CHAPTER X

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THE ARCH LINE.

§ I. We have seen in the last section how our means of vertical support may, for the sake of economy both of space and material, be gathered into piers or shafts, and directed to the sustaining of particular points. The next question is how to connect these points or tops of shafts with each other, so as to be able to lay on them a continuous roof. This the reader, as before, is to favor me by finding out for himself, under these following conditions.

Let _s_, _s_, Fig. XXIX. opposite, be two shafts, with their capitals ready prepared for their work; and _a_, _b_, _b_, and _c_, _c_, _c_, be six stones of different sizes, one very long and large, and two smaller, and three smaller still, of which the reader is to choose which he likes best, in order to connect the tops of the shafts.

I suppose he will first try if he can lift the great stone _a_, and if he can, he will put it very simply on the tops of the two pillars, as at A.

Very well indeed: he has done already what a number of Greek architects have been thought very clever for having done. But suppose he _cannot_ lift the great stone _a_, or suppose I will not give it to him, but only the two smaller stones at _b_, _b_; he will doubtless try to put them up, tilted against each other, as at _d_. Very awkward this; worse than card-house building. But if he cuts off the corners of the stones, so as to make each of them of the form _e_, they will stand up very securely, as at B.

But suppose he cannot lift even these less stones, but can raise those at _c_, _c_, _c_. Then, cutting each of them into the form at _e_, he will doubtless set them up as at _f_.

[Illustration: Fig. XXIX.]

§ II. This last arrangement looks a little dangerous. Is there not a chance of the stone in the middle pushing the others out, or tilting them up and aside, and slipping down itself between them? There is such a chance: and if by somewhat altering the form of the stones, we can diminish this chance, all the better. I must say "we" now, for perhaps I may have to help the reader a little.

The danger is, observe, that the midmost stone at _f_ pushes out the side ones: then if we can give the side ones such a shape as that, left to themselves, they would fall heavily forward, they will resist this push _out_ by their weight, exactly in proportion to their own

## particular inclination or desire to tumble _in_. Take one of them

separately, standing up as at _g_; it is just possible it may stand up as it is, like the Tower of Pisa: but we want it to fall forward. Suppose we cut away the parts that are shaded at _h_ and leave it as at _i_, it is very certain it cannot stand alone now, but will fall forward to our entire satisfaction.

Farther: the midmost stone at _f_ is likely to be troublesome chiefly by its weight, pushing down between the others; the more we lighten it the better: so we will cut it into exactly the same shape as the side ones, chiselling away the shaded parts, as at _h_. We shall then have all the three stones _k_, _l_, _m_, of the same shape; and now putting them together, we have, at C, what the reader, I doubt not, will perceive at once to be a much more satisfactory arrangement than that at _f_.

§ III. We have now got three arrangements; in one using only one piece of stone, in the second two, and in the third three. The first arrangement has no particular name, except the "horizontal:" but the single stone (or beam, it may be,) is called a lintel; the second arrangement is called a "Gable;" the third an "Arch."

We might have used pieces of wood instead of stone in all these arrangements, with no difference in plan, so long as the beams were kept loose, like the stones; but as beams can be securely nailed together at the ends, we need not trouble ourselves so much about their shape or balance, and therefore the plan at _f_ is a peculiarly wooden construction (the reader will doubtless recognise in it the profile of many a farm-house roof): and again, because beams are tough, and light, and long, as compared with stones, they are admirably adapted for the constructions at A and B, the plain lintel and gable, while that at C is, for the most part, left to brick and stone.

§ IV. But farther. The constructions, A, B, and C, though very conveniently to be first considered as composed of one, two, and three pieces, are by no means necessarily so. When we have once cut the stones of the arch into a shape like that of _k_, _l_, and _m_, they will hold together, whatever their number, place, or size, as at _n_; and the great value of the arch is, that it permits small stones to be used with safety instead of large ones, which are not always to be had. Stones cut into the shape of _k_, _l_, and _m_, whether they be short or long (I have drawn them all sizes at _n_ on purpose), are called Voussoirs; this is a hard, ugly French name; but the reader will perhaps be kind enough to recollect it; it will save us both some trouble: and to make amends for this infliction, I will relieve him of the term _keystone_. One voussoir is as much a keystone as another; only people usually call the stone which is last put in the keystone; and that one happens generally to be at the top or middle of the arch.

§ V. Not only the arch, but even the lintel, may be built of many stones or bricks. The reader may see lintels built in this way over most of the windows of our brick London houses, and so also the gable: there are, therefore, two distinct questions respecting each arrangement;--First, what is the line or direction of it, which gives it its strength? and, secondly, what is the manner of masonry of it, which gives it its consistence? The first of these I shall consider in this Chapter under the head of the Arch Line, using the term arch as including all manner of construction (though we shall have no trouble except about curves); and in the next