46 TIMBER. 



this is said to shear out along the fiber. In the same manner, if the 

 shoulder A B G D in fig. 30, is pushed oft' along B D, it is sheared, 

 and if B B and G B are each 1 inch, the surface thus sheared off is 

 1 square inch, and the weight necessary to do this represents the 

 shearing strength per square inch of the particular kind of wood. This 

 resistance is small when compared to that of tension and compression. 

 In general wet or green wood shears about one-third more easily 

 than dry wood; a surface parallel to the rings (tangent) shears more 

 easily than one parallel to the medullary rays. The lighter conifers 

 and hard woods offer less resistance than the heavier kinds, but the 

 best of pine shears one-third to one-half more readily than oak or 

 hickory, indicating that great shearing strength is characteristic of 

 "tough" woods. 



Resistance to shearing along the fiber. 



Per 



square 

 inch. 



(1) Locust, oak, hickory, elm, maple, ash, birch 



(2) Sycamore, longleaf, Cuban, and shortleaf pine, and tamarack 



(3) Tulip, basswood, better class of poplar, Norway, loblolly and white pine, spruce, red cedar 



(4) Softer poplar, hemlock, white cedar, fir , 



Pounds. 

 1 1, 000 

 600 



400 

 = 400 



■Over. 2 Lessthan. 



Note. — Resistance to shearing, although a most important quality in wood, has not been satisfacto- 

 rily studied. The values in the above table, taken from various authors, lack a reliable experimental 

 basis and can be considered as only a little better than guesswork. 



INFLUENCE OF WEIGHT AND MOISTURE ON STRENGTH. 



It has been stated that heavy wood is stronger than lighter wood of 

 the same kind, and that seasoning increases all forms of resistance. 

 Let us examine why this is so. 



Since the weight of dry wood depends on the number of fibers and 

 the thickness of their walls, there must be more fibers per square inch 

 of cross section in the heavy than in the light piece of the same kind, 1 

 and it is but natural that the greater number of fibers should also offer 

 greater resistance, i. e., have the greater strength. 



The beneficial influence of drying and consequent shrinking is two- 

 fold: (1) In dry wood a greater number of fibers occur per square inch, 

 and (2) the wood substance itself, i. e., the cell walls, become firmer. A 

 piece of green longleaf pine, 1 by 1 inch and 2 inches long, is only about 

 0.94 by 0.9G inch and 2 inches long when dry ; its cross section is 10 per 

 cent smaller than before, but it still contains the same number of fibers. 

 A dry piece 1 by 1 inch, therefore, contains 10 per cent more fibers than 

 a green piece of the same size, and it is but fair to suppose that its 

 resistance or strength is also about 10 per cent greater. 



The influence of the second factor, though unquestionably the more 

 important one, is less readily measured. In 100 cubic inches of wood 



1 This imperfect assumption is used only for comparison. 



