Besides the ribbed half dome just described, there is still another type to be seen in the Lady chapel of the church of Saint Martin-des-Champs at Paris [(Fig. 65)]. Its plan is a trefoil and the vault is made up of a series of segments of domes with salient ribs marking their intersections. As far as construction is concerned, there is really no change from that of the more common half dome, for the courses of masonry are still horizontal and the ribs merely serve as centering and as a means of subdividing the surface to be vaulted and clearly marking the lines of intersection. The vault would stand equally well were the ribs removed and is, in structural character, very similar to the celled domes of the Villa Adriana at Tivoli and of S.S. Sergius and Bacchus at Constantinople.
Fig. 61.—Saint Martin-de-Boscherville, Saint Georges.
“Groined Half Domes”
Another form of apse vault of which there would seem to be a number of examples prior to the introduction of ribbed vaulting may perhaps be termed the “groined half dome.” It is a vault resembling a segmental dome except that the segments do not run down to a common impost, but form a series of window cells not unlike those of a groined vault but not running all the way to the vault crown. The earliest of these vaults appears to be that in the crypt of Saint Laurent at Grenoble (Isère) (sixth century).[348] Rivoira has shown[349] that Roman prototypes of this form can be found in the so-called “Temple di Siepe” (second century) at Rome, the vestibule of the Villa Adriana at Tivoli (125-135) and elsewhere. There are also a number of Romanesque examples. Of these, one is in the chapel off the south transept of Saint Nicholas at Caen (1080-1093),[350] while another is to be found in Saint Andrew’s chapel at Canterbury cathedral (cir. 1110).[351] These vaults closely resemble the true Gothic chevet which was soon to follow them, and they might seem to be its prototypes were it not for the fact that their construction is of an entirely different character. All are built of small stone or rubble and were undoubtedly laid up on a wooden centering with no particular regard for the direction in which the masonry courses ran, or possibly with these courses like those in a half dome. The construction was thus a combination of half dome and groined vaulting and not at all of the ribbed type. That they may, however, have been of influence in the development of the true chevet will be later suggested.