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Although very frequently used over the crossing of Romanesque churches, the dome on squinches is seldom found over the bays of the nave. There is in fact no distinct school in which this method is employed and the examples of its use are widely scattered. The principal one is, perhaps, the cathedral of Notre Dame at Le Puy (Haute-Loire), which dates from the eleventh and twelfth centuries. Unlike the domed churches of Perigord it is of basilical plan with side aisles. The nave is in six bays with broad arches opening into the aisles and a triforium arcade above them. Across the nave are transverse arches separating the bays. The four toward the east are semicircular, the remaining two are pointed in elevation. These arches rise from imposts nearly or quite as low as those of the nave arcade, and walls are built upon them to the level of the string-course above the triforium. Six rectangular bays,—or seven including the crossing,—of practically square plan are thus formed and each is covered with a dome. In the western bays,—which are at least a century later than those at the east end and therefore more advanced in structure,—a clerestory wall is erected with a single window in its north and south walls, and openings corresponding to windows from one bay to the next above the transverse arches, to secure a good distribution of light (Fig. 5). Across the upper corners of these four walls and rising from the same level as the window heads, are arches with half domed triangular niches beneath them, converting the square into an octagon and furnishing the impost for the domes.[26] These are octagonal in elevation as well as plan and are laid up in flat panels, or gores, which meet at the crown (Fig. 6). It is a type of dome admirably suited to its impost since it presents none of the awkward appearances of a circular dome on an octagonal base.[27] It is also very practical from a structural standpoint. Since the gores are flat, the stone cutting is far less elaborate than in a hemispherical dome, and the gored dome has the further advantage of great flexibility since it may be flattened or raised at the crown, placed over a square bay or one with any number of sides, and made equilateral or with gores of different widths, all with great facility. Furthermore, when the naves are of reasonable width, as in most churches with side aisles, the thrust of the dome is very slight and its downward pressure is not excessive.

Fig. 5.—Le Puy, Cathedral.

Fig. 6.—Le Puy, Cathedral.

But with all its structural advantages, a system like that at Le Puy was not a satisfactory solution of nave vaulting. The transverse arches were necessarily so far below the surface of the dome that the continuity of the nave as a whole was destroyed, and the appearance was rather that of a series of lantern towers or crossings juxtaposed than of a single homogeneous vault.

The side aisles of Le Puy are of less importance than the nave, though the fact that some of their bays were vaulted, or revaulted, at nearly every period of mediaeval architecture makes them interesting for a study of consecutive methods. In the bays to the east the vaults are groined on stilted, round headed transverse arches in the early Romanesque manner, while the succeeding bays have pointed transverse arches with groined vaults closely resembling those of the school of Bourgogne, and the bays nearest the west end have ribbed vaults, in one case with the early heavy-torus rib, in another with the light rib of pointed section of a late Gothic rebuilding.

Fig. 7.—Poitiers, Saint Hilaire.

Although not the basis of a school of Romanesque architecture, the cathedral of Le Puy was not without its influence. This is especially apparent in the large church of Saint Hilaire at Poitiers (Figs. 7, and 8), which was constructed with very broad nave and aisles,—both covered with wooden roofs,—after a disastrous fire of 1018, and dedicated in 1059. In 1130 the vaulting of this church was undertaken, the result being a most unusual edifice. As the nave was too broad to be easily covered by a vault of single span, it was subdivided by lofty and slender piers and arches into a central portion consisting of square bays,[28] and narrow rectangular bays forming veritable inner aisles on either side. These narrow bays were covered with groined vaults directly above the original clerestory windows which thus continued to light the newly formed nave. Domes were then placed over the square central bays as had been done at Le Puy, but instead of the niche-head-squinch and the practically equilateral octagonal dome, small conical trumpet arches were employed at Saint Hilaire, and the gores of the dome rising from these were much narrower than the four remaining panels. This gives the dome rather the character of a cloistered vault with its corners cut off than of a dome properly speaking. Since the clerestory is below the level of the transverse arches upon which the domes of Saint Hilaire are built, the interior has a loftier and less broken appearance than that of Notre Dame-du-Puy. But even so the effect is not remarkably pleasing.

The side aisles of Saint Hilaire (Fig. 8) are quite as interesting in their vaulting as the nave. A single broad aisle on either side, which apparently opened into the nave through lofty arches rising almost to the clerestory, and which probably had transverse arches with ramping walls carrying half gable roofs, was altered when it was determined to vault the church. In doing this, two arches with a solid wall above were placed under each of the original arches of the nave arcade, a slender column built up in the center of each of the original bays, and upon the pseudo-double side aisles thus formed, compound groined vaults were constructed in a manner best understood from the photograph (Fig. 8).

Except for those just mentioned there are but few Romanesque churches,—outside of Italy and Sicily,—in which the nave is covered by a series of domes.[29] But because of the powerful Byzantine influence, these latter

Fig. 8.—Poitiers, Saint Hilaire.

countries contain a large number of churches of semi-Byzantine, semi-Romanesque character, some of which are as late as the thirteenth century.[30] Most of these are so distinctly Byzantine that they do not properly fall within the province of this book, in spite of their late date; but others, like the cathedral of Molfetta,[31] have a vaulting system quite closely allied to the Romanesque.[32] In this particular cathedral, a nave of three square bays is covered by three domes, one on flattened spherical pendentives, the others on niche-head-squinches. Two of them rise from drums and unlike their Byzantine prototypes, they are all of stone.[33] Moreover, the side aisles are covered with half tunnel vaults on full transverse arches, the crown of the vaults together with the nave walls above them acting as admirable buttresses for the domes. A system not quite so logical exists in the aisles of the church of San Sabino at Canosa (1100), where there are full tunnel vaults which do not serve so adequately as buttresses.