12 AMERICAN FOREST TREES 



weak. It may sustain a heavy load without breaking, but when it fails, 

 the break is sudden and complete. A tough wood behaves differently, 

 though it may not be as strong as a brittle one. When a tough wood 

 breaks, the parts are inclined to adhere after they have ceased to sustain 

 the load. Hickory is tough, and in breaking, the wood crushes and 

 splinters. Mesquite is brittle, and a clean snap severs the stick at once. 



Builders of houses and bridges, and the manufacturers of articles 

 of wood, study with the greatest care the stiffness, elasticity, strength, 

 toughness, and brittleness of timber. Its chief value may depend 

 upon the presence or absence of one or more of these properties. Take 

 away hickory's toughness and elasticity and it would cease to be a 

 great vehicle and handle material. Reduce the stiffness and strength of 

 longleaf pine and Douglas fir and they would drop at once from the high 

 esteem in which they are held as structural timbers. Destroy the 

 brittleness of red cedar and it would lose one of the chief qualities which 

 make it the leading lead pencil wood of the world. 



There are recognized methods of measuring these important 

 physical properties of woods, but they are expressed in language so 

 technical that it means little to persons who are not specialists. For 

 ordinary purposes, it is unnecessary to be more explicit than to state a 

 certain wood is or is not strong, stiff, tough and elastic. Some species 

 possess one or more of these properties to double the degree that others 

 possess then. Different trees of the same species differ greatly, and 

 even different parts of the same tree. Most tables of figures which show 

 the various physical properties of woods, give averages only, not abso- 

 lute values. 



Hardness In some woods hardness is considered an advantage, 

 but not in others. If sugar maple were as soft as white pine, it would 

 not be the great floor material it is ; and if white pine were as hard as 

 maple, pattern makers would not want it, door and sash manufacturers 

 would get along with less, and it would not be the leading packing box 

 material in so wide a region. 



It is generally the summer growth in the annual rings which makes 

 a wood hard. The summenvood is dense. A given bulk of it contains 

 more actual wood substance and less air and water than the spring- 

 wood. For the same reason, summerwood gives weight, and a relation- 

 ship between hardness and weight holds generally. It may be added 

 that strength goes with weight and hardness, but it is not a rule without 

 apparent exceptions. 



Some woods possess twice or three times the hardness of others. 

 Among some of the hardest in the United States are hickory, sugar 

 maple, mesquite, the Florida ironwoods, Osage orange, locust, persim- 



