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Lesson the Third

"There is no great disadvantage, then, in employing wood' newly cut for roof-timbers, which are generally left exposed. They dry where they are. They warp, but do not perish of dry-rot.

"As we shall not be able to find wood absolutely dry for your sister's house, we shall leave the floor-joists visible, and endeavour by simple and economical means to render them not unsightly.

"But you ought to be well acquainted with the qualities of timber. I will not tell you that nature has caused these large vegetable growths which we employ to grow for our pleasure or use. Nature is, I think, very little concerned as to whether the oak or the fir would serve any of our purposes; and if human intelligence has been able to take advantage of these materials that spring up before us, it is after having recognized and verified their properties by experience. Unfortunately, it would seem as if the results of this experience did not tend to increase; and judging from the way in which building-timber is most frequently employed, we might be led to suppose that we are less informed than were our predecessors, or that we have lost that habit of observation with which they were familiar.

"Wood, being composed of fibres more or less lax or compact, possesses a, considerable power of resistance to a pressure exerted along these fibres, but is easily bent or crushed under a pressure exerted across these same fibres. Thus a log of wood 4 inches in diameter and a yard or so long, placed on end, will support, without being crushed or contorted, a pressure of 40,000 lbs.; whereas this weight will break or crush it if placed horizontally, as you would crush a reed under your foot. Take a thoroughly sound bit of straw, 4 inches long, and place your finger on one end of it, holding the straw vertically on a table; you will have to press pretty strongly on it to bend it, while the least pressure on the same straw placed horizontally will flatten it. The straw is a tube. A tree consists of a series of tubes, some enveloping others. The more numerous, close and fine these tubes are, the more does the trunk resist pressure, either in the direction of its length or its thickness. But this shows us that to enable the wood to retain its power of resistance we must employ it as nature gives it to us; and in fact this was done formerly. Each piece of timber was cut from a tree of larger or smaller size, as the case required, but they did not split the tree lengthwise to get several pieces of timber; for the heart being harder and more compact than the sap-wood (which is the spongy envelope beneath the bark), and the concentric layers of wood being the closer and tougher in proportion to their nearness to the bark, if you split a tree in two lengthwise one of its faces is much more resisting than the other, the equilibrium is disturbed, and flexure is easily produced under a weight. The outer layers, being the more recent, are more spongy and lax in texture than the older layers that are near the heart; con sequently the process of drying makes these outer layers shrink more than the inner : hence curvature. Let A (Fig. 14) be a split piece of wood; the layers B are harder and more compact than those marked C, which contain more moisture and whose fibres are softer. In drying,

therefore, this piece of wood will present a hollow bend on the outer side, as I show at D. If the wood is left entire, as at E, the effects of drying will neutralize each other, and the piece will remain straight.

"Look at this old roof, whose rafters are framed (Fig. 15) : the wall plates, A, are cut from small trunks, the heart being in the centre. It is the same with the rafters B, the tie-beams C, the collars D, the king-posts E, the foot-pieces F, and the foot-posts G; all these pieces, therefore, have preserved their rigidity, and none of them has been bent, because they were used dry and were unsplit trunks. Ob serve, on the contrary, this purlin, H, placed on this truss, I, of recent date; it is bent not so much on account of the weight of the rafters it supports as because it is split and the carpenter has unadvisedly turned the heart on the inside. If he had done the contrary, - if the heart had been placed next to the rafters, - this purlin would prob ably not have bent, perhaps have even become more rigid - that is to say, it would be convex on the outer side. Carpenters, however, are but men, and they do not care to give themselves trouble when they think they can avoid it. The man that put this purlin here found it more convenient to place it on its sawn side than to turn it the other way with the flat under the rafters.

"Considering this quality of wood, and of oak especially (whose internal fibres are harder and closer than the outer layers), when we have to place a piece of wood horizontally on two points of support or posts, and wish to give it all the strength possible to bear a weight acting on its centre, we saw it in two lengthwise, and turning the flat faces outside, bolt these two pieces together, as shown here (Fig. t6). Then as the heart-wood is outside, and the two furnished with good heads and nuts, they must remain straight; the tendency to curvature in the one neutralizing that in the other, these two opposing forces tend to make the piece more rigid, so that, if you take a piece of timber that is slightly bent naturally and then place these two pieces with their hollow downwards, - that is, after having placed one upon the other, putting the tail of one against the head of the other, - you will have given this piece of wood all the resisting power of which it is capable.

"It is in this way that clips and all coupled pieces should be placed. Here, for example (Fig. t8), you see a pair of clips where the sawn faces have been turned outside to replace a decayed tie-beam. We call clips those pieces of wood which, in pairs, usually clench two or more parts of a framfng. These clips, A, hold fast by means of notch ings, the blades B, the king-post C, and the two struts D. Iron bolts with screw-nuts tightly hold the notchings of the clips, like a pair of jaws, against the timbers which have to be kept in their place. But this is enough for to-day, and you will have plenty to do to make a fair transcript of this lesson in carpentry between now and this evening."

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