though
sometimes of great use in bridges and iron roofs (a succession of such
circles may be seen, for instance, in the spandrils at the Euston Square
station); but, from its constructional value, it becomes the best form
in which to arrange spandril decorations, as we shall see hereafter.

Sec. VIII. The height of the load above the arch is determined by the
needs of the building and possible length of the shaft; but with this we
have at present nothing to do, for we have performed the task which was
set us. We have ascertained, as it was required that we should in Sec. VI.
of Chap. III. (A), the construction of walls; (B), that of piers; (C),
that of piers with lintels or arches prepared for roofing. We have next,
therefore, to examine (D) the structure of the roof.

CHAPTER XIII.

THE ROOF.

Sec. I. Hitherto our enquiry has been unembarrassed by any considerations
relating exclusively either to the exterior or interior of buildings.
But it can remain so no longer. As far as the architect is concerned,
one side of a wall is generally the same as another; but in the roof
there are usually two distinct divisions of the structure; one, a shell,
vault, or flat ceiling, internally visible, the other, an upper
structure, built of timber, to protect the lower; or of some different
form, to support it. Sometimes, indeed, the internally visible structure
is the real roof, and sometimes there are more than two divisions, as in
St. Paul's, where we have a central shell with a mask below and above.
Still it will be convenient to remember the distinction between the part
of the roof which is usually visible from within, and whose only
business is to stand strongly, and not fall in, which I shall call the
Roof Proper; and, secondly, the upper roof, which, being often partly
supported by the lower, is not so much concerned with its own stability
as with the weather, and is appointed to throw off snow, and get rid of
rain, as fast as possible, which I shall call the Roof Mask.

Sec. II. It is, however, needless for me to engage the reader in the
discussion of the various methods of construction of Roofs Proper, for
this simple reason, that no person without long experience can tell
whether a roof be wisely constructed or not; nor tell at all, even with
help of any amount of experience, without examination of the several
parts and bearings of it, very different from any observation possible
to the general critic: and mo