CHAPTER XII

THE ARCH LOAD

§ 1. IN the preceding inquiry we have always supposed either that the load upon the arch was perfectly loose, as of gravel or sand, or that it was Mont-Cenisian, and formed one mass with the arch voussoirs, of more or less compactness.

In practice, the state is usually something between the two. Over bridges and tunnels it sometimes approaches to the condition of mere dust or yielding earth; but in architecture it is mostly firm masonry, not altogether acting with the voussoirs, yet by no means bearing on them with perfectly dead weight, but locking itself together above them, and capable of being thrown into forms which relieve them, in some degree, from its pressure.

§ 2. It is evident that if we are to place a continuous roof above the line of arches, we must first fill up the intervals between them on the tops of the columns. We have at present nothing granted us but the bare masonry, as here at a, Fig. 35, and we must fill up the intervals between the semicircle so as to obtain a level line of support. We may first do this simply as at b, with plain mass of wall; so laying the roof on the top, which is the

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[Version 0.04: March 2008]