Page:Charles Moore--Development and Character of Gothic Architecture.djvu/34

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course, stayed in both of these ways. The lateral masses of wall act as buttresses, and the superincumbent masonry tends to overcome the side pressure by its weight. Where a space
FIG 3. between two parallel walls is roofed over by a barrel vault the continuous side pressures, which would tend to overthrow the walls, are, in Roman constructions, met by thickening the walls enough to resist the thrusts—the extra thickness given for this purpose forming a continuous abutment. The walls in Roman buildings are also sometimes weighted by heavy masonry above the springing of the vaults. In Roman buildings of several stories, such as the Flavian amphitheatre (section, Fig. 3), the walls of the lower stories are enormously thick, and the vault pressures are further stayed by the weight of the walls above. The top story has no vault, and the weight of its thinner wall helps to maintain the stability of the vault below. By such massive masonry employed in this double way, the pressures of Roman vaults are much more than met. In the case of Roman intersecting vaults, like those of the Basilica of Constantine, the thrusts, instead of being continuous, as in the foregoing instance, are concentrated upon those points (a, plan, Fig. 4) from which the groins spring, where they are met by walls set across the aisles and dividing them into separate compartments. These walls are, of course, true buttresses in disguise. The compartments of the aisles are covered by barrel vaults springing from the dividing walls, and thus, having their axes at right angles to the side walls of the building, they exert no thrust upon these side walls, and consequently no external stays are required. Thus the buttress employed by the Romans was not a buttress pure and simple, devised to meet a side pressure with economy, as well as efficiency, and openly confessed as a functional member. They always contrived to arrange the plans of their buildings so that some of the enclosing or