Page:A History of Art in Ancient Egypt Vol 1.djvu/198

112 their lateral joints are vertical. (Fig. 76.) When the units of such vaults are properly proportioned they are stable in themselves, and they have no lateral thrust.

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.)

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