366 



FORESTRY INVESTIGATIONS U. S. DEPARTMENT OF AGRICULTURE. 



Itesults of tetts in bending, at rupture. 

 I'm mils per square inch.] 



a Actual tests on ' dry " material not reduced for moisture. 



UKI.AT1ONS OF WEIGHT AND STRENGTH. 



That within the same species the strength of wood varied with the dry weight (specific gravity), i. e., that 

 the heavier stick is the stronger, has been known for some time. That this law of variation held good not only for 

 a given species, but irrespective of species for the four principal piues of our Southern .States was indicated in 

 Circular 12 of this Division. This fact becomes the more important in practical application, as the wood of these 

 species of pines so far can not be distinguished at all by its anatomical structure and only with difficulty and 

 uncertainty by other appearances, while in the lumber market substitution is not infrequent. It will therefore bo 

 best with these pines, where strength alone is desired, to inspect the material by weight (specific), other things 

 being equal, disregarding species determination. 



While this result of the exhaustive series of tests reasonably well demonstrated for these pines may be 

 considered of great practical value, we can now extend the application of the law of relation between weight and 

 strength a .step farther, and state as an indication of our tests that probably in woods of uniform structure strength 

 increases with specific weight, independently of species and genus distinction, i. e., other things being equal, the 

 heavier wood is the stronger. We are at present inclined to state this important result with caution, only as a 

 probability or Indication, until either the test material and tests can be more closely scanned, or more carefully 

 planned and minutely executed series of detail tests can be carried on to confirm the truth of what the wholesale 

 tests seem to have developed. 



In the following two diagrams the average strength of the different species in compression endwise and 

 bending, as found in the preceding tables, has been plotted with reference to the dry weight as given in preceding 

 table. 



Considering that these tests and weight determinations (especially the latter) were not carried on with that 

 finesse which would be required for a scientific demonstration of a natural law, that other influences, as crossgrain, 

 unknown defects, and moisture conditions may cloud the results, and that in the averaging of results undue consid- 

 eration may have been given to weaker or stronger, heavier or lighter, material, the relaxation is exhibited even by 

 this wholesale, method with a remarkable degree of uniformity bordering on demonstration. 



An exception is apparent in the oaks in that they do not exhibit this relation of weight and strength with 

 reference to other species, and also with less deliniteness among the various .species of oak in themselves. The 

 structure of oak wood being exceedingly complicated and essentially different from that of the wood of all other 

 species under consideration, it may reasonably be expected that it will not range itself with these. 



