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Rh filled up and hardened, becoming duramen or heartwood, the remainder being alburnum or sapwood. The sapwood is softer and lighter in colour than the heartwood, and can generally be easily distinguished from it. In addition to the strengthening of the wood caused by the drying up of the sap, and consequent hardening of the rings, there is another means by which it is strengthened—that is, by the compressive action of the bark. Each layer, as it solidifies, expands, exerting a force on the bark, which eventually yields, but in the meantime offers a slight resistance, compressing the tree throughout its bulk. The sapwood is generally distinctly bounded by one of the annual rings, and can thus be sometimes distinguished from stains of a similar colour, which are caused by dirty water soaking into the timber while it is lying in the ponds. These stains do not generally stop abruptly upon a ring, but penetrate to different depths, colouring portions of the various rings. The heartwood is stronger and more lasting than the sapwood, and should alone be used in good work. The annual rings are generally thicker on the side of the tree that has had most sun and air, and the heart is, therefore, seldom in the centre.

Seasoning Timber.

Advantages of Seasoned Timber.—Seasoned timber differs from unseasoned principally in having the sap and moisture removed; this makes it drier, lighter, and more resilient or springy. It is less liable to twist, warp, or split. The advantages of using seasoned timber are that it works more easily under the saw and plane, and retains its size and shape after it leaves the hands of the carpenter or joiner. Unseasoned stuff warps and shrinks, and, besides being unsightly, is liable to cause failures in structures of which it may form a part; it is also very liable to decay from putrefaction of its sap.

Natural Processes of Seasoning Timber.—Timber produced from a newly felled tree is full of moisture, and this must be extracted by drying or seasoning. Timber cut down in the autumn, after the sap has

Fig. 127.—Hardwood Stacked for Seasoning.

formed the new layers of wood, is best seasoned by cutting it into planks and stacking them horizontally in open order under cover, exposed to a free current of air, and protected from ground moisture. Hard woods are generally stacked with thin strips between them, placed transversely every 2 ft. or so (Fig. 127), and soft woods by laying them on edge with spaces between, the direction being crossed in adjacent courses. The time occupied is, say, two years. Balk timber is best seasoned by putting it under water in a running stream for a few weeks, then stacking it loosely with some protection from sun and rain. These are termed natural processes. For protecting the stacked timber from the action of the sun and high winds, a shed with open ends,