Variable Qualities of Wood.
The choice is additionally complicated by the very great variation found in the strength and characteristics of trees of exactly the same species, and also of different portions cut from the same tree. The nature of the site upon which a tree is grown exercises a marked influence upon its properties, while as a general rule, it may be taken that the greater number of annual growth rings per inch, the greater the strength. It is also a general rule that up to certain diameters, the timber contained in that part of the tree the greatest distance from the pith, or centre, is the stronger.
The wood obtained from the base of a tree is heavier than that at the top, and one finds the influence of this in the necessity for balancing and alternating the different laminæ of air-screws before gluing.