MECHANICAL PROPERTIES OF WOODS GROWN I1ST UNITED STATES. 9 



SUMMERWOOD. 



The amount of summerwood is expressed in per cent of the entire 

 cross section. (See definition of summerwood, p. 23.) It is measured 

 along a representative radial line. 



In many species the proportion of summerwood is indicative of the 

 density; and different proportions of summerwood are usually accom- 

 panied by different densities and strength values. When the change 

 from springwood to summerwood is not marked or the contrast 

 between them is not sharp, no accurate measurement can be made 

 and the results have no practical value. 



In southern yellow pine and Douglas fir, one-third or more summer- 

 wood, except when associated with rapid irregular growth, indicates 

 material of a quality suitable for use as structual timber. 



MOISTURE CONTENT. 



Moisture content is the weight of water contained m the wood, 

 expressed in per cent of the oven-dry weight of the wood. Moisture 

 content is determined by weighing a small section of the test specimen 

 and then drying it at 100° C. in freely circulating air until its weight 

 becomes constant; the loss of weight is then divided by the dry weight 

 to give the proportion of moisture, and this is usually expressed in 

 per cent of the dry weight. Consequently, "moisture" as deter- 

 mined includes any other substances besides water volatile at 100° C. 

 which may be in the wood. 



The various species differ widely as to the amount of moisture 

 contained in the wood of the living tree. For example, white ash and 

 black locust are always comparatively dry; black ash and the oaks 

 have about twice, and chestnut and buckeye three times, as much 

 water as white ash. The coniferous species also show wide range in 

 moisture content. White and red cedars are comparatively dry; 

 cypress and white fir contain large amounts of water. 



Moisture content sometimes varies with position in the tree. Most 

 coniferous species have a large proportion of moisture in the sapwood 

 and a much smaller proportion in the heartwood. In some the heart- 

 wood is very wet at the base of the tree, but comparatively dry higher 

 up. Most hardwoods, or broad-leaved species, show a fairly uniform 

 distribution of moisture throughout the tree. 



SPECIFIC GRAVITY. 



Specific gravity is the weight of any given substance divided by the 

 weight of an equal volume of pure water at its greatest density. 



Obviously, the weight of wood in a given volume changes with the 



shrinkage and swelling caused by changes in moisture. Consequently, 



specific gravity is an indefinite quantity unless the circumstances 



under which it is determined are specified. Each of the columns 



91728°— Bull. 55S— 17 2 



