This section is from the book "Cyclopedia Of Architecture, Carpentry, And Building", by James C. et al. Also available from Amazon: Cyclopedia Of Architecture, Carpentry And Building.
In speaking of wood we are accustomed to use certain words to express our idea of its mechanical properties, or of its probable behavior under certain conditions. Thus we say that a wood is hard, or tough, or brittle, or flexible, and frequently we use these terms without having a clear understanding of just what they mean. A very brief discussion of some of these properties or characteristics of lumber will now be given in order that we may see what peculiarities of structure or of growth cause them.
If a block of wood is struck with a hammer when lying on a bench, the hammer-head will make an impression or dent in the wood, which will be deeper or shallower according as the wood is soft or hard. A wood is said to be very hard when it requires a pressure of about 3,000 pounds per square inch to make an impression one-twentieth of an inch deep. A hard wood requires only about 2,500 pounds to produce the same effect. Fairly hard wood will be indented by a pressure of 1,500 pounds, and soft woods require even less. Maple, oak, elm, and hickory are very hard; ash, cherry, birch, and walnut are hard; the best qualities of pine and spruce are fairly hard, and hemlock, poplar, redwood, and butternut are soft.
"Toughness" is a word which is often used in relation to timber, and implies both strength and pliability, such as is found in the wood of the elm and the hickory. Such timber will withstand the effect of jars and shocks which would cause other woods like pine to be shattered.
Timber is said to be flexible when it bends before breaking instead of breaking off short, or, in other words, a flexible wood is the opposite of one which is brittle. The harder woods, taken from the broad-leaved trees, are usually more flexible than the softer woods, taken from the cone-bearing trees. The wood of the main tree trunk is more flexible than that of the limbs and branches, and moist timber is more flexible than dry wood. Hickory is one of the most flexible woods.
Most woods split very easily along the grain, especially when the arrangement of the fibers is simple, as in the conifers. In splitting with an axe, the axe-head acts as a wedge and forces the fibers apart, so that usually the split runs along some distance ahead of the axe. Hard woods do not split so easily as do soft woods, and seasoned wood not so easily as green wood, while all timber splits most easily along radial lines.