HARDNESS OF WOOD. 47 



substance the material of the cell walls takes up about 50 cubic inches 

 of water and. thereby swells up, becoming about 150 cubic inches in 

 volume. In keeping with this swelling the substance becomes softer 

 and less resistant. In pine wood this diminution of resistance, accord- 

 ing to experiments, seems to be about 50 per cent, and the strength of 

 the substance therefore is inversely as the degree of saturation or 

 solution. 



HARDNESS AND SHEARING ACROSS THE GRAIN. 



When the solid steel plunger P in fig. 32 descends on the piece of 

 wood io, the first effect is to press it into the wood of the upper surface 

 without affecting the interior or lower part. The wood is thus tried 

 with regard to its hardness. If a perforated steel plate is substituted 

 for the solid plate the elfect of the plunger is at first the same, but 

 soon the fibers some dis- 

 tance from the steel are 

 seen to bend, and finally 

 the piece of wood fails 

 in shearing across the 

 grain. Hardness and 

 shearing across the grain 

 are closely related. The FlG " 32 - Test in hardness and 8hearing across lbe grain - 

 former is the more important quality, however, since abrasion and 

 indentation, the two failures in hardness, are the common cause of 

 loosening of tenons in the mortise, of the handle in the ax, etc. 



Heavy wood is harder than lighter wood; the wood of the butt, 

 therefore, is harder than that of the top; the darker summer wood 

 harder than the light-colored spring wood. Moisture softens, and sea- 

 soning, therefore, hardens wood. 



Placing the rings vertical helps the wood to resist indentation. 

 Though harder wood resists saw and chisel more than softer wood, the 

 working quality of the wood is not always a safe criterion of its hard- 

 ness. 



The following indicates the hardness of our common woods : 



1. Very hard woods requiring over 3,200 pounds per square inch to 

 produce an indentation of one-twentieth inch: Hickory, hard maple, 

 osage orange, black locust, persimmon, and the best of oak, elm, and 

 hackberry. 



2. Hard woods requiring over 2,400 pounds per square inch to pro- 

 duce an indentation of one-twentieth inch: Oak, elm, ash, cherry, 

 birch, black walnut, beech, blue beech, mulberry, soft maple, holly, 

 sour gum, honey locust, coffee tree, and sycamore. 



3. Middling hard woods, requiring over 1,000 pounds per square inch 

 to produce an indentation of one-twentieth inch: The better qualities 

 of Southern and Western hard pine, tamarack and Douglas spruce, 

 sweet gum, and the lighter qualities of birch. 



