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But it is perhaps somewhat too costly: the thing to be done is to carry the sand or gravel with brick voussoirs, six inches thick, or, at any rate, with the least thickness of voussoir which will be safe; and to do this requires peculiar arrangement of the lines of the arch.
Let a, Plate III., be the shell of a pointed arch with loose loading above; and suppose you find that shell not quite thick enough; and that the weight bears too heavily on the top of the arch, and is likely to break it in: you proceed to thicken your shell, but need you thicken it all equally? Not so; you would only waste your good voussoirs.
Much beautiful sculpture is, in the best Byzantine buildings, half lost by being put under soffits; but the eye is led to discover it, and even to demand it, by the rich chasing of the outside of the voussoirs. It would have been an hypocrisy to carve them externally only.
The pebbles and gravel above will now shoot off it right and left, as the bullets do off a cuirassier's breastplate, and will have no chance of beating it in. If still it be not strong enough, a farther addition may be made, as at c, now thickening the voussoirs a little at the base also.
An arch consists of a series of wedge shaped blocks, known as voussoirs, arranged in a curve, and so locking one another together that unless the abutments from which the arch springs give way, it will not only carry itself, but sustain a heavy load.
As to whether they were of pisé, like those of Khorsabad, the Greek writers tell us nothing. From what we know of the habits of the Chaldæan builder we may conclude that they were true arches with voussoirs either of bricks burnt in the kiln, or so well dried that they were almost as hard and durable as those that had passed through the fire.
XXXI.; when it is half up, b, and when three quarters up, c. There will be an infinite number of arches between these, but we will take these as sufficient representatives of all. Then a is the low arch, b the central or pure arch, c the high arch, and the rays of the sun would have drawn for us their voussoirs.
It would seem decidedly grand to use for the corners substantial blocks of hewn stone, sandstone, granite, marble, or porphyry, channelled and chamfered, rock-faced, tooled, rubbed, or decorated; key-stones and voussoirs embellished with your monogram or enriched by any other charming device you choose to invent; bands of encaustic tile, brilliant in color and pattern, belts of sculptured stone, and historic tablets, if you fancy and can afford them.
The perpendicular side walls passed into the vault without any preparation, and the arch when complete had no inward projection and no structural ornament but the inner faces of the carefully placed voussoirs; as all the bricks were of the same size and shape something more than their slightly trapezoidal form was required to keep them in place, and a softer clay was used to bind them together.
And farther, the problem which is to be solved by the arch builder is not merely to carry this weight, but to carry it with the least thickness of shell. It is easy to carry it by continually thickening your voussoirs: if you have six feet depth of sand or gravel to carry, and you choose to employ granite voussoirs six feet thick, no question but your arch is safe enough.
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