KELATIOK 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 maximum 

 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 was made. 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. AU 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. Each 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, 1(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 aU 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 wiU 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 



