The left hand half of this diagram represents a portion of the vault in perspective, including the entire spandrel solid (if I may be allowed the expression), which is contained by the two semi-diagonal ribs AD AE and the wall.
ABC is the transverse rib, and on the right hand of the diagram the vertical section through this rib is exhibited.
GH is the string moulding upon which the clerestory windows rest and the arch which contains these windows springs from LK at a considerable height above the springing, A, of the vault ribs.
This is a very universal arrangement of clerestory vaults and is productive of great beauty and convenience, but it leads to some difficulty in the form and arrangement of the vaulting surface AKFD, for as this is contained between two arches or ribs ADKF, which spring from different levels, it follows that this surface must be skewed back at K in a very peculiar manner. This is shown by the perspective.
The junction of the solid mass, AKQ, with the clerestory wall is therefore bounded by parallel vertical lines, one of which is AK, and this mass is always built of solid masonry bonded into the wall and forming a part of it.
It is from the level of KQ that the real rib and panel construction of the vault begins, for separate ribs are erected upon the surface of this solid, and connected by vaults of a light material. From below, however, if the vault be painted and decorated, this change of construction at KQ is disguised, or, in other words, the decorative construction of the vault exhibits the rib and pannel from the abacus A upwards, but the mechanical construction is of solid masonry from A to Q, and of rib and pannel work only above this level.
The point Q of this change of construction is commonly at about half the vertical height of the arch, as shown in the drawing, but is not necessarily at the same level as the impost K of the clerestory rib KF.
The peculiar construction of the solid mass AKQ is better shown in the section at CMN. The ribs of the vault converging downwards to C, their mouldings become entangled as it were, in a manner that will be subsequently explained. At a point M, about halfway up the solid, they are, however, freed from each other, and separated by the divergence of the ribs.
Now, between C and M the solid is built of horizontal courses of masonry, generally each of a single stone, and its level beds cut the curved mouldings obliquely in front.
Above M the ribs are each built separately of voussoirs, having their beds properly inclined to meet the axis of curvature P of the rib, and these ribs are backed and united by solid masonry which connects them with the wall, and which appearing between the ribs seems to be a portion of the light vaulting surface, such as is really employed higher up.
From the upper surface N of the solid, each rib NB is still built as from M to N with voussoirs, but upon these ribs rests the light thin vault or pannel, shown in section on the right, and in perspective on the left of the diagram.
It is remarkable that the courses of the vaults are not laid level, but are in most cases made to incline downwards upon the diagonal rib.
Thus in fig. 3 the ridge FD is level, and also the ridge DBE; but the courses of the vaults incline considerably downwards from FK and from QB towards the diagonal rib AD. These courses, in Westminster transepts, are of a light-coloured stone, probably chalk, interrupted, at regular intervals, by a course of a darker stone; and the ridge FD, which has no rib; is also formed entirely of this darker stone, laid in the serrated manner shown by the drawing. The dark courses are rather broader than the light ones, and there are four or five courses of the light between each of the dark.
The surface FKD is also slightly concave or domical, and may therefore have been laid without any centering, since each course would support itself. These peculiarities may all be found with some variations in other vaults of the same age.
What might have been the reason for this downward inclination of the courses it is not easy to say, but it is very common, especially in the earlier examples.
Some have supposed it to have arisen from the courses having been laid to meet the bounding ribs AKF AD at respectively equal distances from their springing, which would certainly produce the effect in question, since the diagonal rib is so much longer than the others, but the downward inclination is greater than that which would arise from this cause. In some examples the slope seems to be derived from the courses having been laid so as to meet the diagonal rib at right angles.
The perspective effect which arises from the arrangement is curious, for the vaulting surfaces ADB ABE are really very nearly coincident with a single surface extended from AD to AE, or in other words, a horizontal rod passed upwards along the backs of the ribs AD AE would very nearly touch the two vaulting surfaces and the back of the rib AB.
But the effect of the inclination of the courses is to make the rib AB appear in perspective as if prominent downwards from such a surface, and consequently gives to the entire solid spandrel FDEA the effect of a kind of fan vault or vault with a polygonal horizontal section.