Fig. 76.—Off-set semicircular arch.
2. Centred vaults. These are true vaults. They are composed of voussoirs, whose lateral joints are oblique, and radiate towards one centre or more. ([Figs. 77, 78, and 79].)
Fig. 77.—Voussoir.
Fig. 78.—Arrangement of voussoirs.
This method of construction is very convenient because it enables the builder to utilize constructive units of very small dimensions, such as bricks. But this advantage has a corresponding drawback. These voussoirs thrust one against another and tend towards disintegration. They are not stable in themselves, and in order to give them stability they must be kept in place by surrounding them with opposing forces which will effectually prevent their setting up any movement in the structure of which they form a part. This function is fulfilled by the wall in Egyptian architecture, which is consequently very thick, but the radiating arch never arrived at such a development in Egypt as to lead to the adoption of any contrivance specially charged with the maintenance of vaults in a state of proper rigidity. The Egyptians not only employed the semicircular arch; they made use, in a few instances, of the pointed form, and many of their underground buildings have roofs cut out of the rock in the form of a segmental vault. The fact that these sepulchral chambers affected the aspect of vaulted halls, can only be explained by the supposition that a similar construction was common in the dwellings of the living.[106]
Fig. 79.—Semicircular vault.