That at Gloucester is a work of extraordinary magnificence. Its great specialty lies in its having two sets of diagonal ribs (Fig. 388), the one on the ordinary system, the other comprising two bays each, and by their intersection defining the position of the “Welsh” groins. Thus the whole of the ordinary diagonal ribs, and half of the others, become mere ornamented mouldings on the surface of the barrel-vault, while to them is added a vast network of liernes, cutting it into an infinity of panels, whose angles are marked by a perfect army of carved bosses. The same was imitated at a later period in the Lady Chapel.

In many cases, as in the western portals at Winchester, the upper portion of the groining assumes forms in which the lines of the true ribs seem almost forgotten. This is done to a very vicious extent in the choir at Wells. I may mention, as a late example of this manner of vaulting, an exquisitely beautiful chamber adjoining the cloister at Windsor.

During this period two practices crept into use, to which it is not easy to assign precise dates, but which worked great changes in the art of vaulting. The one is the use—especially at first for diagonal ribs—of a curve drawn from two centres, which gradually brought down the arches to excessively low proportions. The other practice was the use of portions of the same curve for several, or even all, of the ribs, either throughout their height, or at least for their lower part. The first-named custom was very natural in cases where the height of the vaults was limited by circumstances, as in St. Stephen’s crypt. The diagonal ribs here were struck from three centres to the double rib, and in the next ribs from two to each single rib, and were also slightly segmental; that is to say, that they made an angle with the vertical line. They may be said to be a sort of imitation of the ellipse, as also is the case with the diagonals in the vaulting of the south and west cloisters at Westminster.

The other practice (i.e. the repetition of the same curve for different ribs) is very curious and important in its results. It influenced in an extraordinary degree the plan of the vaulting at its intermediate heights. Professor Willis has called special attention to these half-height plans, as a matter of much importance to the effect.

If vaulting were carried out on perfect theoretical principles, and with the level ridge, these plans would be rectangular. The mere substitution of circular curves, unless made a little segmental, softens the angle of the square, while any modification of these curves produces its effect, one way or another, on this half-height plan. Professor Willis has illustrated this in a very interesting manner from the variations found in the cloisters at Norwich; a work generally of uniform design, but which, having been carried out at several different periods, the habits of the masons had undergone changes which produced very curious effects in this respect.

I may mention that where the ribs are of the same curve the ridges cannot be level; but that the use of the two-centred curve enabled them to repeat the same curve, for the springers, but slightly to change it above by varying the position of the centre, from which the upper part of the curve is struck, so as to make the ridges level where it was desired.

This tendency to the repetition of the same curve led to the development of a most remarkable variety in vaulting, which especially characterised its later history in this country. I allude to that extraordinary form known as Fan vaulting.

It is self-evident that if a number of ribs of equal curvature spring at equal angles from a single pillar, the plan of the vault at any level will be a circle or a portion of a circle; and that they may be bounded at the level of their apex, or at any other level, by a circular moulding, forming the whole into a figure which I have had occasion to mention as having been generated at a very different date and from another cause, in the Norman vaulting of the circular Chapter-house at Worcester,[60] a figure which I have defined as a concave-sided conoid, and likened to the flower of a convolvulus.

Now, the vaulting of any space, if set out in square compartments, and having ribs of similar curvature and at equal distances, may be formed into a number of portions of this figure by merely drawing semicircles and quadrants, as the case may be, from apex to apex of the surrounding arches (Fig. 389).

A remainder of the ordinary vaulting, with rising ridges, exists, however, above this extemporised fan, and the whole has to be dealt with artistically on a system suited to the new form.

The general idea of such treatment of a fan or conoid may be said to be parallel to that of a circular or rose window. The ribs are viewed as radiating mullions, and are made to multiply in number as the circle expands, and usually to terminate round the outer boundary with a series of tracery forms like the heads of two-light windows. A good rose-window of late date, if imagined to be elastic, and drawn out from its centre into a conoidal form, would make a good compartment of fan groining; or a groining fan compressed into a plane would make a good rose-window. The interstices between the fans are filled up in various ways; either by circles of somewhat similar design, which sometimes drop down in little pendent fans, like stalactites from the roof of a cavern; or with a number of circles fitted together; or by continuing the diagonal ribs to their intersection with the ridges, and filling in the triangular spaces with tracery.

It is not, however, essential that the compartments so dealt with should be square in plan. If oblong, the semicircles or quadrants are drawn from the apex of the narrower and lower arches; and from the same centres portions of circles are drawn from the apex of the wider and higher arches, assuming the form of an additional or outer zone of a rose-window, and intersecting on the line of the cross ridges, and thus forming a portion cut out of a great fan (Fig. 390). Or the roof may be described as being formed of very large fans intersecting one another. Such is the famous roof of King’s College Chapel, at Cambridge, but the frustums of fans are there bisected by vast transverse ribs, which were, no doubt, required for strength, owing to the great scale of the vaulting (Fig. 391).

The earliest instance which is known of fan vaulting is in the cloisters at Gloucester, dating from the beginning of the fifteenth century; indeed, Professor Willis thinks the invention due to a school of masons there. It, contrary to the usual practice, is formed with simple arches, instead of four centres. Each fan has but the transverse and wall ribs at its springing. At the next stage it forms the diagonal, then two intermediate ribs, and finally four more on each side of the transverse rib, and ends in little two-light window-like heads, while the space above is filled with four large cusped arches, and as many pear-shaped figures (Fig. 392).

The limits of my lecture will not permit me to follow up this part of my subject as its importance demands; and my means of producing illustrations (for which I have heartily to thank several zealous and talented gentlemen engaged in my office, as well as one of my sons) have exhausted themselves before this laborious phase is reached.

I have already referred to the Gloucester cloisters and King’s College Chapel.

The central compartment or crossing of St. George’s, Windsor, is a most magnificent treatment of an oblong space. The aisles of the same chapel, with the smaller chapels adjoining, are charming examples, as are the aisles and small chapels of Henry VII.’s Chapel, and the cloisters of St. Stephen’s, Westminster. Bath Abbey and Sherborne Minster are thus vaulted, and a whole century later, appearing long after date, comes the beautiful ceiling of the staircase to the hall at Christ Church, Oxford (Fig. 393), a square space groined on a central pillar (Fig. 394).

In the construction of the vaulting of these later periods, we have a curious instance of the manner in which extremes meet. In the earliest specimens of vaulting, all the strength lies in the vaulting surface itself. As time went on, ribs were introduced, one after another, to strengthen and support it, till at length they amounted to a permanent framework of stone, centering on which the vaulting surface lay.

Now, at length, time has its revenge, and the extreme multiplication of ribs led to the loss of their uses; the whole, or nearly the whole, being cut out of the same blocks with the panels; and thus the original system was reverted to; the vaulting surface becoming again the entire structure, and the ribs and panels simply cut as ornaments out of its substance.

The most remarkable production of the fan system of vaulting is the gorgeous central roof of Henry VII.’s Chapel, a work in which ingenuity, perplexity, and beauty are united in the most wonderful manner which can be conceived.

Though it stands quite alone in point of intricacy and magnificence, it is not the first instance of the use of the peculiar system on which it is founded. How Fuller could have attributed its origin to the foreign studies of the King and Bishop Fox, it is difficult to conceive; for, not only is fan groining itself a purely English invention, but the special system of this roof has, so far as I know, only English prototypes.

The earliest of these is the vaulting of the Divinity School at Oxford,[61] finished about 1480. This was subsequently imitated, though with the loss of its leading principle, in the choir of the Cathedral (then the Church of St. Fridewide’s Convent in the same city), a work which, though popularly attributed to Wolsey, is probably of earlier date.

I have often looked in vain for the leading principle on which this wonderful work was designed. Its construction is plain enough; the difficulty is the ideal of its design. Like everything, however, which is founded on reason, its idea once perceived, it becomes perfectly simple, and one then only wonders where the difficulty lay.

It was simply as follows:—First imagine, for argument’s sake, that the architect had intended to divide his space into three spans—a wide and two narrow ones—like the Lady Chapel at Salisbury, or the crypt of the Sainte Chapelle at Paris, supporting the vaulting on ranges of thin pillars (Fig. 395). The setting out of the divisions was about the following. The whole span being divided into five parts, two were given to the width of a bay, one to the imaginary aisle, and three to the central span. The vaulting of the aisles, then, would be in oblongs of double their width set lengthway of the building, and the central span in oblongs half as long again as their width set crossway to the building.

Let us apply to each the rule I have already laid down for the fan vaulting of oblongs. Beginning with the ideal aisles, from the pillars and responds and centres draw the lower circles of the fans reaching the apex of the narrower arches, and the upper circles reaching the apex of the wider arch (Fig. 396). This gives us an oblong fan vault in its most normal form.

Then do precisely the same with the wider or central span, and our space is covered with oblong fan vaulting of the most usual kind. Now, as the lower arch of the central span is identical with the higher arch of the side spans, it follows that the fans in the sides (or aisles) are continued and completed in the central vault, the lower arch of the small vault being also continued round to complete the design. The decoration of the smaller fans then is in two stages, and that of the larger ones in three; but, the design of both being continuous, the one is only an extension of the other.

Had the architect stopped here, no system of vaulting, on the fan principle, or space so divided, could be more systematic or more simple in its ideal.

He had no thought, however, of stopping at so commonplace a stage, and his pillars were designed only to do their work on paper, and then to be erased. The columns were omitted, and their places supplied by pendants; but, as such a roof could not stand for a moment, something must be done to supply the support which the pillars would have afforded.

This was effected by the introduction of strong transverse arches crossing the whole chapel, and springing much lower than the vaulting (Fig. 397). These crossed the narrower spans, striking arbitrarily into their fans, and uniting themselves with the central vaulting. The pendants are not to be looked upon as constructionally interfering with these transverse arches, as they become, in fact, part of them, and the arches may be supposed to pass through them in an imaginary line, so that, as in the case of the Lady Chapel at Caudebec, the pendant is in reality a voussoir, the greater portion of which hangs down below the face of the vaulting; the small arches upwards towards the wall help to strengthen and weight the transverse arches at their weakest part.

In the Divinity Schools (Fig. 398) these great arches show themselves throughout as the supports of the otherwise helpless vault; in Henry VII.’s Chapel they are visible only in the side vaults, which are strutted up from them with strong tracery, but their upper portions penetrate the central vault, and become concealed from view.

The same system is carried into the apse (Fig. 399), and that with the most surprising skill. The apse is supposed to be a portion of an entire octagon, with an aisle supported by eight small columns, of which two are lost by its conjunction with the straight part of the chapel. These columns being converted into pendants, the structural arches supply, as before described, the support demanded, but in this case they converge to the central part of the octagon.

The treatment of this point in detail cannot intelligibly be described in words. It is, perhaps, the most consummately skilled piece of designing to be found in the whole range of Mediæval vaulting.

I have now completed my running (and all too rapid) sketch of the arched and vaulted systems of Mediæval architecture, though purposely leaving to another occasion the subject of domes. The limits of three lectures have only sufficed to give a somewhat cursory glance at its salient points, leaving the treasures of its detail to be searched out by the zealous student.

No subject in the whole history of architecture is so remarkable, or would more richly repay the investigator. I commend it to your individual study, and will only add that our own country is more rich in the variety of its vaulting than any other, and that London is especially well supplied with objects of study, containing, as it does, excellent examples of nearly every variety of vaulting, from the stern severity of the work of King Edward the Confessor in the substructures of his monastic buildings at Westminster, to that gorgeous and astonishing work which I have just been describing, and of which we may boldly assert (whatever may be our individual preferences), that the world does not contain its equal.

LECTURE XVI.

The Transition.

Strongly as my tendencies towards our own Mediæval architecture draw me towards the modes of vaulting which prevail in our own ancient buildings, and which formed the subject of my later lectures, I am bound to admit that the noblest of all forms by which a space can be covered is the dome; and, much more than this, that of all architectural forms it is the most sublime and the most poetic, and is susceptible of, and demands, the highest artistic treatment. I deplore, therefore, its non-existence in our old English architecture.

This regret, however, is diminished by the abundant evidence we possess that the dome, though absent from English buildings, was by no means held to be alien from the contemporary architecture of neighbouring countries, inasmuch as we possess it in highly developed forms over a large part of France, in Germany, and in Italy, erected at the same periods with many of our great Mediæval edifices.

If, then, I am departing from the line I had been taking in tracing out the history of old English architecture, I am not only supplying a hiatus in that history, but I trust that I may be able to offer suggestions for a more practical object—the supplying of that hiatus in our revived and redeveloped Neo-mediæval style.

In a former lecture, after defining a vault as the covering of a rectilinear space produced by the motion of an arch parallel to itself, I defined a dome as the covering of a circular space produced by the revolution of an arch round its central vertical axis. It follows that, if the arch so revolving is semicircular, the resulting dome is a hemisphere.

The revolving arch may, however, be of any form which an arch can assume. It may be elliptical, parabolic, hyperbolic, cycloidal; or it may be a pointed, a horse-shoe, or an ogee arch. Any one of these, or other forms of arch revolving on its centre, will generate a dome of its own sectional form. The plan, too, in spite of my definition, need not be a circle; it may be an ellipse, or of other forms.

I will not at this stage admit of square-planned, polygonal, or other straight-sided domes, because it may be doubted whether they are genuine domes at all, or whether they are not figures resulting from the intersection of a certain number of ordinary vaults. Anyhow, these are not generated by the revolution of an arch, so that if they are domes, my definition is at fault.

Simple, however, as is the hemispherical dome, it does not appear to have been historically the primeval type; for, strange to say, the earliest known domes assume, in section, the form of a lofty pointed arch. I refer specially to that of the supposed tomb of Agamemnon at Mycenæ, and also to some portrayed on the Assyrian sculptures.

These would hardly come within the range of this lecture, were it not so curious a fact that the earliest form of the dome foreshadows the very use to which I would now especially desire to accommodate it.

It is easy to perceive why the pointed arch was adopted in these primeval domes. They were not built with radiating joints, but in overhanging courses; and it will readily be seen on comparing their sections, that for this mode of construction the pointed arch is much more favourable than the circular; while, as soon as ever the radiating system was adopted, the semicircle at once became the simplest and most obvious section.

It is, however, strange to observe how little has come down to us of the history of domes. From the so-called tomb of Agamemnon—some eleven centuries before the Christian era—we have scarcely any certain evidences of their history till we arrive at the Pantheon, erected in the reign of Augustus, or, as Mr. Fergusson thinks, considerably later. Yet, as that dome is still the widest (built of solid materials) which exists, and as both in its construction and its architecture it is in a high degree artificial, and evinces a period of advanced development, it follows that it must be the representative of a long series of antecedent domes progressing from the crude idea onwards to this, the very highest developed form of the simple dome; for we shall presently see that there are other forms scarcely foreshadowed by even this magnificent structure.

I may here remark that the dome, like the arch and the vault, having great outward pressure, requires either a vast amount of sustaining wall carried up considerably above the springing level, or, in the absence of this, a tie of metal at, or somewhat above, its springing, or perhaps several of such ties at different heights. Unlike, however, the arch and the vault, it is independent of a keystone, each circular course of its structure forming a horizontal arch and keying itself. It may, consequently, be erected without the aid of centering, and may be discontinued at any level, leaving a central opening or eye.

The Pantheon is the great type of Roman domes (Fig. 400). It is a simple rotunda of 142 feet internal diameter, the wall being some 20 feet thick. The wall is about 72 feet high to the springing of the dome, and continues above that level about 28 feet more.

The dome is a semicircle, but has an open eye at its apex of nearly 30 feet in diameter (Fig. 401).

The dome, as viewed externally, is buried by the wall which rises above its springing to fully a third of its height, and above this rises a sort of attic crowned by six gradenæ, burying nearly an equal height, so that the dome as an external feature is far from conspicuous, appearing as a mere flattened disc.

Internally, however, it forms a covering of the noblest character. Its internal surface is deeply coffered by panels of four orders, in depth dividing the circumference into twenty-eight parts, and its height, up to about two-thirds of the distance from the springing to the crown, into five parts, the upper portion being plain. These vast panels or coffers, the larger range of which exceed twelve feet in diameter, are curiously arranged as to their sectional recessing, so as to appear perfectly symmetrical to the eye of a spectator standing beneath the centre of the dome.

The wall, up to the springing of the dome, is beautifully decorated with rich architecture in marbles of varied colour, and it cannot be doubted that the cupola—the very soul of the design—was embellished in a manner fully proportioned to the beauty of its sustaining wall; indeed, it is thought to have been coated with gilded bronze.[62] When thus perfect, it must have formed an interior of surpassing beauty, lighted as it was solely through the central eye, and the light tempered by the linen veil stretched across its rich bronze cornice, which still remains round the opening, and retains vestiges of gilding. The Pantheon can scarcely be called a daring effort of construction, because its vast solidity seems to defy all doubts as to its duration. It would be, however, absurd to suppose it to have been an early effort; for it is, as I have already said, so artificial in its construction, as clearly to prove it to be the result of long-continued practice.

The walls, which I have described as being 20 feet thick, are so only in theory, for practically they are hollowed into innumerable cells, some of them forming beautiful architectural recesses, and others merely constructional hollows.

The dome itself is constructed in a manner evincing long-continued practice; for it differs toto cælo from the normal mode of construction. It is shown differently by different authors; in fact, it has probably never been sufficiently exposed to obtain complete information as to its curiously complicated construction. As far, however, as I can ascertain, it seems to be in two thicknesses. The inner thickness consists of the framework of the coffers, which is of brick, and the filling-in of the coffers, which is of rubble or concrete. This would form the centering on which the outer shell was built, which is a curious tissue of arches, each rising from the crowns of those below it, and so disposed as to concentrate the pressure upon points in the wall which intervene between the cells. The spaces between these arches are filled in (so far as I can gather) with rubble or concrete. The whole was probably covered externally with plates of bronze or of marble.

The antique Roman dome of the so-called Temple of Minerva Medica—named from the discovery of a statue of that deity among its ruins, but now supposed by some to have been, like the Pantheon itself, the great hall of some public baths—greatly resembles the Pantheon in its general idea, but differs in this essential particular, that its surrounding wall is not circular, but decagonal (Fig. 402).

At a later date, as we shall presently see, this peculiarity would have been seized upon as the suggestion of another type of dome, of which I shall have subsequently to treat. As a matter of fact, the transition from the polygonal prism below to the nearly hemispherical dome above, is got over by “rule of thumb,” rather than on any true system. The vertical sides of the wall do intersect the dome in arched forms; but neither are these forms the true sections of a plane with a sphere, nor have they been used as architectural features, as in later times; but have been afterwards, so far as I can judge, obliterated by the incrustation of the dome with plaster, so as to slur over a union of forms which the architect had fallen into accidentally without appreciating its true results.

The dome is surrounded by gradenæ much as in the Pantheon. It is not lighted by an eye, but by ten windows, surrounding what we should call the clerestory. Beneath these are ten arches piercing the surrounding wall; indeed, reducing it to small anglepiers. One of these was devoted to the entrance, the other nine to semicircular recesses, of which five seem to have contained basins for water, and four to have opened by means of colonnades into exedræ or surrounding buildings.[63] It may be mentioned that this form was in after times extensively imitated. The span of this dome is about 80 feet. Its date is not known.

There are other domes not differing materially from those already described, but which it would extend my lecture unduly to dwell upon. One called the Torre dei Schiavi, in the Via Prænestina, is rather like the Pantheon on a very small scale, though lighted by round clerestory windows instead of a central eye. The Temples of Vesta, both at Rome and at Tivoli, consist of circular walls surrounded externally by a peristyle. The cell of each is supposed to have been covered by a dome, though roofed over. A parallel case, but in a more complex form, exists in what is called the Temple of Jupiter in Diocletian’s Palace at Spalatro (Fig. 403). In this case the exterior of the cell, with its peristyle, is octagonal, but the interior, with the dome, round. The latter has a complicated construction of fan-shaped arches throughout, scarcely any part being constructed of horizontal curves. Next, perhaps, in date, yet at once displaying similarity of idea with a significant change in the carrying out, is the tomb of St. Constantia, the daughter of Constantine (Fig. 404).

A Christian church, in its early form, has been familiarly described as the pagan temple turned inside out. To convert the ideal temple into the ideal church, the wall and the colonnade must change places. So completely is this the case that some of the earlier commentators on Vitruvius were completely puzzled between the wall of the temple and the peristyle. They assumed that the latter must be within the wall, as in their own churches, and based their remarks on this supposition.

The comparison between these almost contemporary works, the Temple of Jupiter in Diocletian’s Palace at Spalatro, and the tomb of St. Constantia at Rome, exactly illustrates this change. In the one the solid wall forms the circle and carries the dome, and the colonnade is external (See Fig. 403), in the other the colonnade forms the inner circle and carries the dome, while the wall becomes external, an aisle taking the place of the peristyle (See Fig. 404). The colonnade is doubled to support the massive clerestory whence the dome springs, and the whole assumes the type of one form of Christian church, which henceforth became of frequent occurrence. The dome, in this instance, ceases to be an external feature, being covered over by a conical roof. I may add that the peristyle is repeated in the old manner as an additional feature beyond the wall of the aisle. The baptistery at Nocera is similar in distribution to the tomb of St. Constantia, and may be of similar date. It is ruder, however, and loses much beauty by the omission of the clerestory, and the admission of light through the haunches of the dome. Among the innumerable remains of domes of the older type, I will only mention one more, before proceeding to the second branch of my subject, to which it, in fact, properly belongs. That to which I allude is the baptistery at Ravenna, erected, as it is supposed, about the year 450 (Fig. 405).

This is a very charming building, octagonal in form, yet covered over by a hemispherical dome. Though having no surrounding aisle, the design of its sides seems derived from the aisle and clerestory: indeed, it has a clerestory, though the arcade below is rather rudimentary than real. The dome, like that last alluded to, is covered externally by a sloping roof.

The special feature, however, in this dome is that it rests upon an octagonal wall, or rather upon eight arches.

We shall presently see how this was effected in subsequent times, and I will not anticipate that subject, but will content myself with mentioning that this seems to anticipate the Byzantine domes of the succeeding century, as had been the case in two other instances to which I shall have to refer, and as had been nearly the case in the Temple of Minerva Medica.

The domes which we have hitherto considered are exclusively and of necessity carried by circular or other continuous walls. They are consequently supported uniformly throughout their entire circumference, and their use is necessarily limited to the coverings of circular or quasi-circular or polygonal buildings. Had no further development been attained, it would ever have been felt to be a sad deficiency in the scope of architectural facilities that the noblest form of covering should be limited to the least usual and, for most purposes, the least convenient form of apartment. We are happily as far as possible from being left in this dilemma. A very simple application of geometrical thought opened a way by which almost any reasonable form of building may be covered by a dome, or by a series or group of domes.

I will endeavour, as simply as I am able, to explain this important development.

It is a property of the sphere that every possible plane section of it is a circle. It follows that every vertical section of a hemispherical or segmental dome assumes the form of a semicircular or segmental arch. If, therefore, a square be inscribed in the base of a dome, and walls be built on that square, and continued up till they meet the dome, they will intersect with it in four semicircles (Fig. 406). If, instead of walls, you build arches on the sides of that square, these arches will coincide with the curve of the dome where they meet it, and, if strong enough, will carry the portion of the dome remaining between them. If, again, instead of arches, you suppose the dome intersected on the lines of the inscribed square by vaults at right angles to those sides, the result will be the same.

In the first case we have a dome, or a portion of one, covering a square apartment; in the second we have the same covering standing on arches open towards the exterior; in the third, we have a dome covering the intersection of two barrel-vaults, just as is more usually done by groining.

The process, however, is not limited to a square; it is equally applicable to the octagon or any other polygon—indeed, to any figure which can be inscribed in a circle.

The following diagrams (Figs. 407, 408, and 409) will tend better to explain this.

Nor is it necessary that the inscribed figure should be complete, for remnants of the circle may equally well be left between the arches or walls.

Thus, a circular space may be intersected by four vaults of less width than the sides of a square (Fig. 410), leaving portions of the circular walls remaining between them.

The dome, again, may as well be segmental in section as semicircular, in which case the arches supporting it will also be segmental (Fig. 411). Again, the figure inscribed need not be equilateral, so that oblong compartments, such as those customary in the nave of a church, may be domically vaulted.

In all the cases which I have enumerated, I have supposed the result to be literally a portion of the original dome. As it happens, however, we have but few ancient examples of so strict an adherence to principle; though in modern works they are more frequent. The purest specimen I know (if I judge rightly from drawings) is the dome of the little church of SS. Nasario and Celso, at Ravenna, the Mausoleum of Galla Placidia, and of her two husbands, the Emperors Honorius and Valentinian II. This is a dome such as I described as standing between four walls, which intersect it in the form of arches.