6 BULLETIN 676, U. S. DEPARTMENT OF AGRICULTURE. 
SPECIES-LOCALITY AVERAGES. 
The specific-gravity relations given in this bulletin are derived from 
a study of what may be called ‘‘species-locality”’ averages; that is, 
each average represents tests of material of one species from one 
locality. 
There are two principal reasons for using ‘‘species-locality”’ aver- 
ages in preference to the results of individual tests. First, the number 
of individual tests is quite large, amounting in some instances to as 
many as 900 from a single ‘‘species-locality’’, so that an immense 
amount of work is saved by the use of the ‘‘species-locality”’ averages; 
second, if individual tests were used, the ‘‘species-localities’’ having 
larger trees or a larger number of trees would include a larger number 
of tests and would have undue weight in determining the relations. 
The method of analysis used is applicable also to individual tests 
from a single species to determine the specific gravity relations within 
that species. It has been applied to a few of the properties of some 
of the more important species which are used for structural timbers 
where there was a rather large number of test pieces and a con- 
siderable range in specific gravity. 
DETERMINATION OF SPECIFIC GRAVITY. 
Specific gravity of wood, as used herein, is based on the volume of 
the specimens when tested (green or air-dry) and their weight when 
in an oven-dry condition; that is, it is the ratio of the weight of the 
specimen of wood, oven-dry, to the weight of a volume of water equal 
to the volume of the specimen at the time of test. Because of the 
shrinkage which takes place in wood when it is dried, this figure is 
not the true specific gravity of a piece of oven-dry wood. The 
method, however, is easily applied to each specimen tested, and is 
the standard method of the Forest Service for the determination of 
a specific-gravity figure for use in studying the properties of wood. 
MOISTURE CONTENT OF TEST SPECIMENS. 
Both green and air-dry specimens were used in the tests, and the 
relations between specific gravity and strength were determined 
separately for green and air-dry wood. Variations in the moisture 
content of wood have no effect on its mechanical properties so long 
as the wood is thoroughly green; they have considerable influence on 
these properties, however, as soon as the wood becomes air-dry, or 
partially air-dry. Accurate comparisons can not be made between 
the properties of two lots of air-dry specimens unless they were tested 
at the same moisture content or adjustments made in the strength 
figures for difference In moisture content. 
