VAULT 1 (Fr. voute, Ital.

volta, Ger. Gaavolbe)  , in architecture, the term given to the covering over of a space with stone or brick in arched form, the component parts of which exert a thrust and necessitate a counter resistance: In the case of vaults built under the level of the ground, the latter gave all that was required, but, when raised aloft, various expedients had to be employed, such as great thickness of walls in the case of barrel or continuous vaults, and cross walls or buttresses when intersecting vaults were employed . The simplest kind of vault *is that known as the barrel, wagon Or tunnel vault; which is generally semicircular in section, and may be"regarded as a continuous arch, the length of which is in excess of its diameter; like the arch (q.v.), the same provision is required as regards its temporary support whilst the voussoirs constituting one of its rings are being placed in position, for until the upper voussoir, or keystone, is introduced it is not self-supporting . At the present day, when timber of all kinds is easily procurable, this temporary support is given by centring, consisting of a framed truss with semicircular or segmental head, which carries the voussoirs until the ring of the whole arch is completed and is then, with a barrel vault, shifted on to support other rings; in early times, and particularly in Chaldaea and Egypt, where timber was scarce, other means of support had to be contrived, and it would seem that it was only in Roman times that centring was regularly employed . The earliest example known of a vault is that found under the Chaldaean ziggurat at Nippur in Babylonia, ascribed to about 4000 B.C., which was built of burnt bricks cemented with clay mortar . The earliest tunnel vaults in Egypt are those at Requaqnah and Denderah, c . 3500 B.C.; these were built in unburnt brick in three rings over passages descending to' tombs: in these cases, as the span of the vault was only 6 ft., the bricks constituting the voussoirs were laid flatwise, and adhered sufficiently to those behind to enable the ring to be completed without other support; in the granaries built by Ramessu II., still in part existing behind the Ramesseum, at Thebes, the span was 12 ft., and another system was employed; the lower part of For the form of safe so called see SAFES . the arch was built in horizontal courses, up to about one-third of the height, and the rings above were inclined back at a slight angle, so that the bricks of each ring, laid flatwise, adhered till the ring was completed, no centring of any kind being required; the vault, thus formed was elliptic in section, arising from the method of its construction . A similar system of construction was employed for the vault over the great hall at Ctesiphon, where the material employed was burnt bricks or tiles of great dimensions, cemented with mortar; but the span was close upon 83 ft., and the thickness of the vault was nearly 5 ft. at the top, there being four rings of brickwork . It is probable that the great vaults of the Assyrian palaces were constructed in the same way, but with unburnt bricks dried only in the sun: one of the drains discovered by Layard at Nimrud was built in rings sloping backwards . From the fact that each Assyrian monarch on his accession to the throne commenced his reign by the erection of a palace, it is probable that, owing to the ephemeral construction of these great vaults, half a century was the term of their existence . This may also account for the fact that no domed structures exist of the type shown in one of the bas reliefs from Nimrud (fig . I); the tradition of their erection, however, would seem to have been handed down to their successors in Mesopotamia, viz. to the Sassanians, who in their palaces at Serbistan and Firuzabad built domes of similar form to those shown in the Nimrud sculptures, the, chief difference being that, constructed in rubble stone and cemented with mortar, they still exist, though 'probably abandoned on the Mahommedan invasion in the 7th century .

In all the instances above quoted in Chaldaea and Egypt the bricks, whether burnt or sun-dried, were of the description to which the term "

tile" would now be given; the dimensions varied from 18or coin. to to in., being generally square and about 4 to 2 in. thick,, and they were not shaped as voussoirs, the connecting medium being thicker at the top than at the bottom . The earliest Egyptian examples of regular voussoirs in stone belong to the XXVIth Dynasty (c . 65o B.C.) in the additions made then to the temple of Medinet-Abou, and here it is probable that centring of some kind was provided, as the vaults are built in rings, so that the same centring could be shifted on after the completion of each ring . The earliest example of regularly shaped voussoirs, and of about the same date, is found in the cloaca at Graviscae in Etruria, with a span of about 14 ft., the voussoirs of which are from 5 to 6 ft. long . The cloaca maxima in Rome, built by Tarquin (603 Lc.) to drain the marshy ground between the Palatine and the Capitoline Hills, was according to Commendatore Boni vaulted over in thelet century s.e., the vault being over Boo ft. long, to ft. in span, with three concentric rings of voussoirs . So far, all the vaults mentioned have been barrel vaults, which, when not built underground, required continuous walls of great thickness to resist their thrust; the earliest example of the next variety, the intersecting barrel vault, is said to be over a small hall at Pergamum,in Asia Minor, but its first employment over halls of great dimensions is due to the Romans . When two semicircular barrel vaults of the same diameter cross one another (fig . 2) their intersection (a true ellipse) is known as a groin, down which the thrust of the vault is carried to the cross walls.; if a series of two or more barrel vaults intersect one another, the weight is carried on to the piers at their intersection and the thrust is trans- - mitted to the outer FIG . 2 . cross walls; thus in the Roman reservoir at Baiae, known as the piscina mirabilis, a series of five aisles with semicircular barrel vaults are intersected, by twelve cross aisles, the vaults being carried on 48 piers and thick external walls . The width of these aisles being only about 13 ft. there was no great difficulty in the construction of these vaults, but in the Roman Thermae the tepidarium had a span of 8o ft., more than twice that of an English cathedral, so that its construction both from the statical and economical point of view was of the greatest importance . The researches of M .

Choisy (L'Art de bdtir chez les Romains), based on a minute examination of those portions of the vaults which still remain in situ, have shown that: on a comparatively slight centring, consisting of trusses placed about to ft. apart and covered with planks laid, from truss to truss, were laid—to begin with—two layers of the Roman brick (measuring nearly 2 ft. square and thick); on these and on the trusses transverse rings of brick were built with longitudinal ties at intervals; on the brick layers and embedding the rings and cross ties concrete was thrown in horizontal layers, the haunches being filled in solid, and the surface sloped on either side and covered over with a tile roof of low pitch laid direct on the concrete . The rings relieved the centring from the weight imposed; and' the two layers of bricks carried the concrete till it had set . As the walls carrying these vaults were also built in concrete with occasional bond courses of brick, the whole structure was homogeneous . One of the important ingredients of the mortar was a volcanic' deposit found near Rome, known as pozzolana, which, when the concrete had 'set, not only made the concrete as solid as the rock itself, but to a certain extent neutralized the thrust of the vaults, which formed shells equivalent to that of a metal lid, the Romans, however, do not seem to have recognized the extra-ordinary value of this pozzolana mixture, for they otherwise provided amply for the counteracting of any thrust which might exist by the erection of cross walls and buttresses . In the tepidaria of the Thermae and in the basilica of Constantine, in order to bring the thrust well within the walls, the main barrel vault of the' hall was brought forward on each side and rested on detached columns, which constituted the principal architectural decoration . In cases where the cross vaults intersecting were not of the same span as those of the main vault,, the arches were either stilted so that their soffits. might be of the same height, or they formed smaller intersections in the lower part of the vattit;, in both of these cases, however, the intersections or groins were twisted, for which it was very difficult to form a centring, and, moreover, they *ere of disagreeable effect` though every attempt was made to mask this in the decoration of the vault by panels and reliefs modelled in stucco . The widest hall vaulted by the Romans was that of the throne room in the palace of Diocletian on the Palatine Hill, and this had the enormous span of too ft., its. thrust being counteracted by other halls on, either side with buttresses outside: •In provincial towns and in -other • parts of ; the, Roman Empire, where the material pozzolana was not procurable, the Romans had to trust to‘ their mortar. as a cementing medium, but this, though excellent of its kind, was not of sufficient cohesive strength to allow of the erection ofvaults of more than about 40 ft. span, which were generally built in rubble masonry . There still . exist in Asia Minor.. and Syria some vaulted halls, generally attached to -thermae, which are . carried on walls of great- thickness . There were ftraay varieties of the Roman vault, whether continuous or intersected, such as those employed oyer the corridors on the Colosseum and the theatre of Mar tellus, but in these eases the springing of the vault was above the summit of the arches of the main front, so that there was no intersection; on the other hand, over the corridors were either elliptical- or semicircular,- or over the staircases, -rising vaults, all of which were more difficult to construct; there were also numerous sfrlutions of vault over circular halls, of which that of the Pantheon was the most important example, having a diameter of 142 ft., and over the hemicycles, which were sometimes of great size; that known as Canopus in Hadrian's villa at Tivoli had a diameter of 75 ft.,- and was vaulted over with a series of ribs, between which were alternating rampant flat. and semicircular webs and cells; in the Same villa- and in Rome were octagonal halls with various other combinations of vault . Another type. of vault not yet referred to is that of the Tabularium arcade where the cloister• vault was employed . Fig . 3 compared - with fig .

2 will show the difference; in the former the angles of intersection are inset, and inthe latter they -~ are groins with. pror :jecting angles at the

base, which die away at the summit . The -vault of the basilica, , commenced .by.: - Diocletian and completed by . - Con- stantine, was the last The diagram (fig . 4) shows the outlines of the solution of the problem . If hemispherical dome is cut by -four vertical planes, the. intersection gives four, semicircular arches; if cut in addition by a horizontal plane tangent to the top, of these arches, it describes a circle; that portion of the sphere which is below this circle and between-the 'arehessfarming a spherical" spandril, is the Pendentive (fig . 5), and its radius is equal to the diagonal of the square on whichthe four arches rest . - Having obtained -a• circle for the base-of the dome, it is not necessary that the upper portion of the dome should - --------- ---FIG . 5.—AA,pendentive . spring from the same level as the arches, or that its domical surface should be a continuation of that of the pendentive . The first and second dome of Sta Sophia apparently fell down, so that Justinian determined to raise it, possibly to give greater lightness to the structure, but mainly in order to obtain increased light for the interior of the church . This was effected by piercing it with forty windows—the effect of which was of an extraordinary nature, as,the light streaming through these windows gave to the dome the appearance of.being suspended in the air . The pendentive which carried the dome rested on four great arches, the thrust of those crossing the church being counteracted by immense buttresses which traversed the aisles, and 'the other -two partly by smaller arches in the apse, the thrust being carried to the outer walls, and to a certain ex-tent by the side walls Which Were built under the arches .

From the description given by

Procopius we gather that the centring employed for the grs'ntafthes consisted of a wall erected to support them during their erection . The construction of the pendentive§ is not known, but it is surmised that to the top of the pendentives they were built in horizontal courses of brick, projecting one over the other, the projecting angles being cut off afterwards and covered with stucco in which the mosaics were embedded ; this was the method employed in the erection of the Periaordiandomes, to which we shall return; these, however, were of leas diameter than tho'se' of Sta Sophia, being only about 40 to 6o ft. instead of i.oy ft . The apotheosis of Byzantine architecture, in fact, was reached in Sta Sophia, for although It formed the model on which all subsequent' . Byzantine churches were based, so far at their plan was concerned, no domes approaching' the former in dimensions were even&'I't`empted . The principal difference in some later examples is that which took place in the form of the pendentive on which the dome was carried . Instead of the spherical, spandril of Sta Sophia, large niches were formed in the angles, .as in the mosque of Damascus, which was built by Byzantine workmen for the Sherif al Walid in A.D . 705; these gave, an .octagonal `base ion Which the hemispherical dome rested (fig . 6) ;, or again, as in the ,4anian palaces iof Serbistan and Firuzabad of the 4th a Sth*'ten- tury of our, era, when a a of con- '-centric arch rings, projecting one in front'` ,%~ - of the other, were built, giving also an c octagonal base; each of these pendentives is known as a squinch . There is one other remarkable vault, also built by Justinian, in the church of S . Sergius and lacchus in Constantinople . The Central area of this church Was octagonal on plan; and the dome is divided into. sinteen compartments; of these eight consist of broad flat bands rising FIG . 6.—BB, niche or from the centre of each of the walls, squinch pendentive. and the alternate eight are Cohcave cells over the angles of the octagon; which externally and internally give to the roof the appearance of an umbrella .

Although the dome constitutes the principal characteristic of the Byzantine church, throughout Asia Minor are numerous examples in which the naves are vaulted with the semicircular barrel vault, and this is the type of vault found throughout the

south of France in the r rth and r2th centuries, the only change being the occasional substitution of the 'pointed barrel vault, adopted not only on account of its exerting a less thrust, but because, as pointed out by Fergusson (vol. ii. p . 46), the roofing tiles were laid directly on the vault and a less amount of filling in at the top was required . The continuous thrust of the barrel vault in these cases was met either by semicircular or pointed barrel vaults on the aisles, which had only half the span-of the nave; of this there is an interesting example in the chapel of St John in the Tower of London—and sometimes by half-barrel vaults., ,The great thickness of the walls, however, required in such constructions would seem to have led to another solutio i roll , .s . .s of the problem of roofing over churches with incombustible material, viz. that which is found throughout Perigord and La Charente, where a series of domes carried on pendentives covered over the nave, the chief peculiarities of these domes being the fact that the arches, carrying them form 'part of the pendentives, which are all built in horizontal courses . The intersecting and groined vault of the Romans . was, employed in the early Christian churches in Rome, but only over the aisles, which were comparatively of small span, but in these there was a tendency to raise the centres of. these vaults, which became slightly domical; in all these cases centring was employed . Reference has been made to the twisting of the groins in Roman work, where the intersecting barrel vaults were not of the same diameter; their construction must at all times have been somewhat difficult, but where the barrel vaulting was carried round over the choir aisle and was intersected, as in St Bartholomew's, Smithfield,, by semicones, instead of cylinders, it became worse and the groins more complicated; this would seem to have led to a change of system, and to the introduction of a new feature, which completely revolutionized the construction of the vault . Hitherto the intersecting features were geometrical surfaces, of which the diagonal groins were the intersections, elliptical in form, generally weak in construction and often twisting (Plate I. fig . 13) . The medieval builder reversed the process, and set up the diagonal ribs first, which were utilized as permanent centres, and on these he carried his vault or web, which henceforward took its shape from the ribs . Instead of the elliptical curve which was given by the intersection of two semicircular barrel vaults, or cylinders, he employed the semicircular arch for the diagonal ribs; this, how-ever, raised the centre of the square bay vaulted above the level of the transverse arches and of the wall ribs, and thus gave the appearance of a dome to the vault, such as may be seen in the nave of Sant' Ambrogio, Milan . To meet this, at first the trans-verse and wall ribs were stilted, or the upper part of their arches was raised, as in the Abbaye-aux-Hommes at Caen, and the abbey of Lessay, in Normandy . The problem was ultimately solved by the introduction of the pointed arch for the trans-verse and wall ribs—the pointed arch had long been known and employed, on account of its much greater strength and of the less thrust it exerted on the walls .

When employed for the ribs of a vault, however narrow the span might be, by adopting a pointed arch, its summit could be made to range in height with the diagonal

rib; and, moreover, when utilized for the ribs of the annular vault, as in the aisle round the apsidal termination of the choir, it was not necessary that the half ribs on the outer side should be in the same plane as those of the inner side; for when the opposite ribs met in the centre of the annular vault, the thrust was equally transmitted from one to the other, and being already a broken arch the change of its direction was not noticeable . The first introduction of the pointed arch rib would seem to have taken place in the choir aisles of the abbey of St Denis, near Paris, built by the Abbe Suger in 1135, and it was in the church at Vezelay (1140) that it was extended to the square bay of .the;porch . Before entering into the question of the web or stone shell of the vault carried on the ribs, the earlier development of the great vaults which were thrown over the naves of a cathedral, or church, before the introduction of the pointed arch rib, shall here be noted . As has been pointed out, the. aisles had already in the early Christian churches been covered over with groined vaults, the only advance made in the later developments being the introduction of transverse ribs'. dividing the bays into square-compartments; but when in the lath century I Transverse ribs under the vaulting surfaces had been employed from very early times by the Romans, and utilized as permanent stone centrings for their vaults; perhaps the earliest examples are those in the corridor of the Tabularium in Rome, which is divided into square bays, each vaulted with a cloister dome . Transverse ribs are also found in the Roman Piscinae and in the Nymphaeum at Nimes; they were not introduced by the Romanesque masons till the t t th century.the first attempts were made to vault over the naves, another difficulty presented itself, because the latter were twice the width of the aisles, so that it became necessary to include two bays of the aisles to form.: one square bay in the nave: This, was, an immense space to vault over, and, moreover, it ,followed that every alternate pier served no purpose, so f as the support of. the nave vault was concerned, and''this we ld~seem to hav+ suggested an alternative, viz. to provide a supplementary tib across the church and between the transverse ribs . This resulted in what is known as a sexpartite, or six-celled vault, of which one of the `earliest examples is found in the Abbaye-aux-Hoinmes (S . Etienne) at Caen . This church, built by William theConqueror, was originally constructed to carry a timber roof only, but nearly a century later the upper part of the nave walls were partly rebuilt, in order that it might be covered with a vault . The immense size, however, of the square vault over the nave necessitated some additional support, so that an intermediate rib was thrown across the church, dividing the square compartment into six cells; ' and called the sexpartite vault (fig . 7); this was adopted in the cathedrals of Sens (1170), Laon (1i9s), Noyon (r190), Paris' (1223—35),. and Bourges, (125o) . The, intermediate rib, however, had the disadvantage of partially obscuring one side : of the clerestory windows, and it threw unequal weights on the ' alternate piers, so that in the cathedral of S ' (1205) a quadripartite (fig . 8) or four-celled vault was introduced, the width of each bay being half the span of the nave, and corresponding therefore with the aisle piers .

To this there are some exceptions, in Sant' Ambrogio, Milan, and

San Michele, Pavia (the original vault) and in the cathedrals of Spires, Mainz and Worms, where the quadripartite vaults are nearly square, . the intermediate piers of the aisles being of much smaller dimensions . In England sexpartite vaults exist at Canterbury (1175) (set out by William of Sens), Rochester (r aoo), Lincoln (1215), Durham (east transept), and St Faith's chapel, Westminster: Abbey . In the earlier stage of rib 'vaulting, the arched ribs consisted of independent or • separate voussoirs down to the springing; the difficulty, however, of working the ribs separately led to two other important changes: (I) the lower part of the transverse diagonal and wall ribs were all worked out of one stone; and (2) the lower courses were all made horizontal, constituting what is known as the tas-de-charge (q.v.) or solid springer . Fig . 9 is a diagram made by Professor Willis taken from the south transept of Westminster Abbey . The horizontal courses rise to N. or about half the height of the vault, but the ribs are freed from one another from the point M . The tas-de-charge, or solid springer, had two ad- vantages: (I) it enabled the stone courses to run straight through the wall, so as to bond the whale together much better; and (2) it lessened the span of the vault, which then required a centring of smaller dimen- sions . As soon as the ribs were completed, the web or stone shell verse and diagonal ribs; P, some English work, as may be seen centre of the same; DE, longi- in fig . 9, each course of stone was tudinal ridge rib; DF, inter- of uniform height from ohe side section of webs; M, top of solid to the other; but, as the diagonal springer; KN. starting level rib was longetlimn either a be of web; LK, Springing of wall transverse or *all rib, the courses rib; EBD, bosses at inter- dipped towards the former, and at section of ribs, the apex of the vault were cat to fit one another . At an early period, in consequence of the great span of the vault and the very slight rise or curvature of the web, it was thought better to simplify the construction of the web by introducing intermediate ribs between the wall rib and the diagonal rib and between the diagonal and the transverse ribs; and in order to meet the thrust of these intermediate ribs a ridge rib was required, and the prolongation of this rib to the all rib hid the junction of the web at the summit, which was not Airways very sightly, and constituted the ridge rib . In France, on the other hand, the web courses were always laid horizontally, and they are therefore of unequal height, increasing towards the diagonal ribs Each course a so was given a slight rise iwth< centre, so as to increase its strength this enabled the French masons to dispense with the intermediate rib, which was not introduced by them till the 15th century, and then more as a decorative than a constructive feature, as the domical form given to the French web rendered unnecessary the ridge rib, which, with some few exceptions, exists only in England . In both English and French vaulting centring was rarely required for the building of the web, a template (Fr, cerce) being employed to support the stones of each ring until it was complete .

In

Italy, Germany and Spain the French method of building the web was adopted, with horizontal courses and a domical form . Sometimes, in the case of comparatively narrow compartments, and more especially in clerestories, the wall rib was` stilted, and this caused a peculiar tvbting of the web, as may be seen in fig . 9, where the springing of the-wall rib is at K: to these twisted surfaces the term " ploughshare vaulting " is given . ' One of the earliest examples of the introduction of the inter-Mediate rib is found in the nave of Lincoln Cathedral, and there the ridge rib is not carried to the wall rib . It was soon found, however, that the construction of the web was much facilitated by additional ribs, and consequently there was a tendency to increase their number, s(Ythat in the nave of Exeter Cathedral three intermediate ribs were provided between the wall rib and the diagonal rib . In order to Mask the junction of the various ribs, their intersections were ornamented with richly carved bosses, and this practice increased on the introduction of another short rib, known as the lierne, a term I France given to the ridge rib . Lierne ribs in English vaults are sort ribs crossing between the main ribs, and were employed chiefly a!,•idecorative features, as, for instance, in the stellar vault (see Plate I. fig . 16), one of the best examples of watch exists in the vault of the oriel window of Crosby Hall, London . The tendency to increase the number of ribs led to singular results in some cases, as in the choir of Gloucester (see Plate II. fig . 17), where the ordinary diagonal ribs become mere ornamental mouldings on the surface of an intersected pointed barrel vault, and again in the cloisters, where the introduction of the fan vault, forming a concave-sided conoid, returned to the principles of the Roman geometrical vault . This is further shown in the construction of these fan vaults, for although in the earliest examples each of the ribs above the tas-de-charge was an independent feature, eventually it was found easier to carve them and the web out of-the solid stone, so that the rib and web were purely decorative and had no constructional or independent functions . The fan vault would seem to have owed its origin to the employment of centrings of one curve for all the ribs, instead of having separate centrings for the transverse, diagonal wall and intermediate ribs; it was facilitated also by the introduction of the four-centred arch, because the lower portion of the arch formed part of the fan, or conoid, and the upper part could be extended at pleasure with a greater radius across the vault .

The simplest version is that foundin the cloisters of Gloucester Cathedral, where the fans meet one another at the summit, so that there are only small compartments between the fans to be filled up . In later examples, as in

King's College chapel, Cambridge (see Plate II. fig . 18), on account of the great dimensions of the vault, it was found necessary to introduce transverse ribs, which were required to give greater strength . Similar transverse ribs are found in Henry VII.'s chapel (see Plate II. fig . 19) and in the divinity schools at Oxford, where a new development presented itself . One of the defects of the fan vault at Gloucester is the appearance it gives of being half sunk in, the wall ; to •remedy this, in the two buildings just quoted, the complete conoid is detached and treated as a pendant . One of the most interesting examples of the fan vault is that over the staircase leading to the hall of Christ Church, Oxford, and here the complete conoid is displayed in its centre carried on a central column . This vault, not built until 164o, is an exceptional example of the long continuance of traditional workmanship, probably in Oxford transmitted in consequence of the late vaulting of the entrance gateways to the colleges . Fan vaulting is peculiar to England, the only example approaching it in France being the pendant of the Lady chapel at Caudebec, in Normandy, In France, Germany and Spain the multiplication of ribs in the 15th century led to decorative vaults of various kinds, but with some singular modifications . Thus' in Germany, recognizing that the rib was no longer a necessary constructivefeature, they cut it off abruptly, leaving a stump only; in France, on the other hand, they gave still more importance to the rib, by making it of greater depth, piercing it with tracery and hanging pendants from it, and the web became a horizontal stone paving laid on the top of these deco-rated vertical webs . This is the characteristic of the great Renaissance work in France and Spain; but it soon gave way to Italian influence, when the construction of vaults reverted to the geometrical surfaces of the Romans, without, however, always that economy in centring to which they had attached so much importance, and more especially in small structures . In large vaults, where it constituted an importanr element in expense, the chief boast of some of the most eminent architects has been that centring was dispensed with, as in the case of the dome at Florence, built by Brunelteschi, and Ferguson cites as an example the great dome of the church at Mousta in Malta, erected in the first half of the 19th century, which was built entirely without centring of any kind .

Fig. to is a plan and section of the vault of Henry VII.'s chapel and fig . 11 a

perspective view, in which it will be seen that the transverse rib thrown across the chapel carries the pendant, the weight of the latter probably preventing a rise in the haunches . There are two other ribbed vaults in India which form no part of the development of European vaults, but are too remarkable to be passed over; one carries the central dome of the Jumma Musjid at Bijapur (A.D . 1559), and the other is the tomb of Mahomined (A.D . 1626-166o) in the same town . The vault of the latter was constructed over a hall 135 ft. square, to carry a hemispherical dome . The ribs, instead of being carried across the angles only, thus giving an octagonal base for the dome, are carried across to the further pier of the octagon (fig . 12) and consequently intersect one another, reducing the cen- tral opening to 97 ft. in diameter, and, by the weight of the tnasonry they carry, serving as counterpoise to the thrust of the dome, which is set back so as to leave a passage about 12 ft. wide round the interior . The internal diameter of the dome is 124 ft., its height 175 ft. and the ribs struck from four centres have their springing 57 ft. from the floor of the hall . The Jumma Musjid dome was of smaller dimensions, on a square of 70 ft. with a diameter of 57 ft., and walls as in the tomb; but any thrust which might exist was counteracted by its transmission across aisles to the outer wall . (R . P .