It is now known that decay in wood is caused by fungi or low forms of plant life which cannot live without a certain amount of water, food, heat and air.

A fence post decays first at the place where it enters the ground, because at that point the conditions are most favourable. If wood can be kept entirely under water, one item—air—is lacking, so the fungous growths cannot exist and the wood will last indefinitely. This has been proved in many instances. One of the old Viking ships was raised from the bottom of the Christiania Fjord, Norway, after having been under water for a thousand years and it was found to be in a perfect state of preservation. Even the rudder oar or steerboard and wooden shields were intact.

As soon as it was brought into the air the process of decay began, and it became necessary to coat it with preservative. It stands today, 103 feet long, in the museum at Christiania. Many other instances of under-water preservation might be mentioned.

The other extreme is also true. Wood which is kept perfectly dry will last indefinitely, as in the case of woodwork taken from the pyramids of Egypt, 3,000 years old, which is found to be perfectly preserved.

But when wood is alternately wet and dry it decays rapidly. A pile driven into the bottom of a tidal river is a good illustration. If such a pile be divided into four sections (see Fig. 233), a is always in the ground, b is always in the water, c is alternately in air and water, d is always in the air. Sections a and b may be considered to be under the same conditions and should last the longest; c should decay first: d would last indefinitely if the atmosphere were always perfectly dry; but humidity and rain, air and heat combined finally bring about decay, and although this part of the pile will last longer than c it will in time decay. Section c should be coated with a preservative.

Various woods under the same conditions act very differently and according to no well understood law. For example, in contact with the soil black locust is our most durable wood. It is very hard, and its life under these conditions is estimated at from ninety to a hundred years. Red cedar comes next, though it is soft wood. Oak decays in a few years; chestnut, much softer, lasts two or three times as long. Our approaching timber famine has induced a study of this subject, since the preservation of wood is becoming an absolute necessity.

It has been found that certain materials put on the wood before it is placed in the ground prolong its life. Coal tar, wood tar, paint, and creosote all help, but creosote has so far proved to be the best. It is one of the by-products of coal tar and is being used extensively by the railroad companies for prolonging the life of ties.

Experiments with creosote have brought out some very interesting facts. It has been found that after being treated with the hot creosote all woods resist decay alike, regardless of their hardness or softness. Consequently, a treated cheap wood will last as long as an originally valuable one. This is a great gain, as it allows us to make use of wood like the poplar which would otherwise be practically of no value.

The various coatings we put on wood, such as paint, varnish, oil, etc., are intended not only to beautify but to preserve it, which they do by filling up the pores and excluding moisture, preventing fungous growths, etc. All of these coatings should be put only on dry wood, else they prevent evaporation of the sap and may hasten decay.

Drying lumber increases its strength, as it has been found by experiment, even as much as 400 per cent. if no checking occurs. When this happens it counteracts much of the gain, and if the wood absorbs moisture once more the strength will decrease. The strength of the various timbers varies greatly, and sap wood is usually weaker than heart wood.

The strains that may be brought to bear on timbers are illustrated in Fig. 234, the arrows indicating the directions in which the forces operate. At a the wood is under tension, the forces at work on it tending to pull it apart. At b the piece is under compression, the forces tending to reduce its length by forcing its fibres together. A pillar supporting a weight is under compression. At c the weight tends to bend the beam. The upper part is under tension, the lower part under compression. This is known as beam action, and depends on whether the beam is supported at one end, as shown, or on both ends. Also it is important to know whether the beam has a uniformly distributed load or whether the weight is at one point only. The problems relative to beam action are largely of an engineering character and involve considerable mathematics.

Shearing is the sliding of one part of the timber along the grain. If a piece of wood is cut to the form shown at d and a weight applied at e, the tendency will be for this upper part to slide down as shown at f. When this occurs, shearing has taken place.

The strength of wood differs in resisting these various strains, the tensile strength being greater than the crushing or compressive strength. Ash, for example, has a tensile strength of 16,000 pounds to the square inch, but its crushing strength is only 6800 for the same size. The tensile strength of dry white pine is 10,000 pounds, its crushing strength 5400 pounds, and its shearing strength varies from 250 pounds to 500 pounds, showing that its weakest point is along the grain. If the young woodworker becomes ambitious enough to think of designing a bridge or large building he can find these figures in any engineers' hand-book. There are so many important factors to be considered that the amateur will do well to go ahead with great caution. Knots and other defects reduce the proportionate strength of large beams greatly, so that it would not be safe to assume that a beam 6 inches square would be 36 times as strong as a piece 1 inch square.

In upright posts of considerable length, not alone the crushing strength must be considered, but a bending action enters into the problem. Wherever the question of danger to life enters, as in a bridge or a house, it is wise to leave a large margin for safety. We realize this fully when we read of a grand stand holding hundreds or thousands of people collapsing under the weight. The architect has also to reckon with still other elements, such as wind pressure and vibration.