10 BULLETIN 556, U. S. DEPARTMENT OF AGRICULTURE. 
of specific gravity figures given in this table is based on the weight of 
the wood when oven dry and on its volume when green or at a 
specified stage of drying. 
SPECIFIC GRAVITY BASED OX VOLUME WHEN GREEX. 
In the determination of the figures for specific gravity based on 
volume when green the test specimens are weighed and measured 
when green. Their oven-dry weight is then computed by dividing 
the weight when green by 1 plus the proportion of moisture, moisture 
being determined as described in previous paragraphs. The specific- 
gravity data based on green volume are more reliable than the data 
based on air-dry or oven-dry volume because they are based on the 
largest number of determinations, and these determinations are 
unaffected by the shrinkage of the wood. Specific gravity so deter- 
mined is, aside from actual strength data, the best criterion of the 
strength of clear wood of any species. 
It has been found that in oak, more than in any other species or 
group of closely related species, pieces of the same density may vary 
widely in mechanical properties. Occasional very dense pieces of oak 
are for some unknown reason low in strength; but in all species 
specimens of low density are invariably weak. 
SPECIFIC GRAVITY BASED OX AIR-DRY VOLUME. 
Specific gravity based on air-dry volume is obtained in the same 
manner as that based on volume when green, except that the volume 
measurements are made after the material has been air dried. The 
data in the tables are less reliable than those for the specific gravity on 
green volume because they are based on fewer determinations and are 
affected by variations in the shrinkage which has taken place. 
SPECIFIC GRAVITY BASED OX OVEX-DRY VOLUME. 
In determining the specific gravity based on oven-dry volume, the 
volume as well as the weight is taken after the specimens are dried 
to a practically constant weight in air at 100° C. The difference 
between specific gravity based on green volume and that on oven-dry 
volume is due to the shrinkage, and one may be determined from the 
other if the shrinkage in volume is known. Specific gravity on oven- 
dry volume = specific gravity based on volume when green -s- .(1 — 
the shrinkage.) 
The specific gravity based on volume when oven dry and the shrink- 
age in volume (see columns 8 and 10, Table 1) determinations were 
made on the same specimens, of which there were usually four from 
each tree. The specific gravity based on green volume was deter- 
mined from a much larger number of specimens and is consequently 
somewhat more reliable. Because these two specific gravities were 
