"_The Romans always concealed their Butments._"[77] "_Oblique Positions are Discord to the Eye unless answered in Pairs, as in the Sides of an equicrural Triangle.... Gothick Buttresses are all ill-favoured, and were avoided by the Ancients._"[78] Such were the opinions of Wren; but how was he to procure stability? The answer is, by the curtain wall. By its dead weight pressing on the walls of the aisles it renders them stable and immobile, free from all danger of thrust, while it conceals the buttresses which render secure the clerestory stage of the building proper. To paraphrase his own words: "_I do not add buttresses, but I build up the wall so high as by the addition of this extra weight, I establish it as firmly as if I had added buttresses._"[79] Thus this wall performs a double function: it is a substitute for buttresses in respect to the aisle walls, and a screen for the actual buttresses of the clerestory stage. Such is the purpose of the upper story. An ingenious critic who did not seem to know this vindicates it on the plea that "uninterrupted altitude of the bulk in the same plane, is absolutely necessary to the substructure of the mighty dome."[80] No doubt the size of the dome requires a proportionate rise in the lower elevations; but the fact remains that the exterior and interior do not correspond. A greater authority than this critic has thus defined good architecture: "The essence of good architecture of any kind is that its constructive system should be put boldly forward, that its decorative system should be such as in no way conceals or masks the construction, but makes the constructive features themselves ornamental."[81] And at his uncle's cathedral of Ely, Wren might have borrowed and worked out an idea which would have silenced all accusation of fraud and deceit. There, in the central part of the choir on the south side, the roof was removed and placed lower down centuries ago, the better to light up certain shrines below. This roof was never restored to its original position; and the upper stage of the wall is pierced with empty windows through which flying-buttresses can now be seen. The effect, though altogether unusual, is far from displeasing; and the spectator who remembers that Wren was perfectly familiar with this building, is led to wonder why he did not by piercing the niches, imitate Ely at St. Paul's.