intersecting pointed vaults. The new constructive principles did not admit of such forms. Gothic vault-forms do not admit of description in geometric terms. They vary according to the spans, the altitudes, the curves, and the points of springing of the arches that compose the rib system, and it is by the forms and relations of these arches only that such vaults can be described. In the vaults of Paris the filling-in consists of successive courses of arched masonry reaching from rib to rib over each triangular space of the plan. The beds of these successive courses are not parallel one with another, but incline variously according as the mason found necessary or convenient in developing the twisted concave surfaces required by the varying spans and positions of the ribs. In early vaults, like these of Paris, the courses usually have a considerable rise near the springing, from the longitudinal rib toward the diagonal; and they become gradually more level as they approach the crown of the vault, where they are more nearly parallel. But perfectly parallel they hardly ever can be, since each course is properly a portion of a surface which is concaved in all directions. The masonry of these vaults, especially in the choir, is perfectly faced and closely jointed.
The vaulting shafts are slender, and rise from the great capitals of cylindrical columns which constitute the piers of the ground-story. The flying buttresses were originally double—that is to say, the piers which divide the double aisles were carried up above the roof, where each one received the head of a flying buttress which sprang from the outer buttress, over the outer aisle, and gave foothold to another flying buttress which spanned the inner aisle and abutted against the great piers. [1] The principle of equilibrium maintained by opposing thrusts is here completely developed. The inert principle is wholly abandoned. The maximum of internal space for circulation and for prospect is attained by attenuation of supports, and if the maximum of size in external openings is not reached, it is not because any obstacles stand in the way, but only because the idea of having the largest possible openings has not yet presented itself to the minds of the builders.
- ↑ The present flying buttresses, consisting of arches which clear both aisles at a single span, are alterations of the thirteenth century.
