6 BULLETIN 676, U. S. DEPARTMENT OF AGEICULTURE. 



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 easUy 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 specunens 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 moistm-e 

 content of wood have no efi^ect 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 specicaens mdess they were tested 

 at the same moisture content or adjustments made in the strength 

 figures for difference in moisture content. 



