During many centuries the architects struggled with this problem of constructing a building that was large enough. The Roman tradition taught them how to build heavy stone walls with very small windows lest the walls lose their strength. On the top of this they then placed a heavy stone roof. But in the twelfth century, after the beginning of the Crusades, when the architects had seen the pointed arches of the Mohammedan builders, the western builders discovered a new style which gave them their first chance to make the sort of building which those days of an intense religious life demanded. And then they developed this strange style upon which the Italians bestowed the contemptuous name of "Gothic" or barbaric. They achieved their purpose by inventing a vaulted roof which was supported by "ribs." But such a roof, if it became too heavy, was apt to break the walls, just as a man of three hundred pounds sitting down upon a child's chair will force it to collapse. To overcome this difficulty, certain French architects then began to re-enforce the walls with "buttresses" which were merely heavy masses of stone against which the walls could lean while they supported the roof. And to assure the further safety of the roof they supported the ribs of the roof by so-called "flying buttresses," a very simple method of construction which you will understand at once when you look at our picture. This new method of construction allowed the introduction of enormous windows. In the twelfth century, glass was still an expensive curiosity, and very few private buildings possessed glass windows. Even the castles of the nobles were without protection and this accounts for the eternal drafts and explains why people of that day wore furs in-doors as well as out. Fortunately, the art of making coloured glass, with which the ancient people of the Mediterranean had been familiar, had not been entirely lost. There was a revival of stained glass-making and soon the windows of the Gothic churches told the stories of the Holy Book in little bits of brilliantly coloured window-pane, which were caught in a long framework of lead. Behold, therefore, the new and glorious house of God, filled with an eager multitude, "living" its religion as no people have ever done either before or since! Nothing is considered too good or too costly or too wondrous for this House of God and Home of Man. The sculptors, who since the destruction of the Roman Empire have been out of employment, haltingly return to their noble art. Portals and pillars and buttresses and cornices are all covered with carven images of Our Lord and the blessed Saints. The embroiderers too are set to work to make tapestries for the walls. The jewellers offer their highest art that the shrine of the altar may be worthy of complete adoration. Even the painter does his best. Poor man, he is greatly handicapped by lack of a suitable medium. And thereby hangs a story.