CHAPTER XVII.
FILLING OF APERTURE.
§ I. Thus far we have been concerned with the outline only of the aperture: we were next, it will be remembered, to consider the necessary modes of filling it with valves in the case of the door, or with glass or tracery in that of the window.
1. Fillings of doors. We concluded, in the previous Chapter, that doors in buildings of any importance or size should have headings in the form of an arch. This is, however, the most inconvenient form we could choose, as respects the fitting of the valves of the doorway; for the arch-shaped head of the valves not only requires considerable nicety in fitting to the arch, but adds largely to the weight of the door,—a double disadvantage, straining the hinges and making it cumbersome in opening. And this inconvenience is so much perceived by the eye, that a door valve with a pointed head is always a disagreeable object. It becomes, therefore, a matter of true necessity so to arrange the doorway as to admit of its being fitted with rectangular valves.
§ II. Now, in determining the form of the aperture, we supposed the jamb of the door to be of the utmost height required for entrance. The extra height of the arch is unnecessary as an opening, the arch being required for its strength only, not for its elevation. There is, therefore, no reason why it should not be barred across by a horizontal lintel, into which the valves may be fitted, and the triangular or semicircular arched space above the lintel may then be permanently closed, as we choose, either with bars, or glass, or stone.
This is the form of all good doors, without exception, over the whole world and in all ages, and no other can ever be invented.
§ III. In the simplest doors the cross lintel is of wood only, and glass or bars occupy the space above, a very frequent form in Venice. In more elaborate doors the cross lintel is of stone, and the filling sometimes of brick, sometimes of stone, very often a grand single stone being used to close the entire space: the space thus filled is called the Tympanum. In large doors the cross lintel is too long to bear the great incumbent weight of this stone filling without support; it is, therefore, carried by a pier in the centre; and two valves are used, fitted to the rectangular spaces on each side of the pier. In the most elaborate examples of this condition, each of these secondary doorways has an arch heading, a cross lintel, and a triangular filling or tympanum of its own, all subordinated to the main arch above.
§ IV. 2. Fillings of windows.
When windows are large, and to be filled with glass, the sheet of glass, however constructed, whether of large panes or small fragments, requires the support of bars of some kind, either of wood, metal, or stone. Wood is inapplicable on a large scale, owing to its destructibility; very fit for door-valves, which can be easily refitted, and in which weight would be an inconvenience, but very unfit for window-bars, which, if they decayed, might let the whole window be blown in before their decay was observed, and in which weight would be an advantage, as offering more resistance to the wind.