Page:Cassells' Carpentry and Joinery.djvu/58

42 diagonally in any longitudinal cut, as in Fig. 170. Oak with twisted fibres will not retain its shape when squared, but is very suitable for splitting up into wall plugs.

Upsets.—These are portions of the timber where the fibres have been injured by crushing, as in Fig. 171.

Waney Edges.—These occur when the top end of the tree is not large enough to hold up to the full size to which the lower end is squared, as shown by Fig. 172. These balks may be used for piling without detriment if the top end be driven downwards.

Wide Annual Rings.—These generally indicate soft and weak timber.

Wind-cracks.—Shakes or splits on the sides of a balk of timber, caused by shrinkage of the exterior surface, as in Fig. 172, are called wind-cracks.

Wet Rot.—Timber that has been lying long in the timber ponds, and subjected to alternations of wet and dry, may be so soft and sodden as to have reached the stage of wet rot. The term "wet rot" implies chemical decomposition of the wood; whereas dry rot is the result of a fungous growth.

Cause, Cure, and Prevention.—Dry rot is a special form of decay in timber, caused by the growth of a fungus, Merulius ladhrymans, which spreads over the surface as a close network of threads, white, yellow, or brown, and causes the inside to perish and crumble. Various causes may combine to render the timber favourable to the growth of this fungus—namely, large proportion of sapwood; felled at wrong season when full of sap; if cut down in the spring or the fall of the year instead of in midwinter or midsummer, when the sap is at rest; stacked for seasoning without sufficient air spaces being left; fixed before thoroughly seasoned; painted or varnished while containing moisture; built into wall without air space; covered with linoleum: exposed to warm, stagnant air, as under kitchen floors. There is no cure when the fungus has obtained a good hold. The worst must be cut out and remainder painted with blue vitriol (cupric sulphate). The best preventive is to use only well-seasoned timber and to keep it well ventilated.

Detection and Treatment of Dry Rot.—When dry rot is suspected in a floor the floor-boards should be lifted at the corners of the room, or at dead ends of passages, or wherever signs of weakness show themselves, and the surfaces of the joists, wallplates, and under side of the floor-boards should be closely examined for fungus, mildew, or any unhealthy sign, such as a brown semi-charred appearance. If any is found, the worst parts should be cut out and renewed, the remainder well scraped over, including the walls, and well washed with a solution of blue copperas (sulphate of copper). If the earth below is found to be damp, a layer of cement concrete should be spread over it, not less than 4 in. thick. Air bricks and ducts should be placed in the walls on opposite sides, to get a through current, as moist, warm, stagnant air is the most potent aid to dry rot; and every endeavour should be made to obtain thorough ventilation. The means of prevention are: Thorough seasoning, free ventilation, creosoting or charring if necessarily exposed to damp earth, and painting with vitriol or cupric sulphate.

The best way to preserve from decay wood that is to be buried in the ground is to creosote the wood; this does not mean painting the wood over with tar, but proper creosoting by the regular process. The butt-end of a post to be placed in the ground may be charred over a wood fire, quenching with water when the wood is charred, say, ¼ in. to ½ in. deep. This will prevent rotting and the attacks of worms, but it is necessary that the wood should be previously well seasoned, or the confined moisture will cause decay. Chloride of zinc and water, about 1 to 4, in which wood is steeped under Sir Wm. Burnett's system (see p. 34), preserves the timber from decay and renders it incombustible. A method sometimes adopted is to bed the posts in cement concrete, but this is not quite so good as creosoting.

Timber trees are usually divided into two great classes:—(a) Soft woods or coniferous woods; (b) hardwoods or leaf woods.