its 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.
Brunel 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.

Sec. II. Now arches, in practice, partake of the nature of the two. So
far as their masonry above is Mont-Cenisian, that is to say, colossal in
comparison of them, and granitic, so that the arch is a mere hole in the
rock substance of it, the form of the arch is of no consequence
whatever: it may be rounded, or lozenged, or ogee'd, or anything else;
and in the noblest architecture there is always _some_ character of this
kind given to the masonry. It is independent enough not to care about
the holes cut in it, and does not subside into them like sand. But the
theory of arches does not presume on any such condition of things; it
allows itself only the shell of the arch proper; the vertebrae, carrying
their marrow of resistance; and, above this shell, it assumes the wall
to be in a state of flux, bearing down on the arch, like water or sand,
with its whole weight. And farther, the problem which is to be solved by
the arch builder is not merely to carry this weight, but to carry it
with the least thickness of shell. It is easy to carry it by continually
thickening your voussoirs: if you have six feet depth of sand or gravel
to carry, and you choose to employ granite voussoirs six feet thick, no
question but your arch is safe enough. But it is perhaps somewhat too
costly: the thing to be done is to carry the sand or gravel with brick
voussoirs, six inches thick, or, at any rate, with the least thickness
of