SPECIFIC GRAVITY. 23 



lated spruce trees. lu a crowded wood, on the contrary, the crowns 

 of the trees are reduced to the uppermost part of then- boles, and 

 this portion therefore obtains more nourishment than their bases ; 

 broader annual zones are therefore produced in the upper part 

 of the bole, which approaches in shape to that of a cylinder. 



The height of the tree also influences matters in this respect, 

 and Baur has shown that spruce and beech trees increase 

 cylindricity of bole up to 60 — 75 feet in height, but that where 

 the height exceeds 75 feet, the cylindricity falls-off ; so that in 

 closely grown, nearly even-aged woods, the form-factor varies 

 with the height of the trees. A similar relation subsists between 

 the shape of the bole and the age of a tree, as the form-factor 

 falls-off for very old trees, especially after they have been heavily 

 thinned in order that they may put on increment, due to in- 

 creased exposure to light. 



Section IV. — Specific Gravity of Wood. 

 1. General Account. 



The specific weight of wood varies considerably under different 

 conditions, not only according to species, but also to locality 

 and the mode of formation, the age of a particular tree, the 

 part of the tree from which any piece of wood is taken, its 

 degree of moisture, amount of resin it contains, and several other 

 factors. 



The mere knowledge of the average specific gravity of any 

 particular piece of wood is not sufficient to determine its 

 weight. 



Physicists distinguish absolute from specific weight, the 

 former being determined by a balance, and depending on the 

 pressure due to gravity which a given mass exerts on any object 

 supporting it. The unit of weight is that of 1 cubic centi- 

 meter of water at its greatest density, 4 C, and is termed the 

 gram. 



Under the term specific gravity is understood the ratio which 

 the weight of a certain volume of wood bears to that of an equal 

 volume of water. The specific gravity also indicates how much 

 heavier or lighter any wood is than water, and whether it can be 

 floated or not. Since a cubic centimeter of water weighs a gram, 



