Buttressing-Ribbed Chevets

Fig. 66.—Vézelay, La Madeleine.

seen in Saint Quiriace at Provins (cir. 1160)(Fig. 31)and in La Madeleine at Vézelay (cir. 1140-1180)(Fig. 66). The latter is of especial interest because it shows some peculiar makeshifts in the matter of construction. Here the choir would seem to have been originally designed to consist of two rectangular bays with four-part vaults and an apse of five sides probably with a chevet like that at Saint Germer.[369]But by the time the western bay of the choir had been built up to the clerestory, it would seem as if a new idea of a seven-part chevet had come in, perhaps from Paris, and the next bay was subdivided so as to give seven equal sides to the new vault. Then to make all the bays of the same scale, the west bay was also subdivided, but this necessarily at the clerestory level, and covered with a six-part vault. This left nine bays for the chevet and as only seven were to be actually included beneath the radiants, a narrow rectangular four-part vault was used over that toward the choir. Therenow remained an apse in all respects like those of Saint Martin-des-Champs and of Saint Germain-des-Pres and it was similarly vaulted with a broken-ribbed vault whose keystone does not lie quite upon the transverse line between the first two piers of the apse proper. The chevet built upon these radiants differs, however, from those in Paris and at Saint Germer in having a decidedly domed up character. In other words, the windows do not rise more than half the distance from the impost of the radiants to their keystone.[370]

This type of chevet as developed at Paris and Vézelay played a large part in subsequent architecture, for out of it would seem to have sprung what will be for convenience termed the buttressing-ribbed chevet. Among the more important early chevets of this type are those over the apses of Noyon[371]transepts, of Saint Remi at Reims(Fig. 64), of Saint Leu d’Esserent (Oise), and of the cathedrals of Sens, Canterbury, Noyon, and others, all probably completed before 1180. Although differing in a number of details, these apses have certain features in common. They all include beneath the chevet the preceding bay of the church, and all have the same arrangement of ribs which are so placed that the two springing from the piers next beyond the apsidal arch on either side form a transverse arch against whose crown all the others abut (Plate II-d.). The object of this arrangement evidently lay in the desire of the builders to construct a distinct transverse arch between the curve of the apse and the rectangular bay included in the chevet and at the same time to employ the two ribs beyond those forming the arch, as buttresses, to offset the thrust of the remaining radiants. Thus when the rectangular bay was larger than those around the curve, as for example in the choir of Soissons cathedral(Fig. 67), the buttressing ribs were longer than the remainder of those forming the vault. This made the bay containing these two ribs precisely like one-half of a six-part vault, and as this method of vaulting was commonly used in the nave and choir of these churches this chevet was a very

Fig. 67.—Soissons, Cathedral.

logical continuation of such a vault. But the builders do not seem to have realized immediately the aesthetic advantage in so planning their churches that such chevet vaults should come next to a six-part vault. At Sens(Fig. 28), however, the perfected use of this new chevet is shown for it is placed directly beyond a six-part bay and its two buttressing ribs are the counterparts of the two diagonals of the sexpartite vault. Once the advantage of such an arrangement was grasped, the churches were in many cases planned to provide for an even number of six-part bays in the choir followed by a chevet which carries the same system into the apse of the church. Thus in the cathedrals of Paris and Bourges, and probably originally in that of Soissons, as well as in other churches with six-part vaulting, this chevet became the standard form of eastern termination and the bay preceding the apse was made sexpartite so that the completed church would be uniform throughout.[372]Moreover the apsidal bays of the later chevets,as for example that at Soissons(Fig. 67)were frequently so planned that the radiants from the piers next beyond the ribs forming the transverse arch containing the keystone, were exact extensions of the buttressing ribs. In other words, except for the subdivision of the eastern bay into three window cells, the chevet corresponded to a true six-part vault inscribed in the space formed by the last bay of the choir and the polygonal-sided apse.

Notwithstanding the fact that the buttressing-ribbed chevet was primarily suited to churches with six-part vaulting, it was by no means confined to these for it is found in a large number which were from the beginning planned for four-part vaults. Among these is the cathedral of Rouen, in which the chevet is of distinctly six-part type with a full-sized choir bay included beneath the vault,[373]and the cathedral of Reims in which all the bays of the chevet are of practically the same size, as in the early churches which gave rise to this form of apse vault. Reims is thus an example of the perseverance of the design of a seven-sided chevet including one bay with parallel walls and yet of the same size as those forming the curve.[374]

But while pleasing in appearance when used in combination with six-part choir vaults, the chevet with buttressing ribs was not so satisfactory in churches with four-part cross-ribbed vaulting of rectangular plan. A reference to the vault of Soissons cathedral(Fig. 67)[375]will illustrate the faults of such a combination. These lie largely in the three-part vaulted bay. In the first place, though its window cells are practically the same width as those in the remainder of the choir, their crown lines run out at an awkward angle,[376]instead of being practically perpendicular to the outer walls as in the remaining bays of the apse and all those of four-part type. Secondly, the great, triangular, transverse severy is much larger than any of the others in the church and is thus unpleasing when contrasted with them, besides being more difficult to construct because of its larger size. It is not surprising to find, therefore, that a fourth form of chevet was developed and used extensively in churches with four-part vaulting. Thischevet, which will be termed diagonal-ribbed, is perhaps the most important distinct type developed in Gothic architecture.

Fig. 68.—Chartres, Cathedral.

It has already been noted that there were a number of early apses covered with an elementary kind of chevet which was formed by the use of two diagonal ribs over the semicircle of the apse in exactly the same manner as similar ribs were used in rectangular four-part vaulting. Such a vault as this may have been the prototype of the slightly more developed form to be seen in the radiating chapels of the cathedral of Noyon (before 1167)[377]and in the chapel at the end of one aisle of Notre Dame at Étampes (Seine-et-Oise) (cir. 1160). This latter has one extra rib added in what would have been the eastern bay of such a four-part apse vault subdividing it into two window cells and thus producing a four-celled chevet[378](Plate II-e.). It is exactly this principle, applied on a larger scale and with a further subdivision of this outer bay, which may be seen in such chevets as those of Chartres cathedral(Fig. 68)and Saint Étienne at Caen (first quarter of thirteenth century)(Fig. 70).[379]Of these, the one at Chartres has the more primitive character, for all of its seven bays are on the curve of a semicircle and thus none of the choir proper is included beneath the chevet (Plate II-f.). As a result of this increased number of bays, the intersection of the two diagonal ribs which form the first two radiants on each side, lies at a point comparatively near the keystone of the apsidal arch. This gives a certain uniformity to the size and character of the bays, but the vault is not yet perfect, for the ribs are still noticeably different in length, and more important than this the crowns of the window cells are at an awkward angle with the exterior wall. These faults are, however, much less marked in Saint Étienne, where the apse is greater than a semicircle—though even this chevet is not of the perfected diagonal-ribbed type, since it has no wall ribs and, moreover, is used over an apse of semicircular instead of polygonal plan like those of the developed Gothic period. An example of the perfected vault may be seen, however, above the apse of Amiens cathedral(Fig. 69). Here there are but five bays of the chevet along the curve of the apse proper, the remaining two being continuations of the choir walls (Plate II-g.). The diagonal ribs which determine the position of the keystone are therefore precisely such ribs as those in the remainder of the chevet except that the bay in which they lie is of smaller size than those preceding it and thus forms a gradual transition to the still smaller bays comprising the apse proper. As a result of this arrangement of ribs at Amiens, the keystone of the vault is so placed that it not only renders all the radiants of practically equal length but also makes the crown lines of each window cell so nearly perpendicular to the wall as to give a most symmetrical effect to the entire vault. Such a chevet constitutes the finest method of apse vaulting developed in Gothic architecture and in fact may well be considered the most perfect type conceivable, at least from the point of view of appearance. It loses a little in structural character through the fact that the first ribs do not abut the four eastern radiants at as firm an angle as in the previous chevet type,[380]but the advantage gained in the more symmetrical character of the vaulting severies makes up in large degree for this possible fault.

Fig. 69.—Amiens, Cathedral.

Nevertheless it may have been a feeling on the part of the builders that there was a lack of abutment to the west of the keystone which led to the introduction of one or more short ribs at this point in a number of chevets of various dates throughout the Gothic era. Thus in the apse of Saint Étienne at Caen(Fig. 70),[381]of Saint Trophîme at Arles, and of the cathedral of Notre Dame at Mantes, a single rib runs out from the keystone of the chevet to that of the apsidal arch. (Plate II-h.). Nor was this rib a continuation of a ridge rib in the choir, for in the instances just cited no such rib was employed. One is to be seen in a number of churches, among them such widely separated examples as San Saturnino at Pamplona,[382]Westminster Abbey,[383]and Saint Alpin at Chalons-sur-Marne.[384]All of these churches have diagonal-ribbed chevets, but there are instances of a short rib running to the apsidal arch even where the vault is of the buttressing ribbed type, as for example in the cathedral of Barcelona,[385]where it would seem to have been used to subdivide the great triangular transverse cell of the vault even more than to provide further apparent abutment for the other radiants (Plate II-i.). Even in chevets of the first type with ribs radiating from the keystone of the apsidal arch, a rib is occasionally added in the bay preceding this vault, as for example in Saint Pierre-le-Guillard at Bourges (fifteenth century vaulting), where this short rib runs out only to the crown of the six-part vault with which the last bay of the choir is covered (Plate II-j.). Occasionally, too, a church like the cathedral of Moulins (Allier) (1468-1508), with a ridge rib the length of the choir, is terminated by a chevet with radiating ribs which thus receive apparent abutment at their keystone (Plate II-k.).

Fig. 70.—Caen, Saint Étienne.

A similar purpose of providing apparent abutment would seem to account for the unusual form of the chevets of Bayeux cathedral (thirteenth century), and Sant’ Antonio at Padua (after 1232) in which all the radiants which ordinarily stop at the keystone are carried through against the face of the apsidal arch. At Bayeux there are two such ribs (Plate II-l.)[386]and at Padua, three (Plate II-m.). The latter is also exceptionally interesting in the form of its chevet which is really a combination of the diagonal and the buttressing ribbed type.

Although there are occasional instances like the one at Barcelona, in which the transverse severy of a buttressing ribbed chevet is subdivided only by a ridge rib, it is far more common to find a more extensive subdivision of this bay when such subdivision was undertaken at all. Moreover, it is an interesting fact that many of the elaborated chevet vaults—for it may be noted here that the apse vault was elaborated just as were those in the remainder of the church edifice—are fundamentally based upon the simple chevet with buttressing ribs.

Of these vaults with added ribs, perhaps the simplest are those in which the western bay is subdivided by the introduction of a ridge rib running about half way to the crown of the apsidal arch and there met by two tiercerons rising from the imposts of this same arch (Plate II-n.). A good example appears in the cathedral of Bayonne (Basses-Pyrénées) (after 1213), and another in that of Saint Quentin (Aisne) (commenced 1257), while the same subdivision of this severy in combination with other subdivided cells is to be seen in the Marien-kirche at Stargarde (Germany) (fourteenth century) (Plate IV-d.).

A second and unusual division of this severy appears in the cathedral of Saint Jean at Perpignan (Pyrénées-Orientales) (1324-1509),[387]where the customary three-part bay containing the buttressing ribs also contains two diagonals precisely like those in a four-part vault (Plate II-o.). A similar arrangement, with the addition of a ridge rib (Plate II-p.), may be seen in the church of Saint Jean at Ambert (Puy-de-Dôme) (fifteenth and sixteenth centuries). Such subdivisions as these last two quite evidently had for their object not merely a reduction in the size of the spacesto be vaulted but also an effort to retain the buttressing-ribbed type of chevet and still obtain a window cell which would not have the warped surface characteristic of this form.

Fig. 71.—Chalons-sur-Marne (near), Notre Dame-de-l’Épine.

A still more elaborate subdivision of the rectangular vaulting bay appears in the chevet of Notre Dame-de-l’Épine near Chalons-sur-Marne (1419-1459)(Fig. 71), where this bay contains no diagonals at all but is divided by a series of tiercerons and short ridge ribs in a manner best understood from the plan (Plate II-q.). But it is the subdivision of the window cells of the apse proper which is of especial interest at l’Épine, for the method here employed was very widely extended in the later Gothic period. It consists in the introduction into each of these cells of a short ridge rib running from the central keystone to a point about half way to the window crowns where it is met by two tiercerons which rise from the impost of the principal ribs of the chevet on either side of the window. The apparent object of the system is to so subdivide the vault surface asto break up its compound or ploughshare curves and substitute smaller panels whose surfaces are simpler to construct exactly as in the similar nave vaults previously described. This purpose does not show to advantage at l’Épine, where the awkward adjustment between the vault panels and the window heads would seem to indicate that the apse was originally designed for a simple form of chevet with no added ribs. Better examples could be cited, among them Saint Severin at Paris. Such an arrangement of window cells as that in these vaults practically converts the chevet into a ribbed half dome pierced with lunettes which do not rise to its crown. This may clearly be seen from a study of the apse of Saint Jacques at Antwerp (probably sixteenth century), where the vault is unusual in the omission of all the true radiating ribs (Plate II-r.). As a matter of fact such ribs were no longer of value since they did not mark the intersection of two vault panels but merely lay along a surface which is almost precisely like a section of a half dome. The tiercerons are still important since they mark the intersection of the window lunettes and carry the weight of the vault down to the piers. They are therefore retained. Thus, while the absence of radiants in Saint Jacques might seem to make this vault structurally less correct than that of l’Épine in reality such is not the case.

Once it became the custom to introduce extra ribs into the chevet, this portion of the church underwent the same treatment as the vault of the nave or choir. Thus in England, to cite only extreme cases of elaboration, the later Gothic produced such vaults as those of Tewkesbury Abbey (between 1325 and 1350),[388]in France, such pendant types as that of Saint Pierre at Caen (probably early sixteenth century), and in Germany such a choir and apse as that of Freiburg cathedral (late fifteenth century)(Fig. 72).[389]The last named is especially interesting as showing the low point reached in rib vaulting for its ribs have almost no function as supporting members, some of them being actually free from the vault panels and are merely used to form a decorative pattern upon a vault which would stand equally well were they entirely removed. Such chevets are, in many cases, clever examples of stone cutting and decorative design but they are lacking in fundamental structural character.

Fig. 72.—Freiburg, Cathedral.

Thus far the discussion of chevets has been distinctly from a structural point of view, but there remain certain other differences between these vaults which are worthy of remark. In the first place, there is the matter of the number of cells comprised in the chevet. The standard during the best Gothic period was seven, though five was a frequent number and quite often nine are found (Plate II-s.), as for example, in the apse of San Francesco at Bologna, Saint Martin at Ypres, Belgium, and that of Béziers (Hérault), cathedral (1215-1300).[390]In the smaller churches and in the radiating chapels there are frequently three. Moreover, when the apse has a central pier,[391]there are an even number of bays and thus four and six-celled chevets are employed. That in Saint Pierre at Caen, for example,has four bays all on the curve, and that in Notre Dame at Caudebec-en-Caux (Seine-Inférieure) (fifteenth and sixteenth centuries) (Plate II-t.) has only two bays thus placed, a fact which gives an angular character to the apse which is far from pleasing.[392]As for the chevets with six cells, they are of very infrequent occurrence, though one is to be seen in Saint Pierre at Auxerre (Plate II-u.). A chevet with the unusual number of eleven cells is to be seen in the church of La Chapelle-sur-Crécy (thirteenth century).[393]In construction, this chevet is similar to one bay of such an eight-part vault as that at Provins, Saint Quiriace[394]with its easternmost cell divided into five parts.

An interesting question arises in this connection as to why the central pier was employed in the mediaeval church. It is not common, yet it occurs frequently enough and over a sufficient space of time to prove that it did not lack a certain amount of popularity. Thus an apse with such a pier is to be seen in the early Romanesque church of Vignory (Haute-Marne) (consecrated cir. 1050-1052), where it is covered by a half dome, and again at Morienval (Oise)(Fig. 77), where there are ribs beneath a similar vault.[395]Throughout the Gothic period, this plan of apse surmounted by a chevet occurs in an even larger number of examples and toward the close of the period becomes quite popular. Leaving out of consideration the origin of the employment of a central eastern pier, which would seem most difficult to ascertain, it is at least interesting to note the effect which a chevet with a central rib presents when compared with the more usual type. If, for example, the apse of Saint Pierre at Auxerre be compared with that of the cathedral of Reims, the advantage and disadvantage of the two methods from the point of view of appearance may be seen. The most displeasing feature of the apse of Reims lies in the fact that its central arch and window, being seen in their full width, seem disproportionately wide in comparison with those on either side, while at Auxerre there is no window shown in its full width with the result that the transition is apparently more gradual from the ends to the center of the apse.On the other hand, the apse of Reims permits the addition of a lady chapel with an arch on the major axis of the church.[396]Altogether it is largely a question of personal preference which would seem to have guided the builders, combined, perhaps, with some considerations based upon the size of the apsidal curve and as to how many divisions would give the most pleasing form to the apsidal arches. As far as the actual construction of the chevet is concerned, the plan with a central pier made no essential difference, except possibly in the vaulting of the ambulatory which is discussed in the next chapter.

Another interesting, though minor feature of chevet vaulting, lies in the form of the masonry panels and the position of the imposts of the radiating ribs. The position of the latter varies considerably, though it corresponds in general with the impost level of the transverse arches in the nave or choir of the church. In the best period this was generally somewhat above the sill line of the clerestory windows, but in some of the early churches like Saint Germer(Fig. 63), Saint Quiriace at Provins(Fig. 31), and the cathedral of Bourges(Fig. 76), it is below this line, while in a number of later churches, among them Saint Urbain at Troyes (Aube) (1262-1329)(Fig. 73), it is as high as that of the arches forming the window heads. This last chevet is also important as showing a tendency to do away with the flat wall forming the lower portion of each panel and starting the outward curve of the masonry directly from the extrados of the ribs. Although this detracts somewhat from the beauty of the vault by making the curve of its cells too abrupt, it does prevent large portions of the windows from being concealed and therefore gives a more uniform effect to the clerestory.[397]Such an arrangement of the window cells is to be found even earlier in the chevet of Bayeux cathedral (early thirteenth century), where the rib rises from the clerestory string-course but is kept close against the wall to the impost of the window arches so that the effect produced is much like that at Saint Urbain.

Fig. 73.—Troyes, Saint Urbain.

Another feature of chevet vaulting which varies greatly throughout its history, is the comparative height of the crown of the wall rib, or line of intersection, and that of the main keystone; in other words, of the doming up of the vault panels. In this, there is a very wide divergence all through the Transitional and Gothic periods. Thus among the early chevets it will be noted that in some the doming is slight though noticeable, as at Saint Germer(Fig. 63), in others it is very pronounced, as at Vézelay(Fig. 66), while in others the crown of the cells actually curves downward toward the central keystone. This is an exceptional type, of which there is an example in Saint Remi at Reims(Fig. 64). Naturally enough, the

Fig. 74.—Angers, Cathedral.

vault which is most highly domed up exerts the least outward thrust and is thus most easily supported. It is not surprising, therefore, to find this form a favorite where large windows were not required in the apse or where there was no ambulatory or but a low one. This may perhaps explain its use in the south of France in the cathedral of Béziers (Hérault), as well as its popularity throughout Italy, where it may be seen on an exceptionally large scale in the cathedral of Milan. Certain of these domed up chevets may also be attributed to the type of nave vault developed in the locality in which they are found, as, for example, the chevet of Angers cathedral(Fig. 74), which is very highly domed, with the small torus ribs of the region forming the radiants beneath it. As a matter of fact, such a chevet as this differs from a ribbed-lobed-dome only in having its masonry courses running at right angles to the supporting walls. Its pressures are almost all downward with but little outward thrust though the arrangement of the masonry courses and the shape of the vault cells servesto concentrate both thrusts and pressures upon the ribs and piers instead of along the whole curve of the outer walls, thus rendering perfectly safe the introduction of large windows.[398]

Fig. 75.—Auxerre, Cathedral.

Still another interesting characteristic of certain chevet vaults is the presence of openings from one cell to the next in the lower portion of the panels between them. The simplest of these are to be seen in the cathedral of Auxerre (choir finished 1234)(Fig. 75), and it seems very reasonable from their square shape, comparatively small size, and their position at the beginning of the curve of the vault cells to assume that they were intended to hold wooden beams, used, quite possibly, as supports for scaffolding or centering for the rest of the vault. Whatever their use, they may be the prototypes of such larger openings as those in the cathedral of Bourges (after 1215)(Fig. 76), which may not only have been usedin a similar manner but which, from their circular shape and moulded character, supply a certain amount of decoration to this part of the vault and even serve in a slight degree to distribute the light from its windows over a larger area.[399]An even greater amount of decoration is obtained by the use of tracery in the similar openings in the cathedral of Orleans (begun 1630), which are of larger size and of a generally triangular shape.[400]The final development of such tracery panels may be seen in the Brunnenkapelle of Magdeburg cathedral (fourteenth century)[401]where the apse vault proper becomes practically a flat ceiling the entire space between it and each of the ribs being filled with tracery.

Fig. 76.—Bourges, Cathedral.

It is not the province of this essay to enter into a discussion of the origin of the ambulatory and its introduction into the church plan.[402]It is sufficient to note that a passage around a semicircular apse appears even in Roman times in the imperial tribune of the so-called stadium of Domitian on the Palatine at Rome which dates from the second century A.D.,[403]and that a similar passage was added around the apse of San Giovanni in Laterano by Pope Sergius II (844-845).[404]

Such ambulatories were mere service galleries, not directly connected with the apse and in fact shut off from it by a solid wall, but when once adopted as a feature of the church plan, the ambulatory rapidly became an aisle around the apse corresponding in all respects to that which flanked the rectangular nave or choir.[405]It was natural, therefore, that this added aisle should have been vaulted and such is the case in the two earliest ambulatories of any size which still exist, namely, those in Santo Stefano at Verona (end of tenth century) and the cathedral of Ivrea (973-1001 or 1002),[406]while the early ambulatories in France, like those of Saint Martin at Tours (end of eleventh century) and the cathedral of Clermont-Ferrand, which have unfortunately been destroyed, were doubtless also vaulted.

That the form which such vaulting assumes owes its origin to that of the concentric aisles in earlier buildings of circular plan would seem a most natural supposition since the problems in the two cases were precisely alike. As a matter of fact, a comparison shows that all or nearly all the methods of vaulting developed in the Roman or Byzantine period for the aisles of circular buildings were tried by the Romanesque builders when they added an ambulatory to the semicircular apses of their churches.

The principal Roman type would seem to have been the annular tunnel vault. An excellent example is to be seen in the amphitheatre at Nîmes in which the builders have even employed transverse arches of stone beneath the vault of brick.[407]Similar in character, though later in date and without transverse arches, is the fourth century annular vault of Santa Costanza in Rome. It is not surprising, therefore, to find the annular tunnel vault in a number of the earliest Romanesque ambulatories as, for example, at Ivrea and in the lower story of Santo Stefano at Verona, both dating from the close of the tenth century, and somewhat later at Vignory in France and in the gallery of the Tower chapel in London.[408]The annular tunnel vault never became in any sense a popular form, however, probably because it necessitated an impost above the level of the apsidal arches and exerted a continuous thrust throughout its whole extent. It is more often to be found in crypts, as in Saint Wipertus near Quedlinburg (936)[409]and in Chartres cathedral (1020-1028)[410]where there were no structural problems of support, or else with its imposts lowered and cut by lunettes into an interpenetrating form which is really an elementary groined vault and is later discussed.

Besides these annular vaults, there are a few examples of ambulatories with half tunnel vaults which may owe their origin to the desire of the builders to keep the outer impost of the vaults as low as possible and still raise the inner line above the apsidal arcade.[411]In any event such an ambulatory is occasionally found in churches where the aisles also are half-tunneled, as, for example, in the abbey church of Montmajour (cir. 1015-1018)[412]and in the twelfth century church of Saintes.[413]Though this type of vault apparently has no pre-Romanesque prototype, it is perhaps possible that the concentric aisle of the circular church of Rieux-Merinville (Aude) (eleventh century)[414]affords an earlier example of its use over a space of similar plan. There is also an interesting use of a half-tunnel vaulted triforium above the ambulatory and abutting the half dome of the apse which opens into it through five arches, in the church of Loctudy (Finistère) twelfth century.[415]

There are, however, circular buildings of the Byzantine and Carolingian periods with vaulted aisles which may well have furnished the prototypes for other methods of ambulatory vaulting which the Romanesque builders employed. One of these is the Royal Chapel at Aachen (796-804), in which the aisles are two stories high with the lower story covered by groined vaults of alternately square and rectangular plan with no transverse arches separating the bays.[416]

Although there appear to be no Romanesque churches with ambulatories of exactly this type, there are a number which are composed of triangular sections of an annular vault alternating with groined bays of practically square plan. One of these is the upper ambulatory of Santo Stefano (end of tenth century) at Verona, while a similar arrangementmay be seen in the concentric aisle of the crypt of Saint Bénigne at Dijon (Côte d’Or) (1002-1018).[417]Moreover, the type at Aachen of alternate square and triangular groined bays, is to be seen at Paris with the addition of transverse arches between the bays, in Saint Martin des Champs (cir. 1136) and at Gloucester in the beautiful ambulatory of the cathedral (1089-1100). Furthermore, this alternation of square and triangular bays was of quite frequent occurrence in the ribbed vaulted ambulatories later described.

The gallery of the Palatine chapel at Aachen is covered in still another manner by a series of ramping tunnel vaults alternately triangular and square in plan and springing from a series of transverse arches. Although never exactly copied in ambulatory vaulting, a similar system in which ramping groined vaults displace the simple tunnel form appears in the gallery of the north transept of San Fedele at Como (twelfth century)[418]while the system of ramping the vault had still another application in the trapezoidal groined vaults of San Tommaso at Almeno-San-Salvatore,[419]the evident object being to get a slant above the vaults suitable for an exterior roof which might rest directly upon them. But if ramping tunnel vaults were not used over the ambulatory, there are at least two instances of the employment of expanding transverse tunnel vaults in this position and these may well be products of the Aachen type. The ambulatory at Vertheuil[420]affords an example dating from about the middle of the twelfth century, which must soon have been followed by the gallery of the cathedral of Notre Dame at Mantes (beg. in 1160?).[421]Here the vaults are similar, but on a much larger scale, and with quite different transverse supports consisting of lintels, each resting upon two columns placed between the apsidal piers and the outer walls.[422]

All of the ambulatory types thus far described were but occasionally used in the Romanesque period. Far more common, and in fact the standard form, is that of simple four-part groined vaults over bays of trapezoidal plan. Here again the plan at least has a Byzantine prototype in the church of San Vitale at Ravenna where the concentric aisle is divided into trapezoids, though these in turn are cut by the radiating niches of the central nave and the groined vaults employed are therefore of irregular form.

Even without any prototypes, however, this arrangement of bays is a direct outcome of the use of an annular tunnel vault intersected by lunettes or transverse tunnels opposite the apsidal arches. Such vaults may in fact be seen at a comparatively early date in the churches of Bois-Sainte-Marie (Saône-et-Loire) (twelfth century), Champagne (Ardèche), and Preuilly-sur-Claise (Indre-et-Loire), and in a reversed sense at Saint Savin (Vienne) (cir. 1020-1040) where there is an early instance of a simple annular vault cut by expanding transverse tunnel vaults whose intrados at the smaller end corresponds to that of the apsidal arches but whose crowns rise higher than that of the vault which they intersect. There are no transverse arches and yet the vault is really composed of a series of trapezoidal bays. The ambulatory of Saint Sernin at Toulouse (choir consecrated 1096) shows this same system in its fully developed form. There are still no transverse arches, but the vault is no longer interpenetrating but fully groined, yet with practically level crowns, so that it still has the general form of intersecting tunnel vaults.

It was far more common, however, for the Romanesque builders to separate their trapezoidal bays by transverse arches, though their use would seem to have been optional rather than to indicate a more developed architectural type, since they are found at an early date in the ambulatory of Saint Philibert at Tournus (Saône-et-Loire) (1009-1019), where the form of the vault would otherwise be of interpenetrating type. It is, in fact, less developed than that at Saint Sernin, the transverse panels being considerably lower than the concentric portion of the vault thus forming simple lunettes above the window heads. In such a vault, the transverse arches are structurally valuable only in so far as they make possible the erectionof the vault in sections and consequently serve as permanent centering and as a stiffening member between the apsidal piers and the outer walls. In the fully developed vaults with transverse arches, like those at Paray-le-Monial these arches serve still another purpose. Here it is evident that the vault was laid up in sections, for each bay is domed up at the crown and the transverse arch not only carries a little of the weight of the vault but also conceals what would otherwise be an awkward intersection line between one bay and the next. With this doming up of the vault crown and the use of pointed transverse arches to replace the awkward stilted form, the vault of Paray-le-Monial marks the highest point possible before the introduction of the diagonal rib in the Transitional and Gothic periods.

It has already been stated that it is not the purpose of this paper to discuss the origin of ribbed vaulting. In fact, it is rather the intention to accept the conclusions of Mr. Porter in his “Construction of Lombard and Gothic Vaults” that this innovation arose from the necessity for providing a centering where wood was not to be easily obtained or where the shape of the bays or their position in the church made a permanent centering of stone or brick far superior to, and easier of construction than, a similar centering in wood.[423]Accordingly the fact that some of the earliest ribbed vaults appear over the ambulatory is readily explained by the trapezoidal shape of the vaulting bays, for which a wooden centering would have been especially difficult to construct.

Of these rib-vaulted ambulatories, the earliest which has come down to us would seem to be that of the little church of Morienval (Figs. 77, 78, 79), which probably dates from about 1120-1130. A study of this ambulatory shows most clearly the gradual changes and adjustments which mark the development of perfected rib vaulting from its groined prototype. In size this is an insignificant work and yet historically most important. Perhaps its first noticeable feature lies in the use of slightly pointed apsidal arches(Fig. 77), showing that the builders grasped in at least a

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