§ 1. Hitherto our inquiry 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.