its cleavability, while 

 all "bent" ware, from the wicker basket to the one-piece felly or ship's 

 knee, involves its flexibility. 



STIFFNESS. 



If 100 pounds placed in the middle of a stick 2 by 2 inches and 4 feet 

 long, supported at both ends, bend or "deflect" this stick one-eighth of 

 an inch (in the middle), then 200 pounds will bend it about one-fourth 

 inch, 300 pounds three-eighths inch, the deflection varying directly as 

 the load. Soon, however, a point is reached where an additional 100 

 pounds adds more than one-eighth inch to the deflection — the limit of 

 elasticity has been reached. Taking another piece from the straight 

 grained and perfectly clear plank of the same depth and width, but 8 

 feet long, the load of 100 pounds will cause it to bend not only one- 

 eighth inch, but will deflect it by about 1 inch. Doubling the length 



