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Updated: May 23, 2025
For when arches with voussoirs are sprung from the ends of beams, or from the bearings of lintels, in the first place they will discharge the load and the wood will not sag; secondly, if in course of time the wood becomes at all defective, it can easily be replaced without the construction of shoring.
§ I. In the preceding enquiry we have always supposed either that the load upon the arch was perfectly loose, as of gravel or sand, or that it was Mont-Cenisian, and formed one mass with the arch voussoirs, of more or less compactness. In practice, the state is usually something between the two.
This arch line the architect has first to conceive and shape in his mind, as opposed to, or having to bear, certain forces which will try to distort it this way and that; and against which he is first to direct and bend the line itself into as strong resistance as he may, and then, with his voussoirs and what else he can, to guard it, and help it, and keep it to its duty and in its shape.
§ X. The circular window furnishes an exception to the common law, that the bars shall be vertical through the greater part of their length: for if they were so, they could neither have secure perpendicular footing, nor secure heading, their thrust being perpendicular to the curve of the voussoirs only in the centre of the window; therefore, a small circle, like the axle of a wheel, is put into the centre of the window, large enough to give footing to the necessary number of radiating bars; and the bars are arranged as spokes, being all of course properly capitaled and arch-headed.
§ VI. Now the arch line is the ghost or skeleton of the arch; or rather it is the spinal marrow of the arch, and the voussoirs are the vertebræ, which keep it safe and sound, and clothe it.
So the arch line is the moral character of the arch, and the adverse forces are its temptations; and the voussoirs, and what else we may help it with, are its armor and its motives to good conduct. § VII. This moral character of the arch is called by architects its "Line of Resistance."
The triforium arcade has round arches with coupled colonnettes of red marble on the face and varied caps; the voussoirs are alternately red and grey; and a string with carved leaf pattern, much like that at Traù, runs along the triforium, between the nave arcade and the balustrade. The nave arcade terminates at each end with a single arch.
Likewise in houses where piers are used in the construction, when there are arches composed of voussoirs with joints radiating to the centre, the outermost piers at these points must be made broader than the others, so that they may have the strength to resist when the wedges, under the pressure of the load of the walls, begin to press along their joints towards the centre, and thus to thrust out the abutments.
§ XII. The voussoirs, in despair, go over to the classical camp, in hope of receiving some help or tolerance from their former enemies. They receive it indeed: but as traitors should, to their own eternal dishonor. It had been better they had died ten times over, in their own ancient cause, than thus prolonged their existence.
But the voussoirs then concede, on the other hand, so much of their dignity as to receive a running ornament of foliage or animals, like a classical frieze, and continuous round the arch.
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