§ 1. On the subject of the stability of arches, volumes have been written, and volumes more are required. The reader will not, therefore, expect from me any very complete explanation of its conditions within the limits of a single chapter. But that which is necessary for him to know is very simple and very easy; and yet, I believe, some part of it is very little known, or noticed.

We must first have a clear idea of what is meant by an arch. It is a curved shell of firm materials, on whose back a burden is to be laid of loose materials. So far as the materials above it are not loose, but themselves hold together, the opening below is not an arch, but an excavation. Note this difference very carefully. If the King of Sardinia tunnels through the Mont Cenis, as he proposes, he will not require to build a brick arch under his tunnel to carry the weight of the Mont Cenis: that would need scientific masonry indeed. The Mont Cenis will carry itself, by its own cohesion, and a succession of invisible granite arches, rather larger than the tunnel. But when Mr. Brunei tunnelled the Thames bottom, he needed to build a brick arch to carry the six or seven feet of mud and the weight of water above. That is a type of all arches proper.