It is therefore not surprising that though fifty years after its introduction the groined vault was generally adopted throughout Western Europe, and even in the East, the success of the flying buttress was infinitely more gradual and restricted. Thus, in the North, the multiplication of great religious monuments built, or even rebuilt on the new lines, was simultaneous with the construction in the South of vast churches on the old principles. The adventurous builders of the North had eagerly adopted the new division of churches into several aisles, all with groined vaults, the vault of the great central nave relying upon exterior flying buttresses for resistance to its thrust. In the South, on the other hand, architects were prudent, either through instinctive resistance to, or deliberate reaction from, the innovating influence, or by way of fidelity to an ancient tradition. They built with a single aisle, wide and lofty; the vaults were indeed supported by ribs, but their thrusts were received by powerful buttresses inside the walls, the projections thus formed being further utilised for the construction of chapels in the intervals. This latter system, which has the incontestable merit of perfect solidity, recalls the construction of the Basilica of Constantine, or of the tepidarium in the Baths of Caracalla. The stability of the edifice was ensured by the resistance of masses at the imposts, and the whole principle of construction formed, as it were, a protest against the miracles of equilibrium so much in favour among the Northerners. The new system of vaults supported by flying buttresses made very slight way in the South. It appears but rarely, and in the few instances where it is used has entirely the air of a foreign importation. Even in the cradle of its origin, it took root slowly and with difficulty, for its first applications were not without disaster. Lacking that mathematical knowledge which is the mainstay of the modern architect, the experimental skill shown by the thirteenth-century builder in constructing his vaults, and then in neutralising their thrusts by flying buttresses reduced to the legitimate function of permanent struts, was little short of miraculous. For it must be borne in mind that the thrust of these vaults, and the strength of the flying buttresses, varied of necessity according to their span, and the resisting powers of their materials. It was only by dint of long gropings in the dark that the necessarily empirical formulæ of the innovators were gradually transformed into recognised rules, and this knotty problem of construction received no positive solution till the last years of the thirteenth, or more emphatically, the first years of the fourteenth century. While even then the solution could claim no universal acceptance, for what was comparatively easy in countries where stone abounds became difficult, if not impossible, in districts where such a material as brick was the sole resource of builders.