MECHANICAL PROPERTIES OF WOODS GROWN IN 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 in 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. 556—17 2 
