JANUAET 22, 1915] 



SCIENCE 



125 



conditions in the stanfl rrom -which it comes. 

 "Were it not, therefore, that mechanical 

 properties can be tied up with some definite 

 forest conditions and correlated with some 

 readily visible expression of tree growth, 

 such as the number of rings per inch or the 

 specific gravity of the wood, timber would 

 be too much of an indefinite quantity for 

 architects and other users of wood to handle 

 with perfect safety. To find such a rela- 

 tion is just what the foresters have been 

 attempting to do and most of the studies of 

 the strength of wood have been with the 

 view of establishing certain relations be- 

 tween the mechanical, physical and anat- 

 omical properties of the wood. Some of 

 these relations I may mention here. 



One of the earliest relations which for- 

 esters 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 gravity, 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 exception, 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 correlation between the specific 

 gravity and the resistance to compression 

 end-wise (parallel to the grain) is appar- 

 ently beyond question. Thus by the specific 

 gravity the resistance to compression end- 

 wise can be readily determined. The com- 

 pression end-wise equals 1,000 times the 

 specific gravity minus 70, when the mois- 

 ture contents of the wood is 15 per cent., or 

 C = 1,0008 — 70. 



Since in construction work the most de- 



sirable wood is the one which possesses the 

 highest strength at a given weight, the ratio 

 between the compression strength 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 sub- 

 stantiates the law that the specific gravity 

 of wood determines its mechanical prop- 

 erties. 



Another relation which has been fairly 

 established is that between the resistance 

 to compression end-wise and the bending 

 strength of timber. (By the resistance 

 compression end-wise, therefore, the bend- 

 ing strength of timber can be determined.) 



One of the other properties of wood, 

 namely hardness, was found to have a defi- 

 nite relation to the bending and compres- 

 sion strength of wood and this fact tempts 

 the conclusion that by hardness alone all 

 other mechanical properties can be deter- 

 mined. 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 summer wood 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 proportion of fibro-vascular bundles, 

 their thickness and length, the proportion 

 of thick-walled cells, medullary rays, etc. 

 The anatomical structure in its turn is 

 probably determined by the combination of 

 two factors — the amount of nourishment 

 in the soil and the intensity of transpira- 

 tion. The mechanical properties of wood 

 come, therefore, within the control of the 

 forester who raises and cares for the forest. 



There is another field of scientific en- 

 deavor in which foresters in this country 

 may claim some credit. This is in the field 



