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Lesson the Second

"Is it all clear to you? Well, let us go and look at that little flight of steps which perhaps you have never attentively examined. It is 4 feet 3 inches wide, which was large enough to afford a passage to the queues of wine. Observe (Fig. I I) : the ramping vault is com posed of as many arches, one above another, as there are steps; that is extremely well managed, solid and easily built. In fact, when the stone steps are laid, over above them is successively fixed the same wood centre which, of course, is raised at each step; and upon this centre an arch is built, which is quickly done, as the stones are worked ready. In this way the arches follow the section of the steps, and the centre being shifted - after each arch is `keyed - to the next step commencing from the bottom, two men can turn five or six of these arches in one day, so that if there are twelve steps, this ramping vault may be built in two days. Look, I will show you how this construction should be denoted in perspective and geometrical section in your résumé to-day - A and B.

"Let us go up to the ground floor. Look at the efflor escence resembling cotton wool on the interior of the walls : it is the saltpetre which forms inside the stone, and, through the humidity of the ground, crystallizes on the surface. This saltpetre affects the stone injuriously, ulti mately eats it away, and throws off any painting that we might endeavour to use as a counteractive on the interior surface. Mastic cements are made to stop the effects of the saltpetre, but these means only delay its appearance for a short time without curing the evil, and this cement soon falls off in a crust. It is therefore necessary in build ing, especially in the country, to prevent the damp of the ground from rising up in the walls, and to stop it at the ground level. The interposition of a layer of pitch beneath the plinth has sometimes been tried, in order to prevent the absorption of. damp by the stones - or what is called capillary attraction - but this method is very inefficient. The pitch oozes out under the pressure, as it does not harden sufficiently to bear that pressure, or it decomposes and combines with the lime. The best plan is to lay a course of slates in the mortar-bed between the first lower courses of the plinth. The slate effectually hinders that effect of capillary attraction, and the damp is unable to rise in the walls.

"Now observe this front wall in the court : it forms a protuberance at the floor level of the first story. We call that a bulging of the wall. Instead of preserving its vertical plane, as it should have done, it has bulged out; and why? Because it has been thrust out by a force acting from within outwards. What is that force? It might be an arch; but there is no arching on the ground floor. It can therefore be only the floor. It is not clear at first sight how a floor, which is a horizontal plane, can thrust; for to thrust, we must suppose the floor to expand in one direction, which cannot be. But see what happens. Give me your best attention.... Formerly, to compose a floor, large beams were laid from wall to wall, and upon these beams lighter pieces of timber, called joists; then on these was laid a bed of earth, gravel, or sand, and upon that a surface of mortar to receive the tiling. This made a very heavy mass. Now, as a piece of timber, even of considerable section, bends in time under its own weight - . that is to say, from being straight becomes curved - its tendency to bend will be proportionally greater when it is weighted. The more it bends, the more powerful its

thrust upon the inner surface of• the walls in which it has its bearing. It is this pressure upon the interior surface that tends to thrust the wall outwards. But if, as in this case, in order to relieve the bearing of the beams, struts of wood have been put underneath (Fig. 12), this effect of thrust is all the more sensible because the arm of the lever is longer. You do not quite understand, I see. A sketch will make it clear to you. Let A be the section of the Wall, or, if you will, its thickness. If the beam bends according to the line C D, there occurs a pressure at D, which produces a thrust at F and the rounding of the wall, as indicated by the dotted curves. Supposing even that in lieu of the strut E we have a stone corbel, the effect produced will be the same, though less forcible, unless the tail of the corbel reaches through the wall, as you see marked at I, and this tail K is weighted in such a manner that the weight neutralizes the pressure which the beam exerts at the end L. This has not been done here, where instead of the wood strut, a corbel was put. This corbel has but a middling hold in the wall, and the latter, formed of small stones not very well built, has not sufficient 'cohesion to resist the thrust exerted by the deflection of the beams. But why, you will ask me, has this effect been produced at the floor level of the first story and not above? Because, by the effect of the bulging we find here, the wall has inclined above towards the inside, and has thereby squeezed the second floor - its surfaces being placed, by"theirvery inclination, perpendicularly to the curve line of the upper beams, as I indicate to you at NI, exaggerating the effect for the purpose of making it clearer.

"You see that each detail merits attention, and that the -builder ought to have a good reason for everything he does.

"In work of every kind we learn to avoid faults only by analysing and searching into their causes and ascertain ing their effects. To become a good builder, therefore, it is not enough to familiarize one's self with rules of construction, which cannot provide for all contingencies; we must see and observe much, and ascertain defective points in buildings that have been tested by time; just as physicians become able to determine what a good constitution is only by studying diseases and their causes. For the most part we appreciate what is good only through observing what is bad;' if, in the absence of the bad, we are able to admit that there is such a thing as the good. An old proficient in architecture, who, when I was about your age, was so kind as to aid me with his advice, used often to say to me : I can tell you, my dear fellow, what you must avoid in the art of building; - as to explaining to you in what the good and the beautiful consist, you must find out that yourself. If you are a born architect, you will know well enough how to discover it; if not, all that I could show you, all the examples I could place before you, would not give you talent.' And he was right. The sight of the finest works in architec ture may pervert the minds of students, if it's not been explained to them how the authors of these works suc ceeded in making them beautiful by having avoided such or such faults.

"But you have enough to write out for to-day. Make a fair copy of these sketches opposite your text, and we will examine it this evening."

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wall, floor, thrust, effect and beams