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. MORIENVAL 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 [Illustration: FIG. 77.--MORIENVAL, CHURCH.] rudimentary way the advantage to be gained in thus bringing these arches up to a point where they would be nearly, at least, on a level with the crown of a semicircular formeret. The use of these formerets or wall arches is a second advance in this vault at Morienval, and though these are unnecessarily heavy and in two orders (Fig. 78) they do reduce the width of the vaulting bays and furthermore they clearly define the wall line of the panels and may even have aided in the support of the wooden centering or _cerce_ on which the severies were laid up. They do not apparently support the actual masonry of the cell, which, as is clearly shown in the southwest bay, does not follow the curve of the formeret.[424] The transverse arches (Fig. 78) show little structural advance, for they are still round headed. They are however highly stilted yet in addition to this the builders have found it necessary to pile their crowns with masonry in the manner already described in connection with the vaults at Bury.[425] It is in the use and [Illustration: FIG. 78.--MORIENVAL, CHURCH.] [Illustration: FIG. 79.--MORIENVAL, CHURCH.]