272 COMPARATIVE ARCHITECTURE. more extensive edifices being built, a new method was gradually evolved. In seeking to diminish the size of the piers and thick- ness of the walls, it was necessary for the architects of this period to find a mode of construction more homogeneous and more capable of resistance, and to avoid the expense of labour which the carrying of material of large size involved. The walls, therefore, became of secondary importance, their place being occupied by stained glass windows, and the support of the structure was effected entirely by means of buttresses or short walls placed so as best to resist the thrust of the vaulting. Vaulting. — The method was an extension of the Roman- esque system, which was evolved from that of the Romans (page 224) and consisted of a framework of independent ribs, which were first constructed and which supported thin panels of stone. The difficulties of vaulting oblong compartments were now overcome by the introduction of the pointed arch, which was used to cover the shorter spans, while the semicircular arch was still used for some time for the diagonal ribs. The ribs became permanent centres on which the panels or " infilling " of thin stone could rest, and enabled the building to be erected all at once or in parts without disadvantage to the solidity of the edifice. As indicated on Nos. logand 141, the pressures of the vaults were transmitted to the angles of each compartment by the diagonal ribs. Such pressures are of two kinds : oiitwai'ds by the nature of the arch, and dowmvards by the weight of the material, the resultant of the two being in an oblique direction. The increase of the number and variety of ribs and the consequent form of the vaults (No. 11 1 d) during the three centuries of Gothic architecture is one of the most fascinating studies of the style. The invention of painted glass was an important factor in the development of the style, for traceried windows came to be looked upon merely as frames in which to exhibit painted transparent pictures displaying the incidents of Bible History. Neither the painted sculpture and hieroglyphics of the Egyptian temples, the colored and sculptured slabs of the Assyrian palaces, the paintings of the Greek temples, nor the mosaics and frescoes of the Byzantine and Romanesque periods produced color effects that can be compared with the brilliancy and the many-tinted splendours of the transparent walls of a Gothic cathedral. In the north and west of Europe, where painted glass was the principal mode of decoration, the walls were kept internally as flat as possible, so as to allow the windows to be seen internally in every direction, all the mechanical expedients of buttresses and pinnacles being placed externally. Eurther, when by the grouping of windows and the subsequent forma- tion of nuillions and tracery, the entire screen wall between
