FORESTRY GEAVES. 267 



of wood have been with the view of establishing certain relations 

 between the mechanical, physical, and anatomical properties of the 

 wood. Some of these relations I may mention here. 



One of the earliest relations which foresters have established with 

 a fair certainty is that between the specific gravity of the wood and 

 its technical qualities. Some of the foresters even go so far as to 

 claim that the specific gravity of wood is an indicator of all other 

 mechanical properties and that the strength of wood increases with 

 the specific gravit}^, irrespective of the species and genus. In other 

 words, the heavier the wood, all other conditions being equal, the 

 greater its strength. Even oak, which formed apparently an excep- 

 tion, has been recently shown to follow the same law. If there is 

 still some doubt that the specific gravity of wood can be made a 

 criterion of all mechanical and technical properties of wood, the cor- 

 relation between the specific gravity and the resistance to compres- 

 sion endwise (parallel to the grain) is apparently beyond question. 

 Thus by the specific gravity the resistance to compression endwise 

 can be readily determined. The compression endwise equals 1,000 

 times the specific gravity minus 70, when the moisture content of the 

 wood is 15 per cent, or 6^=1,000 S—70. 



Since in construction work the most desirable wood is the one 

 which possesses the highest strength at a given weight, the ratio 

 between the compression streng-th and the specific gravity was found 

 to express most clearly the strength of wood. This ratio, however, 

 increases with the increase in the specific gravity, a fact which 

 further substantiates the law that the specific gravity of wood 

 determines its mechanical properties. 



Another relation which has been fairly established is that between 

 the resistance to compression endwise and the bending strength of 

 timber. By the resistance compression endwise, therefore, the bend- 

 ing strength of timber can be determined. 



One of the other properties of wood — namely, hardness — was 

 found to have a definite relation to the bending and compression 

 strength of wood and this fact tempts the conclusion that by hard- 

 ness alone all other mechanical properties can be determined. The 

 test for hardness is very simple; it can be made even by a small 

 manufacturer and therefore the whole problem of wood testing would 

 be greatly simplified. Hardness was also found to have a definite 

 relation to the proportion of the summerwood in the annual ring, 

 and consequently to the specific gravity of the wood. The specific 

 gravity of wood is determined by its anatomical structure, by the 

 pro]3ortion of fibro- vascular bundles, their thickness and length, the 

 proportion of thick-walled cells, medullary rays, etc. The anatomi- 

 cal structure in its turn is probably determined by the combination 

 of two factors — the amount of nourishment in the soil and the in- 



