groined vaults over oblong compartments without either doming or stilting, since the crowns of all the arches could be readily brought to the same level whatever their differences of span (Fig. 10).
FIG. 10. But it is important to observe that, in true Gothic, oblong vaults are never constructed upon ribs which all spring from the same level, and whose crowns all reach the same height. Other exigencies, which will be explained in the next chapter, stood in the way of so constructing them. True Gothic vaults are always, to some extent, both stilted and domed. But though full advantage of the pointed arch, in affording any height with any span, could not be taken, its introduction was a great help, and it gave a powerful stimulus to constructive invention.
With the reduced thrust of the vault, secured by the pointed arch, the volume of the external stays could now be much reduced, and new experiments soon led to their better adjustment. The flying buttresses were brought to bear more directly upon the points of greatest pressure, which were found to be at a higher level than those upon which the abutting arches of the Abbaye-aux-Dames and of Durham Cathedral had been brought to bear. In order to reach these points it was necessary to make the flying buttresses spring over the aisle roofs, and thus to become marked external features—as in St. Remy at Reims, St. Leu d'Esserent, St. Germain des Prés at Paris, and a few other early Gothic monuments which still retain their original flying buttresses. The vault ribs and the vaults themselves were now also made lighter. And finally the ribs were more closely grouped at their springing—being made to interpenetrate, more or less, by which means the pressures were concentrated to
