Lectures on the rise and development of medieval architecture; vol. 2

by Sir George Gilbert Scott

Fig. 358.

Fig. 359.

It will be better understood by means of Fig. 358, in which a transverse rib, two diagonal ribs, and two wall ribs meet at their common springing line, and so intersect and unite one with another as to produce a section at the base composed of portions of them all. Now, a drawing of this group of ribs will at once show that their combined and united form must extend to some considerable height above the springing; and so far as it reaches, which is often some 8 feet or 10 feet in height, they cannot possess an individual existence ([Fig. 359]). To this height, then, it is customary to build the group of ribs in horizontal courses, and only to commence the radiating arch-joints where the ribs clear themselves one from another, which usually occurs at one level, though in vaults of great irregularity one rib often clears itself at a lower level than another.

In setting out the relative position of the ribs upon the common springing level, great skill and judgment are requisite, or they will clear themselves one of another so irregularly as to cause great difficulty and needless twisting in the filling in of the vaulting surface. If you set out on plan the side lines of two ribs ([Fig. 360]), and lay down the true position of the mouldings of one of them, it is clear that, if the curvature

Fig. 362.

Fig. 360.

Fig. 361.

of both were equal, the second rib should be set out with its back line at an equal distance from the points at which the plans of the adjoining lines would intersect; for, in following the curve, both would at a given height reach a point vertically over that intersection, and so the filling in would have a proper starting-point, which would not be the case if they reached that vertical line at different heights. As, however, the diagonal rib (where the ridges are level) has to travel farther to reach a given height, its springing section has to be set farther back to make it reach the vertical line over this point of intersection at an equal level with the transverse rib. The wall rib in a square vault would be similarly placed with the transverse rib; but in an oblong vault, as it would travel a less distance to reach a given level, its springing section must be placed forwarder than that of the transverse rib, and, of course, greatly forwarder than the diagonal. This is easily adjusted by drawing the curve of the back of the rib, whose position is first determined, drawing against it the vertical line of its intersection with the next rib, and then, from the apex point of the adjoining rib, to draw its curve through the point of intersection, which will give on the springing line the distance backward or forward at which the springing section of that rib should be placed ([Fig. 361]).