MECHANICAL PROPERTIES OF WOODS GROWN IN UNITED STATES. 11 



determined from different specimens the equation given at the end of 

 the preceding paragraph does not hold exactly when applied to the 

 data in columns 7, 8, and 10 of Table 1. 



WEIGHT PER CUBIC FOOT. 



Weight per cubic foot, like specific gravity, is a very indefinite 

 quantity unless the circumstances under which it is determined are 

 specified. The variability is also large, as may be realized from a 

 consideration of the following: The specific gravity of some speci- 

 mens may be twice that of others of the same species; occasionally a 

 piece may contain nearly as much resin as wood ; the moisture con- 

 tent may be as little as 4 or 5 per cent of the dry Weight of the 

 wood in the case of kiln-dry lumber, or it may be as great as 200 per 

 cent in green timber, as is occasionally the case in the sapwood of 

 some of the coniferous species. 



WEIGHT PER CUBIC FOOT GREEN. 



Weight per cubic foot green is the weight per cubic foot of the 

 wood (including moisture) as it comes from the living tree. The 

 various species differ largely as to the wetness of the green wood. 

 The hardwoods as a rule do not exhibit any considerable variation 

 with the position in the tree. The conifers, on the other hand, show a 

 wide variation in moisture content between the heartwood and sap- 

 wood and, in some instances, between wood from the upper and lower 

 parts of the tree. Tamarack and cypress, however, have a compara- 

 tively uniform moisture content throughout the tree. Sugar pine 

 and western larch are frequently very heavy because of moisture and 

 resin at the butt. Longleaf pine and some other species have a very 

 low moisture content in the heartwood, while the sapwood is very 

 wet. When this is the case, young thrifty trees with a large propor- 

 tion of sapwood are much heavier than old overmature trees with a 

 small amount of sapwood. 



Variations of 4 per cent above or below the averages given are to 

 be expected in any lot of material of a species which has fairly uniform 

 moisture content. If the species is one that does not have a uniform 

 distribution of moisture, about twice as great a variation may be 

 expected. Under exceptional conditions the weight of green timber 

 of some of the conifers may vary as much as 30 per cent from the 

 average. 



WEIGHT PER CUBIC FOOT AIR DRY.i 



The weights given for air-dry wood are for wood with 12 per cent 

 moisture. A variation of 4 per cent in any given lot of material 

 even at this moisture content is to be expected. Large timbers 



iSee definition of air dry (glossary, p. 20). 



