XIII. Theoretical Writings on Architecture: 776.
776.
This wall will break under the arch e f, because the seven whole square bricks are not sufficient to sustain the spring of the arch placed on them. And these seven bricks will give way in their middle exactly as appears in a b. The reason is, that the brick a has above it only the weight a k, whilst the last brick under the arch has above it the weight c d x a.
c d seems to press on the arch towards the abutment at the point p but the weight p o opposes resistence to it, whence the whole
pressure is transmitted to the root of the arch. Therefore the foot of the arch acts like 7 6, which is more than double of x z.