Architects who had been trained in the great abbey schools, emboldened by the successes of their forerunners and their own individual experience, raised on every hand vast cathedrals, in which every known development of the system was essayed with unequalled daring. Going on from strength to strength, they eventually abandoned the antique traditions, and disregarding the statical conditions which ensured the solidity of the ancient buildings, they invented a system of construction which is, as it were, merely a skeleton in stone, a stone version of the timbered roof; its characteristic expression was the permanent strut known as the _flying buttress_; its governing idea was equilibrium, for which it provided by architectural stratagems ingenious in the highest degree, but also extremely precarious. Its existence or stability depends for the most part on the quality of the materials and their degrees of resisting power, the essential organs, by which I mean those vital _weight-carrying_ portions, the failure of which would involve the ruin of the whole, being _outside_ the building, and therefore exposed to all those deteriorating influences from which the _load_ they bear, that is to say, the vaults, are protected by walls and roof. [Illustration: 22. SECTION OF A SINGLE-AISLED CHURCH VAULTED ON INTERSECTING ARCHES WITH BUTTRESSES] [Illustration: 23. SECTION OF A THREE-AISLED CHURCH VAULTED ON INTERSECTING ARCHES WITH FLYING BUTTRESSES] The great buildings constructed on these new principles consisted of a central nave with two, or even four side aisles. The huge structure depended for its light first upon low windows in the collateral portions, secondly, upon windows at a much higher level. Hence it became necessary to raise the vault of the central nave, and to give it an abutment in the form of _detached semi-arches_ or flying buttresses. The crowns of these semi-arches impinged the piers at the planes of greatest pressure and received the collective thrust of all the ribs, formerets, transverse and diagonal arches. Their bases rested upon abutments, the strength of which was calculated according to the thrust they had to meet. CHAPTER V ORIGIN OF THE FLYING BUTTRESS