LESSON THE THIRD.
Eugene to Paul, when the latter came into his room,"we visited the cellars and the ground floor; now weshall take a walk among the garrets of the chateau. But first I must show you what' is meant by a roof truss. The simplest truss (Fig. 13) is composed of four pieces of wood : two principal rafters, a tie-beam, and a king-post. The two inclined pieces A are the blades; the horizontal piece B is the tie-beam, and the vertical piece C the king post. The upper ends of the blades meet in the king-post, as I show you in the detail D, - namely, by the means of two tenons E, which fit into two mortises F, and a shoulder G, which make the whole pressure of the timber bear into the notch I. The lower ends of the blades are similarly connected at the two extremities of the tie-beam, as this other detail H shows us. The king post is also connected by a tenon, in the centre of the tie beam, but loosely, and without bearing upon this tie-beam.
When the tenons are let into the mortises, pegs of wood are driven into the holes marked to fasten the whole well. The more pressure there is on the top, ivt, the more the two blades tend to spread at the foot; but these, being fixed at the two ends of the tie-beam, tighten the latter like the string of a bow. The more this tie-beam is strained, there fore, the less it is inclined to bend, and the object of the king-post is only to suspend it by its centre, and to connect the heads of the blades. But between bt and N these rafters may bend under the weight of the roof covering; two struts, 0 P, therefore are added, which arrest this bend ing by bringing the pressure to bear on the king-post, so that the latter is in its turn strained between Nt and P. As wood will not stretch, the point P is fixed, and the two points 0 likewise.
"Now that you know what is the simplest roof-truss, let us go up into the roofs." These roofs were old, and had been repaired and strengthened many times, and formed a complication of timbers difficult enough to understand.
"Formerly,"said Eugene,"that is, more than a cen tury ago, they used to make roofs such as you see here : every rafter was framed, that is, each of the rafters com posed a truss, except the tie-beam, which was introduced only at intervals. Then wood was in plenty, and they scarcely thought of economising it. At present it is less abundant, and there is a difficulty in procuring a con siderable number of pieces of large dimensions. The noble forests that covered the soil of France have been foolishly wasted, and long timbers of heart of oak are rare. It has therefore been necessary to economise them. The expedient has been adopted of placing strong trusses at a distance of about 12 feet from each other. On these
trusses have been placed purlins, which are the horizontal pieces you see on this side; and on these purlins longer or shorter rafters have been placed to receive the lathing for the tiles, or the battens for the slates. But all timber roofing should be fixed upon sleepers, which are those horizontal pieces resting on the top of the walls, which bind and isolate the tie-beams from the masonry; for it is to be observed that timber is preserved for an indefinite time in the free dry air, but soon decays in contact with a moist body, such as stone is. Look here at this piece of wood, almost buried in the masonry; it is nearly reduced to touchwood, while the blade above, which is in the free dry air, is as free from rot as if it were new.
"Formerly upper floors were made by putting joists resting on beams and the walls. These joists and beams remained visible, as you may see still in the kitchen and the large hall on the ground floor, which serves as a store-room. The air therefore could circulate round these timbers, and they might last for centuries. But it was considered that thus exposed they were not pleasant to look at - that they were not clean, and allowed spiders to spin their webs in the interspaces. Laths were there fore nailed under these joists, and this lathwork plastered so as to form what we call a ceiling. Timbers thus inclosed and deprived of air, heated ' (as carpenters call it), that is, they fermented and soon began to decay. In fact, floorings with exposed joists which had resisted the action of time for centuries decayed and broke down in a short time after being inclosed. I may add that formerly, before using timber in building, they took the precaution of leaving it exposed for some years to the action of the sun and rain. They even kept it for some time in water, to free it from the sap (for sap is the ferment which makes wood rot). When the timber, after having been barked and roughly squared, had remained in the open air for five or six years, it was used. But now-a-days we are in a hurry, and make use of timber that has not been cut more than a year. It is not dry, it retains its sap, and if it is then enclosed it ferments rapidly, so that in a few years the largest beams are completely rotten. Prudent architects therefore hesitate to use wood for floors. Yet its use - even if only partially dried - would not entail serious in convenience if it was not covered with plaster. The worst that could happen would be the occurrence of cracks and shrinkings. It would dry when in use, as it would have dried in the open air.