Dry Rol. The defects which have been mentioned above are all of such a kind that they can be readily detected in the timber before it has been put in position in a structure, and, therefore, the use of the timber so affected may be avoided, but dry rot, while it is probably the most common and the most dangerous defect of them all, may start and spread rapidly in timber which appears to be absolutely sound when it is put in place. Dry rot is a disease which fastens itself upon the wood and spreads from one part of it to another, causing it to lose its strength and cohesive power and even to decay altogether. It may he readily seen that this process can lead to most serious results when it takes place in timber which is depended upon to carry heavy loads. Large beams and posts have been known to fail and thereby cause considerable damage solely because of dry rot, and others have been so weakened by the ravages of this disease that they have yielded when subjected to slight fires which would have had very little effect upon them if they had been sound.
The timber in which dry rot is most to be feared is that which is kept alternately wet and dry, while that which is always either entirely submerged in water or absolutely dry appears to be able to last indefinitely without a sign of the disease. For this reason wood piles should always be cut off below the water level. Decay takes place very rapidly when the wood is in a confined position where the gases can not escape. The ends of beams buried in brickwork and the ends of posts fitting into iron caps and bases are examples of such cases, and special precautions should be taken to allow the air to circulate freely around such woodwork wherever this is pos sible. Woodwork which is in contact with wet or damp materials, such as wet concrete or masonry in which the mortar has not dried out thoroughly, is peculiarly liable to dry rot. Wood flooring laid on top of newly-placed concrete slabs and immediately covered with some other substance has been known to rot very quickly. It is also noticeable that this form of decay seems to be hastened by warmth and is more common in the southern climates than in the northern. It may be prevented by introducing into the timber certain salts such as the salts of mercury, also by heating the wood to a temperature above 150° F. and keeping it at that temperature. As precautionary measures, all wood should be thoroughly seasoned before being painted, as good ventilation as possible should be pro vided for it, and it should be kept from contact with anything from which it can absorb moisture. Posts should have a hole about one and one-half inches in diameter bored through them from end to end, and other holes near each end bored through them crosswise, so as to provide for the free circulation of air in the interior of the post.
Wet Rot. There is another form of decay which affects wood in a manner somewhat similar to dry rot, but which takes place in the growing tree. It is known as "wet rot" and is caused by the
wood becoming saturated with water which it may absorb from a swamp or bog. Wet rot may be readily communicated from one piece of wood to another by contact so that it is apt to spread rapidly.
Knots. Knots are more or less common in all timber, and consist of small pieces of dead wood which occupy a place in the body of the log with sound wood all around them. These bits of dead wood have no connection with the living wood about them, so that in the course of time they work loose, and when the log is sawed up into boards the pieces of dead wood fall out leaving round or irregular-shaped holes. Knots are formed at the juncture of the main tree trunk with branches or limbs, while such branches are still voting and green. At such points the fibers of the main trunk, near the place where the branch comes in, do not follow straight along up the trunk, but are turned aside so as to follow along the branch as shown in Fig. G. Frequently such a branch is broken off near the trunk of the tree when it is still young, while the tree itself continues to grow and the trunk increases in size until the end of the branch which was left buried in the main trunk is entirely covered up. 'A feanwhile the end of the branch dies and a knot is formed. The presence of a limited number of knots will not harm a piece of timber which is subjected to a compressive stress so long as they remain in place and do not drop out, but they very greatly weaken a piece subjected to a tension stress or used as a beam. Knots always spoil the appearance of woodwork which is to be polished.
The defects heretofore considered result from the natural growth of the tree and are not at tributable to the handling of the timber after it has been cut, but there are several classes of defects which are caused by the seasoning of the timber and which have little or nothing to do with the growth of the tree. Among these are the actions known as "warping" and "check ing." Warping. This is the result of the evapora tion or drying out of the water which is held in the cell walls of the wood in its natural state, and the shrinkage which naturally follows. If wood were perfectly regular in structure, so that the shrinkage could be the same in every part, there would be no warping, but wood is made up of a large number of fibers, the walls of which are of dif ferent thicknesses in different parts of the tree or log, so that in drying one part shrinks much more than another. Since the wood fibers are in close contact with each other and interlaced, thus making the piece of wood rigid, one part can not shrink or swell without changing the shape of the whole piece, because the piece as a whole must adjust itself to the new conditions; consequently the timber warps.