The equilibrium thus obtained was sometimes so ticklish that a storm of wind, a trifling settlement, or a slight concussion sufficed to occasion a disaster; and many of the daring feats of the masons of the Middle Ages are lost to us, because they dared a little too much and the entire structure collapsed. This happened more often in the middle period of the style than in the earliest, but during the whole Gothic period there is a constant uniform tendency in one direction: thinner walls, wider arches, loftier vaults, slenderer buttresses, slighter piers, confront us at every step, and we need only compare some Norman structure (such as Durham), with a perpendicular (such as Henry VII.'s Chapel), to see how vast a change took place in this respect. _The Principles of Gothic Design._ All the germs of Gothic architecture exist in the Romanesque of the eleventh and twelfth centuries, and became developed as the passion for more slender proportions, greater lightness, and loftiness of effect, and more delicate enrichment became marked. It is quite true that the pointed arch is universally recognised as, so to speak, the badge of Gothic, even to the extent of having suggested the title of Christian pointed architecture, by which it is often called. But the pointed arch must be regarded rather as a token that the series of changes, which, starting from the heavy if majestic Romanesque of such a cathedral as Peterborough, culminated in the gracefulness of Salisbury or Lincoln, was far advanced towards completion, than as really essential to their perfection. Many of the examples of the transition period exhibit the round arch blended with the pointed (_e.g._ the nave of St. David's Cathedral or the Choir of Canterbury), and when we come to consider German architecture we shall find that the adoption of the pointed arch was postponed till long after the development of all, or almost all, the other features of the Gothic style; so as to place beyond question the existence, in that country at least, of "round arched Gothic." Some of the best authorities have indeed proposed to employ this title as a designation for much, if not all, the round arched architecture of the west of Europe, but Scott, Sharpe, and other authorities class mediæval art down to the middle of the twelfth century under the general head of Romanesque, a course which has been adopted in this volume.