§ 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. § 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 more than this, the enquiry would be useless to us in our Venetian studies, where the roofs are either not contemporary with the buildings, or flat, or else vaults of the simplest possible constructions, which have been admirably explained by Willis in his "Architecture of the Middle Ages," Chap. VII., to which I may refer the reader for all that it would be well for him to know respecting the connexion of the different parts of the vault with the shafts. He would also do well to read the passages on Tudor vaulting, pp. 185-193, in Mr. Garbett's rudimentary Treatise on Design, before alluded to.[50] I shall content myself therefore with noting one or two points on which neither writer has had occasion to touch, respecting the Roof Mask.