Fig. 43.—An Oriel Window.
“At K I have drawn the string-course of the first floor, with the projections figured from the centre of the wall, and the set-off course above; also at L, the cornice and its table-course. You will observe that this table-course slopes towards the outer edge, beneath the gutter, so that, in the event of an overflow, the water may run off outside, and not find its way into the wall. This table-course has a throat, a, as well as the string-course, to prevent the water from running down the wall. These mouldings will have to be drawn to their full size for the stone-mason. On the course, M, behind the gutter, will stand the dormer gables, which will admit light to the second floor in the roof. As to the roofing, of which I merely indicate here the base, I will show you what it is to be. Take these sketches, then, and make from them figured drawings to a scale of half an inch to a foot, so that they may be worked from.
“In the meantime, I am going to make a perspective sketch of the projecting bays or loges of the billiard-room and dining-room, with the aid of which you may draw out these details. We shall see how you acquit yourself.
“The English in their country-houses are fond of employing this kind of projecting window. They call them bay or oriel windows, and often construct them on corbelling. Stay—here I have a sketch in my note-book of such a window in a house at Lincoln, dating from the sixteenth century (Fig. [43]). This projecting oriel, supported by a bracket, is terminated by a small terrace forming a balcony on the first story. Observe, by the by, how well-devised this construction is. This part of England possesses stone, but that material is less common than brick. The builder has used the dearer material only for the oriel window, which he could scarcely erect in brick, and for the jambs and lintels of the windows. The rest of the structure is built of brick.
“But,” continued Eugène, “your bay windows project too far to allow of their resting on corbelling.”
“What do you mean by corbelling?”
“It is a projecting piece of construction, not rising from the bottom, but supported by corbels; whence the name corbelling. The weight of the masonry resting on the tail, i.e. the part of the corbels fixed in the walls, allows us to place on their projecting part a construction, which, being less heavy than that resting on their tail, is thus sustained without fear of overbalancing. We must also calculate the length of the arm of the lever—that is to say, the relation of the projection of the corbels to the weight that secures their tail and that which rests on their head. Of course, the further the corbels project the more does the weight placed on their outward extremity tell on that which keeps the balance. So that a very trifling weight placed at the extremity of a greatly projecting corbel might overbalance a heavy mass placed at the tail. Corbels have, therefore, been frequently replaced by squinches—that is to say, a system of masonry which brings the weight at the extremity to bear on the walls. The architect who designed the oriel window I have just shown you did not trouble himself to make such an arrangement. He constructed what is called a bracket—that is, an inverted pyramid, by means of three courses of corbelling; or, if you choose, projecting one beyond the other, so as to obtain a portion of a polygon. On this supporting surface he has erected his window framing, which is scarcely 9½ inches thick. The bracket being built into the wall, supports the window framing, without being over-balanced, on account of the weight of the wall. Closed balconies of this kind were frequently employed during the Middle Ages, because they gave space in the upper stories without encroaching on the street, and afforded side views. And though civic regulations no longer allow us to build these projections in our cities, nothing would hinder us from erecting them when we build in the country. Still, there ought to be a good reason for adopting them. And in our case these corbel constructions serve no purpose, and it will cost us less to raise our bay window direct from the ground.”
Fig. 44.—Bay Window of Billiard-room.