Page:Charles Moore--Development and Character of Gothic Architecture.djvu/93

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openings. Thus Sir Gilbert Scott ^[1] says, "The side arches were sometimes stilted, not from any necessity, but merely to afford greater space for clerestory windows." But that it was not adopted because it was productive of beauty or convenience, nor to afford greater space for clerestory windows, a just consideration of the structural exigencies involved would show beyond question, even if it were not proved by the fact that the same peculiarity is frequent long before the clerestory opening is developed so as to fill the whole space beneath the longitudinal rib. In fact, the opening occupies but a comparatively small portion of this space in all early Gothic buildings, as may be seen in Paris, in Mantes, in Laon, in St. Leu d'Esserent, in the Collegiate Church of St. Frambourg at Senlis, and in many others. Fig. 35, a perspective view of one bay of the clerestory of St. Leu d'Esserent, will illustrate this point. Here the springing, a, of the longitudinal rib will be seen to be above the springing, b, of the main ribs by almost half the vertical height of the vault. It will be seen, too, that the intrados of the flying buttress, visible through the window, meets the pier at the same level. It is well known that the thrusts of the great vault ribs are not confined to their points of springing, but that there is a tendency in the arches, when firmly abutted at these points, to rise at their haunches, in consequence of which they require to be reinforced in these parts. Now the method here employed by which the line, a, b, is made to rise vertically to the level, a, brings the triangular vault surface, b, a, c, into a plane, which is inclined to the pier in the direction of the thrust of the diagonal rib; and as the diagonal rib of the next adjoining compartment, with the corresponding portion of vault surface, is inclined to the same pier in the opposite direction, the obliquity of each pressure is neutralised; and as the haunch of the transverse rib is reinforced by a solid filling-in, a perfect concentration of thrusts upon the pier is secured—the greatest power of these thrusts falling where the flying buttress is brought to bear. ^[2] The horizontal

1. Lectures on the Rise and Development of Medieval Architecture, vol. i. p. 63. London, 1879.
2. This was the intention, but actually the flying buttresses are not brought to bear precisely on the points of greatest thrust in St. Leu d'Esserent, as will be seen farther on.