8. Resonance

If a piece of timber is struck with a hammer a sound is emitted which varies in pitch and character with the shape and size of the stick, and also with the kind and condition of the wood.

A dull, heavy sound indicates decay in the timber. Knots and irregularities in structure also affect the character of the sound emitted.

Thin boards may also be set vibrating by sound waves produced in the air. This property is utilized in the construction of many musical instruments and in architecture in the construction of sounding boards for reinforcing the voice of a speaker or the music of a choir or orchestra.

The ability of a properly shaped sounding board to respond freely to all the notes of an instrument or of the human voice depends first on the structure of the wood and next on the uniformity of the same throughout the board. Sounding boards should be made as thin as practicable, and the wood should be free from knots, cross grain or resinous tracts and thoroughly and carefully seasoned.

"Spruce is the favored resonance wood; it is used for sounding boards both in pianos and violins."

9. Weight Of Wood

The weight of any particular piece of wood depends upon two main factors: The proportion of wood substance contained in the piece and the amount of water contained in the wood.

The weight of the wood substance is practically the same in all woods, viz., about 1.6 times as heavy as water. As the wood cells, however, are in most cases filled with air, this reduces the weight of the wood and causes it to float. When wood is immersed in water for a long time the water soaks into the cells, and when most of them become filled the wood sinks.

When the wall of the wood fibre is very thick the wood sinks whether the cells are empty or not. As the proportion of wood substance in the dark bands of summer wood is much greater than in the lighter colored spring woods, it follows that those woods which contain the greater proportion of summer wood are the heaviest.

The difference in the weight of green wood and that which has been seasoned is due to the quantity of sap contained in the cells of the green wood.

In a thrifty young pine the wood is lightest at the centre of the tree and grows gradually heavier toward the bark; in an old oak the reverse is true.

The weight of wood is in itself an important quality. It assists in distinguishing different woods, and light weight, when coupled with great strength and stiffness, makes the wood especially valuable for many purposes. To a large extent, also, weight indicates the strength of wood, at least of the same species.

"For any given species of timber, and for any given degree of dryness, the strength is almost directly proportionate to the weight"

The weight of kiln-dried wood of different species is given by Mr. Roth as follows:

Approximate Weight Of Kiln-Dried Wood Of Different Species

The weight of ordinary lumber as found in lumber yards will generally average 33 per cent, heavier than the above values.

10. Moisture in Wood.* - "Water may occur in wood in three conditions: (1) It forms over 90 per cent, of the life-giving contents of the living cells; (2) it saturates the walls of all cells, and (3) it entirely, or at least partly, fills the cavities of the lifeless cells, fibres and vessels. In the sapwood of pine it occurs in all three forms; in the heartwood it merely saturates the walls.

"Of 100 pounds of water associated with 100 pounds of dry-wood substance in 200 pounds of fresh sapwood of white pine about 35 pounds are needed to saturate the cell walls, less than 5 pounds are contained in the living cells and the remaining 60 pounds partly fill the cavities of the wood fibres. This latter forms the sap as ordinarily understood. It is water brought from the soil, containing small quantities of mineral salts, and in certain species of trees, as the maple and birch, it also contains at certain times a small percentage of sugar and other organic matter. These organic substances are the dissolved reserve food, stored during winter in the pith rays of the wood and bark; generally but a mere trace of them is to be found. From this it appears that the solids contained in the sap, such as albumen, gum, sugar, etc., cannot exercise the influence on the strength of the wood that is sometimes claimed for them."

*This section is taken largely from Bulletin No. to, U S. Department of Agriculture.

In all exogenous trees the wood next to the bark contains the most water and the centre of the tree the least. In trees forming heart-wood the change from a moist to a drier condition is usually quite abrupt at the sapwood limit; thus in long-leaf pine the wood of the outer 1 inch of the tree may contain 50 per cent, of water, that of the next inch only 35 per cent, and that of the heartwood only 20 per cent.

Different trees, even of the same kind and from the same place, differ as to the amount of water they contain. A thrifty tree contains more water than a stunted one, and a young tree more than an old one, while the wood of all trees varies in its moisture relations with the season of the year.

In the living tree of certain species, and at certain seasons, the sap will flow when the tree is tapped, but from boards, timber, etc., the water does not flow out under normal conditions, but must be evaporated.

When the tree contains clefts or shakes water will sometimes flow from them when the tree is sawed into lumber. From very sappy wood water is forced out whenever the wood is warmed.

Before the living wood can be made suitable for building or other mechanical purposes most of the moisture which it contains must be eliminated. If the sap is not expelled or dried up it putrefies and causes decay. After a tree is cut, if left in a dry place, the moisture will gradually evaporate, and as this takes place the wood shrinks and often cracks; hence it is desirable that the wood should shrink all it will before it is put into a building or a piece of furniture.

II. Seasoning of Timber. - This is simply evaporating the sap and moisture contained in the green wood either by natural or artificial means.

After the log is converted into lumber, the boards, planks or timbers are "stacked " in the lumber yard for seasoning. In building the stacks the pieces are laid in courses or layers, usually about 6 feet wide, and inch strips are placed between the layers so that the air may circulate through the stack. It requires a long time for wood to season in the open air, and it never becomes sufficiently dry to answer for fine interior finishing or for furniture.

Framing lumber, however, is seldom dried in any other way, and it is seldom that it is allowed to stay in the yard for more than three or four months, consequently most of the lumber used in the frame, floors and partitions of ordinary buildings is generally comparatively green, and the seasoning must be completed in the building. It is the shrinkage of the lumber due to this final seasoning that causes most of the cracks in the interior of buildings having wooden Moors and partitions. To prevent these cracks it is very desirable that the building should be commenced in the spring, so that the frame may season during the warm, dry weather of summer.

For special cases, where it is very desirable to have well-seasoned lumber, as for truss timbers or beams supporting brickwork, a search through the lumber yards will often result in finding a few pieces that have been seasoning for several years. The railway corporations and many large manufacturing concerns keep a large stock of lumber constantly on hand, so that it may be kept in the stack several years before using.