§ IX. The second kind of superimposition, lightness on weight, is, in its most necessary use, of stories of houses one upon another, where, of course, wall veil is required in the lower ones, and has to support wall veil above, aided by as much of shaft structure as is attainable within the given limits. The greatest, if not the only, merit of the Roman and Renaissance Venetian architects is their graceful management of this kind of superimposition; sometimes of complete courses of external arches and shafts one above the other; sometimes of apertures with intermediate cornices at the levels of the floors, and large shafts from top to bottom of the building; always observing that the upper stories shall be at once lighter and richer than the lower ones. The entire value of such buildings depends upon the perfect and easy expression of the relative strength of the stories, and the unity obtained by the varieties of their proportions, while yet the fact of superimposition and separation by floors is frankly told. § X. In churches and other buildings in which there is no separation by floors, another kind of pure shaft superimposition is often used, in order to enable the builder to avail himself of short and slender shafts. It has been noted that these are often easily attainable, and of precious materials, when shafts large enough and strong enough to do the work at once, could not be obtained except at unjustifiable expense, and of coarse stone. The architect has then no choice but to arrange his work in successive stories; either frankly completing the arch work and cornice of each, and beginning a new story above it, which is the honester and nobler way, or else tying the stories together by supplementary shafts from floor to roof,--the general practice of the Northern Gothic, and one which, unless most gracefully managed, gives the look of a scaffolding, with cross-poles tied to its uprights, to the whole clerestory wall. The best method is that which avoids all chance of the upright shafts being supposed continuous, by increasing their number and changing their places in the upper stories, so that the whole work branches from the ground like a tree. This is the superimposition of the Byzantine and the Pisan Romanesque; the most beautiful examples of it being, I think, the Southern portico of St. Mark's, the church of S. Giovanni at Pistoja, and the apse of the cathedral of Pisa. In Renaissance work the two principles are equally distinct, though the shafts are (I think) always one above the other. The reader may see one of the best examples of the separately superimposed story in Whitehall (and another far inferior in St. Paul's), and by turning himself round at Whitehall may compare with it the system of connecting shafts in the Treasury; though this is a singularly bad example, the window cornices of the first floor being like shelves in a cupboard, and cutting the mass of the building in two, in spite of the pillars.