In any given piece of wood, then, the weight is determined by two factors, the amount of wood substance and the amount of water contained therein. The amount of wood substance is constant, but the amount of water contained is variable, and hence the weight varies accordingly. Moreover, considering the wood substance alone, the weight of wood substance of different kinds of wood is about the same; namely, 1.6 times as heavy as water, whether it is oak or pine, ebony or poplar. The reason why a given bulk of some woods is lighter than an equal bulk of others, is because there are more thin-walled and air-filled cells in the light woods. Many hard woods, as lignum vitae, are so heavy that they will not float at all. This is because the wall of the wood cells is very thick, and the lumina are small.
In order, then, to find out the comparative weights of different woods, that is, to see how much wood substance there is in a given volume of any wood, it is necessary to test absolutely dry specimens.
The weight of wood is indicated either as the weight per cubic foot or as specific gravity.
It is an interesting fact that different parts of the same tree have different weights, the wood at the base of the tree weighing more than that higher up, and the wood midway between the pith and bark weighing more than either the center or the outside.[4]
The weight of wood has a very important bearing upon its use. A mallet-head, for example, needs weight in a small volume, but it must also be tough to resist shocks, and elastic so as to impart its momentum gradually and not all at once, as an iron head does.
Weight is important, too, in objects of wood that are movable. The lighter the wood the better, if it is strong enough. That is why spruce is valuable for ladders; it is both light and strong. Chestnut would be a valuable wood for furniture if it were not weak, especially in the spring wood.
The weight of wood is one measure of its strength. Heavy wood is stronger than light wood of the same kind, for the simple reason that weight and strength are dependent upon the number and compactness of the fibers.[5]
THE STRENGTH OF WOOD.
Strength is a factor of prime importance in wood. By strength is meant the ability to resist stresses, either of tension (pulling), or of compression (pushing), or both together, cross stresses. When a horizontal timber is subjected to a downward cross stress, the lower half is under tension, the upper half is under compression and the line between is called the neutral axis, Fig. 42.