KELATIOIT OF SHEI^TKAGE, ETC., TO SPECIFIC GRAVITY. 9 



APPLICATION OF THE EQUATIONS. 



Additional data may possibly necessitate the making of some slight 

 changes in the equations given in Table 1 and the diagrams. How- 

 ever, for comparing species and for determining the best utilization 

 of timber, the value of the equations as they are now is not affected 

 by this possibihty. It is to be expected that among a large number 

 of species of widely different structure many will be found which do 

 not satisfy very accurately the average equations connecting the 

 various properties with specific gravity. It is often this variation 

 from an average relation which determines the usefulness of a species 

 for a special purpose. 



As an example of the use to which the table and diagrams may be 

 put, suppose it is desired to obtain the strength ia compression par- 

 allel to the grain of a piece of green hemlock (eastern) grown in the 

 southern Appalachian region. Its specific gravity may be deter- 

 mined by any one of several means which may readily be devised, 

 and we wiU say that it is found to be 0.38. In the table, the ' 'species- 

 locality" which is probably most nearly representative of the region 

 ia question is the eastern hemlock from Tennessee, and of this the 

 maximum crushing strength is 29 per cent above the average for 

 woods of the same specific gravity. To find what an average wood 

 of a specific gravity of 0.38 will stand in compression parallel to the 

 grain, we solve the equation = 6,900x0.38, or turn to figure 1 and 

 read from the curve a maximum crushing strength of about 2,600 

 pounds per square inch. But smce the compressive strength of the 

 Tennessee hemlock was 29 per cent high, it is reasonable to expect 

 that the timber m question will also run about 29 per cent high, or 

 that the value would be about 3,300 or 3,400 pounds per square inch 

 (2,600X1.29 = 3,354). Any of the other properties of the hemlock 

 under consideration may be estimated in a similar manner. 



Again, suppose it is desired to obtaia a wood for a use which 

 requires that it be very strong for its weight in its abfiity to resist 

 a splitting force. Tension perpendicular to grain is the best measure 

 of this. By looking down the column, ''Tension, surface of failure 

 radial," it is found that in abiUty to resist such a force, yeUow buck- 

 eye is 17 per cent stronger when green and 120 per cent stronger 

 when air-dry than is the average wood of the same specific gravity. 

 It would appear at first that yellow buckeye is the most desirable 

 wood for the purpose, but there is another consideration to be taken 

 into account. Tension perpendicular to the grain varies with the 

 square of the specific gravity; and it must be remembered that those 

 properties (such as tension perpendicular to gram, hardness, work 

 values, and compression perpendicular to the grain) which vary with 

 the higher powers of specific gravity show a large increase in strength 

 65977°— 19— Bull. 676 2 



