117. The True Arch
The true arch differs from the corbelled in needing no counterweight. The blocks that form the under surface or soffit of its span are self-sustaining. The true arch thus yields an æsthetic satisfaction which can be attained in no other way, especially when it soars in magnitude. The fundamental principle of the true arch is the integration of its elements. Such an arch is nothing until completed; but from that moment its constituents fuse their strength. Each block has a shape which is predetermined by the design of the whole, and each is useless, in fact, not even self-supporting, until all the others have been fitted with it. Hence the figure of speech as well as the reality of the keystone: the last block slipped into place, locking itself and all the others. The features of the blocks or “voussoirs” which makes possible this integration, is the taper of their sides. Each is a gently sloping piece of wedge instead of a rectangular block. When bricks replace dressed stone, the mortar takes the place of this shaping, being thinner toward the inner face of the vault and thicker toward the interior of the construction.
A true arch in process of erection would instantly collapse if not held up. It can be built only over a scaffold or “centering.” Once however the keystone has wedged its parts together, it not only stands by itself but will support an enormous weight. The greater the pressure from above, the more tightly are the blocks forced together. Instability in a true arch is not due to the bending stress coming from the superimposed mass, as in the corbelled arch or a horizontal roofing. The blocks are subjected only to crushing pressure, which stone and brick are specially adapted to withstand. The weakness of the arch is that it turns vertical into horizontal thrust. With more weight piled on top, the sidewise thrust, the inclination to spread apart, becomes greater, and must be resisted by buttressing. This is what the Hindus mean when they say that “the arch never sleeps.”