connected by vaults of a light material. The decorative construction, however, of the vault, exhibits the rib and panel from the abacus L, upwards. The point N is commonly at about Fig. 662d. M D half the vertical height of the arch, and is not necessarily guided by the impost of any clearstory rib adjoining. M is the general position where the mouldings of the several ribs run clear of one another at the divergence of the ribs. The solid part LM is built of horizontal courses of masonry, generally each of a single stone and its level beds cut the curved moulding, obliquely in front." 2002g. Moller, Memorials, &c., translation 1836, p. 154, notices that, at Cologne, the lower part of the vaulting of the cathedral is formed by horizontal courses of stone projecting from the wall, consequently the ac tual span of the vaulting is proportionally diminished, while, on the other hand, the abutment is in the same degree strengthened. Still more deserving of attention is the manner in which the essential parts are so linked to gether as to be rendered incapable of thrusting or giving way, and therefore of necessity remaining in their original position. Price, in his work on Salisbury Cathedral, 1753, p. 25, quaintly remarks: "And here I beg leave to make a conjecture, that is, that all the springing stones of the vaultings were inserted into the walls at the time of their being erected, and so left till the whole church was roofed and covered in; and then being defended from rains, &c., they fixed their principal ribs and groins, and turned over the vaultings, as having the weight of the superstructure to act instead of a buttment.” 2002h. "Above M," continues Professor Willis, "the ribs are each built separately of voussoirs, having their beds properly inclined to meet the axis of curvature of the rib, and these ribs are backed and united by solid masonry which connects them with the wall, and which, appearing between the rib, seems to be a portion of the light vaulting surface. really employed higher up. From the upper surface N, each rib A is still built as from M to N with youssoirs, but upon these ribs rests the light thin vault or panel-work." 2002i. "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. The reason for it is not easy to explain, but it is very common, especially in the earlier examples. These courses, in the transepts at Westminster Abbey, are of a light coloured stone, probably chalk, interrupted at regular intervals by a course of a darker stone; and the ridge, which has no rib, is also formed entirely of this darker stone, laid in a serrated manner. These 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 of the panel between each rib is also made slightly concave or domical (probably to preserve the effect of being level, as seen from below it), and may therefore have been laid without any centreing, since each course would support itself. These peculiarities may all be found with some variations in other vaults of the same age." 2C02k. "The architect of Leon cathedral," remarks Mr. Street in his work on Gothic Architecture in Spain, p. 110, “ filled in the whole of the vaults with a very light tufa, obtained from the mountains to the north of Leon; so at least I was assured by the superintendent of the works at the cathedral. Some of the material I saw was no doubt tufa; but some of it seemed to me to be an exceedingly light kind of concrete. The vaulting of Salisbury Cathedral is similarly constructed. I do not know whether at Beauvais the same expedient was adopted to lessen the weight." Both at Beauvais and Leon the construction in every part was too light. 20921. Over the vaults was commonly laid a thick irregular course of rubblework, which again is also often covered with a kind of concrete. The vaults of the western compartments of Westminster, and of the south transept and tower of Hereford, are left bare on the upper surface: and these vaults, instead of being built with small brick-like stones, are composed of long thin slabs. The ribs themselves are, in some later examples, formed of a few long bar-shaped voussoirs instead of the small and numerous pieces of the earlier examples. Thus, in the transept at Westminster, L to N consists of 13 or 14 stones; but at the west end of the nave of 6 only. 2002m. Price notices (p. 24) that at Salisbury, "The groins and principal ribs are of Chilmark stone, but the shell, or vaulting between them, is of hewn stone and chalk mixei, on top of which is laid a coat of mortar and rubble of a consistence, probably ground in a kind of mill, and poured on hot, while the lime was bubbling; because by this, the whole is so cemented together, as to become all of one entire substance. This composition is very remarkable, somewhat resembling the pumice-stone, being porous and light, by which it contributes prodigiously to the strength of the whole, and at the same time the least in weight of any contrivance that perhaps was ever used." 2002. "The early moulded ribs are formed as fig. 662f. from St. Saviour's Church, Southwark, the vaulting or panel-work resting only on their backs; but the ribs of later date are rebated for the reception of this work, as shown in fig. 662e." 20020. As early as 1225-50, the square plans for vaults were superseded by oblong ones, which allowed the cross-rib, the groinrib, and the wall-rib to arrive at nearly one level. In the new system the groin-ribs were portions of circles, and the cross-ribs were struck with the same radius; but these vaults were soon considered to be weak, and the cross-ribs were heightened while the groin-ribs were either stilted or (subsequently) sharper pointed. 2002p. As soon as mediaval builders admitted the principle that the strength of arch-stones, like that of beams, is more dependent on the depth than on the width, they reduced the width as much as they could in order not to require a large abacus to the capital. The next step was to resolve all thrusts upon that support into a force acting directly upon it; and consequently to endeavour to make the various pressures, which the pillar has to bear, combine in a point in a stone that should be fully as large in plan as the abacus, and perhaps rest upon others of the same character. 2002q. The operation of deciding the form and place of this stone is very simple after the size of the arch-stones has been determined. Supposing that the work is, as in a cloister, bounded by a wall, and with B out wall-ribs, there will only be a E DO TK N ୧ M 2002r. The use of the semicircle is not an indispensable, but is a naturally convenient step, because the equal quantities so taken by it from all the spans of the ribs leaves undisIturbed in general result all calculations founded upon lines drawn from mathematical points that are taken as centres in a plan made to a small scale; but the plans of the groin-ribs may be placed anywhere upon their respective centre-lines A Fig. 6620. M RR so long as the intersection or junction of the neighbouring lines of the widths of the ribs is secured at some point. This intersection is not an absolute necessity, but it is the means of reducing the size of the abacus; and the point of junction S is that beyond which (working from the wall) the two ribs will be distinct. Taking this point S as fixing a line for the springing, the elevations of the two arches are to be drawn on the intersecting lines; then lines SR and ST drawn perpendicular to the springings, will cut the extrados of each arch at points which decide the level of the top bed of the horizontal work. The mass of work between this bed and the capital will be divided into a convenient number of courses, and the plans of the beds thus fixed are easily drawn from the elevations of the arches; when it will be seen that, if the groin-ribs are less in depth than the crossribs, the former will give a good starting place for the material which is to form the spandrils of the vaults. In a similar manner. the intersections of any number of ribs may be found, and the tertiary and secondary ribs may be successively suppressed in favour of the primary principal ones. Viollet le Duc, Dictionnaire, s.v. Construction, p. 96. Prof. Willis gives the following illustration (fig. 662h.) showing the method of setting out mouldings for vaulting, belonging to the perpendicular period; it is taken from one of the spandrils of a complex vault which formerly covered the extreme north-western compartment of the nave of Canterbury Cathedral, the lower storey of the so-called Lanfranc's tower. The number of ribs being seven required two stones in each of its upper courses at least; fig. 662h. being only a portion of the spandril, contains but four of them. It also shows that the stone had been scored, then rejected, and another set of lines drawn, and actually employed. AB, AC, are the rejected centres, and D) a portion of the first outline. EF, EG, EH, and EI are the true lines drawn, each parallel to its own rib. The average thickness of the courses is about 10 inches. H F B I Fig. 662h 2002. The key-stone or boss-stone was adopted by the medieval architects as a necessary appendage to groin-ribs, because the solidity of the vaulting depends greatly upon the pressure exerted by the key, which must consequently be heavier than any of the archstones; it will necessarily be an extremely large stone, allowing a great part of its mass to be cut away by the sculptor in order to diminish its apparent heaviness. This stone should generally be nearly circular in plan; for if the ribs diverge enough to leave any large space between them, a fracture is almost certain. In cases of such divergence, it is best to design the sculpture so that a mass may occupy the space. This remark, of course, does not apply where there are only two groin-ribs meeting at right angles; but it governs the amount to which the groin-rib should be allowed to be worked in the key-stone. No part of the boss ought to be sunk within a horizontal line connecting the intrados of one rib with that of another; and it is generally desirable that, whether or not the ribs be back-jointed for the filling of the groining, as fig. 662f., the key-stone should have a projection or tail sufficient to stand above the back of the filling. "Every boss-stone," says Professor Willis, "had its upper surface made horizontal, on which were drawn the lines from the axis of the boss in the direction of the respective ribs." The principles here indicated are illustrated in fig. 662i. Fig. 6621. 2002t. In the construction of groined vaulting it has been considered best to fix the keystone on the centreing, before laying the arch-stones, for the sake of the guidance which it affords in the work; the inconveniences of working a boss in its place, and of setting it it already worked, were obviated in the 18th century by leaving its breast smooth, so that a wooden boss, carved at leisure, might be fastened to it with hooks. In the 15th century such a boss was not unfrequently of stone instead of wood. 2002u. A striking feature of the Flamboyant style is the frequent use of pendents in the vaulted roofs of the period. These, however, are not confined to the Continent, for the Tudor period in this country exhibits many splendid instances of their employment, none, perhaps, more gorgeous, or more interesting as regards its construction, than the Chapel of Henry the Seventh. Some of the various examples that exist have been scientifically investigated by Professor Willis, in his paper On the Construction of the Vaults of the Middle Ages, already quoted; and we therefore now proceed merely to indicate the principles upon which the fairy-like system of not only suspending vast bosses from the ceiling was conducted. but that by which these bosses or pendents became in their turn the springers for supporting other vaults as in the beautiful little Lady Chapel at Caudebec in Normandy, and many other examples. A plan is given in the section on PRINCIPLES OF PROPORTION. 2002. This chapel is hexagonal on plan, about 23 feet in span, or from side to side. Fig. 662k. shows the mode by which, from the key-stone of an arch approaching a semicircular form, and suspended or elongated beyond its ordinary depth, support is given for the springing of the vaults of the different bays. On this practice Philibert De Lorme observes, "Les ouvriers ne font seulement une clef au droict de la croisée d'ogives, mais See another section and] Fig. 662k. (plan, figs. 1322 and 1323. aussi plusieurs quand ils veulent rendre plus riches leurs voûtes, comme aux clefs où s'assemblent les tierc. rons et liernes, et lieux où ils ont mis quelquefois des rempants, qui vont d'une branche à l'autre, et tombent sur les clefs suspendues, les unes étant circulaires, les autres en façon de soufflet, avec des guymberges, mouchettes, claire voyes, feuillages, crestes de choux et plusieurs bestions et animaux: qui étoient trouvés fort beaux du temps qu'on faisoit telles sortes de voûtes, pour lors appelées des ouvriers (ainsi que nous avons diet) voûtes à la mode françoise." 2002w. We have shown above the mode of suspending the pendent in a polygonal building. The fig. 6621, by a little consideration, will explain the mode of suspending pendents not centrically situate, as in the case of the ceiling of Henry the Seventh's Chapel, whose date runs coincident with the Flamboyant period. The figure is a transverse section and plan of the vaulting of the building, in which one of the main arches, on which the whole construction-depends, springs just below A, and reaches its summit at B. The voussoirs or arch-stones whereof it consists are marked in their order. The dotted interval from a to b is not to be considered as an interruption of the formation of the arch by the pendent, but may be supposed an imaginary line passing through it, or rather through the arch-stone or voussoir C, whose general form is marked by the bounding letters cdefba; so that, in fact, the pendent is nothing more, as in the case of the Lady Chapel at Caudebec, than a voussoir, a large part whereof hangs down below the face of the vaulting. The voussoirs are out of blocks about 3 feet 6 inches deep; but a considerable portion of the solid below the soffice of the arch is cut away to form the lobes of the cinquefoils. The arch D serves, by its connection with the walls, to stiffen and give weight to the arch where it would be most required, that is, towards the springing. The pendent or voussoir E, on the same block with C, being thus established in its place, serves at, or towards its foot, as a springer for the ribs of a fanwork tracery shown on the plan, whose ribs are, in fact, ribs of a dome, and in construction do not differ from it. Their section is shadowed somewhat lighter than the pendent voussoir. The fanwork round each affords the means of introducing another pendent at its meeting at F in the plan. (This pendent is shown at F in the two sections given in PRINCIPLES OF PROPORTION.) The fan vault is very properly distinguished by Prof. Willis from what he calls the stellar vault, which is formed of ribs that may be, and indeed frequently are, of different curvature, and the rays of the star of different lengths; whereas the fan vault consists of ribs of the same curvature and height, and the summit of the fan is bounded (see the fig.) by a horizontal circular rib, instead of the ends of lozenges forming the points of the star. "The effect of the fan is that of a solid of revolution, upon whose surface panels are sunk: the effect of the star is that of a group of branching ribs." It is manifest that the constructive details of these two sorts of vaulting are vastly different. In the one, the depcndence is upon ribs which support, by rebates on them, the filling in panels; while in the other the principle is similar to that of domevaulting. This will be immediately perceived by reference to the plan G, in which the courses are marked, as also in the part of the section marked H. The plan I shows the tracery of the soffite of the vault. The author above quoted observes, The construction of these fan vaults is in all examples so nearly the same, that they seem to have proceeded from the same workshop, and it is remarkable that, at least as far as I know, there are no Continental examples of them; whereas, of the previous vaults, there are quite as many on the Continent as in England. In France, indeed, the lierne" (ribbed) "vaults are not very numerous; they are confined to small chapels, and their patterns are in general simple. But in Germany and in the Netherlands there is an abundance of them, distinguished, certainly, from ours by local peculiarities, but nevertheless of similar mechanical construction, and requiring the same geometrical methods." 2002r. The introduction of fan vaulting seems to have occurred in the beginning of the 15th century. The first instance wherein the span was considerable is the Dean's Chapel attached to the north-west transept of Canterbury Cathedral. In St. George's Chapel at Windsor, the aisle and central compartment only have fan vaults, the principal vault not being fanwork. The chief works of this kind, of known date (about 1500), are Henry the Seventh's Chapel at Westminster, King's College Chapel at Cambridge, the central tower of Canterbury, and Bath Abbey church. (See PRINCIPLES OF PROPORTION) In the church of St. Etienne du Mont at Paris, we find a remarkable example of the style of the renaissance contending with the expiring flamboyant style. In short, the whole of the interior is a mass of interesting incongruities. The church is cruciform, and at the intersection of the cross is a pendent key-stone, most elaborately wrought, and more than 13 feet deep. It is obvious, in respect of these pendents, that there is no mechanical difference between their pendency and their being insistent, as lanterns are, on domes. |