The Use of A Central Pier in the Apse
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.
Impost Levels of Chevet Vault Ribs
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.
The Shape of the Chevet Cells
