RELATION OF SHRINKAGE, ETC., TO SPECIFIC GRAVITY. 7 
The moisture content of the air-dry material at the time of test 
varied from 8 to 18 per cent. Modulus of rupture and maxinum 
strength in compression parallel to the grain were adjusted to a 
moisture content of 12 per cent before determinations of the relation 
of these properties to the specific gravity wasmade. This adjustment 
was possible because the laws governing the variation of these prop- 
erties with varying moisture content are fairly well established. 
However, in the case of the other strength functions their variation 
with varying moisture content has not been studied in detail and no 
such adjustment is possible with any very great degree of accuracy. 
Consequently, the actual moisture content values as obtained from 
tests have been used in the determination of the relation of these prop- 
erties to specific gravity. 
THE EQUATIONS. 
Table 1 and figures 1 to 4 give equations which represent the 
average relations between specific gravity and each of the mechanical 
properties. All the ‘‘species-locality”’ averages available on any 
particular property were considered in deriving the equations for 
that property. The number of ‘‘species-locality’’ averages from 
which an equation is derived varies from 84 to 178. This variation 
is due to the fact that several of the tests were not used in some of 
the earlier testing work and to the fact that tests have not yet been 
completed on air-dry material for all of the ‘‘species-localities’’ listed. 
Table 1 gives first the equations for shrinkage and for each of the 
strength properties of green and air-dry wood in terms of the specific 
gravity. These equations, as explained in the appendix, are reduced 
to a simple form; and the powers of gravity used are such that. the 
equations may be solved by arithmetical operations and without the 
use of higher mathematics. However, to simplify even further the 
use of the equations, figures 1 to 4 have been prepared for their solu- 
tion. Kach of the curves shown in these diagrams represents the 
equation connecting specific gravity and one of the properties of 
wood. The curves representing the equations for radial, tangential, 
and volumetric shrinkage appear in figure 1(a). In each of the other 
figures, i(b) to 4(d), appear two curves for some one mechanical 
property. One of these curves is for green and the other for air-dry 
material. If the specific gravity is known, the equation value for 
any one or all of the properties of the wood in question may be readily 
determined from the curves without computation. 
The second portion of Table 1 gives what may be termed a measure 
of the accuracy of the respective equations. It is not to be expected 
that all the ‘“‘species-locality”’ averages will satisfy the equation 
exactly or even very closely. Some of the properties are more 
erratic than others, so that one ‘‘species-locality’’ may far exceed 
