51 PROPERTIES OF WOOD. 



(4) According to the breadth of the annual zones and 

 the ratio of spring-wood to summer-wood within an 

 annual zone. 



It is well known that summer-wood is heavier than spring- 

 wood. Foresters, builders, and manufacturers have always 

 adjudged the hardness and weight of wood in accordance with 

 the ratio of the amount of summer-wood to that of spring- wood. 

 They have also considered the question of the width of the 

 whole annual zone, which E. Hartig has recently shown to 

 have no influence on the quality of a wood. Practical experi- 

 ence has decided that in broadleaved woods, the wider the 

 annual zones, the chief increase is in the harder and heavier 

 summer-wood, while in coniferous wood, when the zones 

 become wider, the increase is chiefly in the spring-wood. 

 Hence, in broadleaved woods wide zones, and in conifers 

 narrow zones, imply heavy wood. 



H. Mayr in 1884, in a pamphlet on the wood of Douglas 

 fir, was the first to publish an account of exceptions to this 

 law, which contrasts broadleaved and coniferous woods in such 

 a remarkable manner. He showed that, in spite of an increase 

 in the breadth of the annual zones, no decrease in the sp. 

 weight of the wood followed, but that it even increased. 

 Hartig, Cieslar, and others, proved this later for Douglas fir 

 and other coniferous woods. It has also been demonstrated 

 that in broadleaved woods a breadth of zone of more than 

 6 mm. results in a decrease in weight and hardness, and that 

 in coniferous woods there is a similar decrease when the 

 annual zones are less than 0'5 mm. broad. The observations 

 also show that woods with the same breadth of annual zones 

 are sometimes heavier and sometimes lighter. These excep- 

 tions to the law prove that another natural law exists by which 

 the effects of the former law may be sometimes enhanced, 

 sometimes diminished, and sometimes reversed. This natural 

 law, enunciated by H. Mayr in 1890 ("BieWaldungen von Nord- 

 america ") owing to his own investigations and to the mass of 

 indigenous and foreign wood then available, is as follows: 



Assuming identity of soil, the specific weight and hardness 

 of wood decreases with distance from the optimum climate 



