WKIGHT. 



calculated from it serves to give at once pretty accurately the 

 quantity of timber in any number of trees of that species and age 

 or size class, if we knov? their several girths at breast-height and 

 the respective lengths of their boles. 



Section II. — Weight. 



Weight as a quality of wood affecting its employment is usually 

 of only slight importance. Generally speaking, it need be con- 

 sidered only in the case of superstructures when extra strong 

 supports would be very expensive, and in that of portable articles. 

 But although of itself weight does not always possess much intrinsic 

 importance, it nevertheless merits careful consideration, in that 

 many other qualities of the first moment, such as hardness, dura- 

 bility, combustibility and heating power, swelling and shrinking 

 with varying quantities of moisture, &c., are intimately connected 

 with it. 



The substance proper of all woods is slightly heavier than 

 water, and its weight does not differ much for the different species. 

 As a rule, it is heavier in conifers than in broad-leaved trees 

 amongst which the hardest and absolutely heaviest stand more 

 frequently the lowest. It is heavier in young than in old trees. 



The main cause of difference in the absolute weight of woods is 

 their anatomical structure. The experiments of Theodor Hartig 

 have established for the European woods the following percentages 

 for the space occupied respectively by the solid wood-substance 

 and the water and air contained in the tissues : — 



Wood 

 substance. Water. Ait. 



The hard woods, ... 44-1 24'7 31-2 



The soft woods, including conifers,... 27*0 33-5-31 7 39-5. 40-4 

 For our hardest Indian woods, such as HardwicJda binata iron 

 wood, ebony, &c., we may safely assume at least 50 per cent, as 

 the proportionate space occupied by the solid substance of the wood 

 the amount of air-space diminishing in almost equal measure. 



The denser tissue of the exterior zone of a concentric ring is 

 obviously heavier than the more or less porous tissue of the inte- 

 rior zone. From this it follows (a), that in conifers, in which class 

 of trees, as an almost invariable rule, the width of the outer zone 

 remains practically constant, while increase in rapidity of growth 

 bears entirely on the inner zone, the weight of the wood is gener- 

 ally inversely proportional to rapidity of growth ; (6), that in 

 broad-leaved species, in which the largest and most numerous 



