DETAILS OF CONSTRUCTION.
In this article a few details of construction will be briefly considered, and illustrations will be given of actual arches and centers.
Backing is masonry of inferior quality laid outside and above the arch stones proper, to give additional security. The backing is ordinarily coursed or random rubble, but some times concrete. Sometimes the upper ends of the arch stones are cut with horizontal surfaces, in which case the backing is built in courses of the same depths as these steps and bonded with them. The backing is occasionally built in radiating courses, whose beds are prolongations of the bed-joints of the arch stones; but it usually consists of rubble, laid in horizontal courses abutting against the arch ring, with occasional arch stones extending into the former to bond both together. The radial joints possess some advantages in stability and strength, particularly above the joint of rupture; but below that joint the horizontal and vertical joints are best, since this form of construction the better resists the overturning of the arch outward about the springing line. Ordinarily, the backing has
a zero thickness at or near the crown, and gradually increases to the springing line; but sometimes it has a considerable thickness at the crown, and is proportionally thicker at the springing.
It is impossible to compute the degree of stability obtained by the use of backing; but it is certain that the amount ordinarily employed adds very greatly to the stability of the arch ring. In fact, many arches are little more than abutting cantilevers; and it is probable that often the backing alone would support the structure, if the arch ring were entirely removed.