of great interest, to which some consideration is given in the book before us, but in regard to which the fullest analysis is still to be found in Choisy's remarkable study, 'L'Art de 'bâtir chez les Byzantins.' To build an arch in one plane between two counterforts or buttresses, it is absolutely necessary to erect in the first instance a centreing, generally of timber (in modern work iron is often used), constructed so as to give the form of the underside or intrados of the arch which is to be erected. The archstones or voussoirs are then built up with their lower edges resting on the centreing, and supported by it until the circle of the built arch is complete and it becomes self-supporting; and, after giving time for it to settle as much as it will, to come to ' its bearings,' and for the mortar to consolidate, the centreing is removed, and the arch remains an independent structure. But, as has already been observed, a dome is a structure under different conditions from an arch in one plane, in that its courses of masonry or bricks form arches horizontally as well as vertically, and it can be stopped at any point in its height so long as the courses are complete all round, and remain a stable construction. An arch built in one plane can be carried up for a certain portion of its curve without scaffolding, and remain stable; but that can only be done up to the point at which the frictional resistance of the voussoirs to sliding, coupled with the cohesive strength of the mortar, is sufficient to counteract the vertical weight of the materials; in other words, the incomplete arch must fall over as soon as it comes to a point at which the joints are too near the vertical for the stones to retain their position by the combined vis inertia of friction and mortar-grip. And with a dome this is not the case: finish it up to any point in its height, and there you may leave it. Instances have been known of domes, though not on a very large scale, being built without any centreing by means of cutting all the stones with a recess or 'joggle' on each face, so that each ring of the masonry could be hung on to the one below it, and when the horizontal circle was completed became a firm foundation to hang the next course on to, and so on in rotation. This would be, however, a hazardous course to employ in a dome on a large scale, and, moreover, a very costly one, requiring an exceedingly accurate cutting and fitting of an enormous number of stones in a rather complicated shape. The Byzantine builders appear to have employed a much simpler, but perfectly effective, system. Instead of forming the joints of

the successive courses of the dome normal to (at right angles to) the curve of its section, they laid them, at the commencement and for a considerable way up, at a much flatter angle, so that, with the additional cohesive assistance of mortar, each horizontal ring could be laid on the one below it, and the stones (or flat bricks rather) be thus sustained in position till the circle was completed, when they formed the bed to lay the next circle on. As the top of the dome was approached, it would be necessary to tilt the joints more towards the vertical to prepare for the final closing in at the top, but at this point it would be easy to construct a small timber centreing supported on the lower portion of the dome already built, and build the crown of the dome on that in the same manner as with an ordinary arch. The great point was that the waste of time and material, and the cost involved in building up a complete system of temporary centreing for the whole of a large dome, were thus avoided, and the greater portion of the dome was built out into the air, so to speak, forming its own support as it went up. That such a method of building requires considerable nerve, both on the part of the architect who plans and is responsible for it, and the artificers who carry it out, cannot be denied; and if the bricklayer of Justinian's day was as reckless as the English workman becomes during the progress of a large structure, it might be a matter of some practical interest in connexion with the problem to know how many men were killed over the building of St. Sophia's dome; on this point history is silent. Still, there is no real reason why any should have been if they were properly careful, any more than there was in the case of the Forth Bridge, which had its deathroll of (if we remember right) something over fifty men, all of whom probably succumbed either to their own carelessness or that of a comrade. On the other hand, it must be felt that there is something very grand and very fascinating in this bold and virile method of construction, which is real building' in the true sense of the word; making the permanent materials of the structure do their own work in the manner suitable to their nature, instead of depending upon a secondary and temporary erection of other materials in the first instance.

Somewhat the same principle was applied, in a still more curious fashion, in the erection of circular arched roofs over a longitudinal space, what are called in architecture wagon vaults,' of which the top of a brick tunnel is the most apt illustration. These also were built by the Byzantines with

out scaffolding, in a series of arches of large, thin, flat bricks built in successive parallel arches with their thin edges downwards and their flat sides contiguous. A wall or a strong arch of pretty thick masonry having been built as the commencement, at one end of the intended vault, one of the thin brick arches was commenced to be built against it, the thin broad bricks being kept in their place by the adhesive power of the cement between their sides and the thick arch or wall until the keystone was in place, when the thin arch became a permanent structure, and the bricks for the next arch were placed against it in turn, and so on till the vault was completed as far as it was desired to carry it. But as it was evidently found difficult to retain the bricks in their place while building by the mere adhesive force of cement, the device was adopted of tilting the thin arches somewhat sideways, instead of building them vertically, so that each leaned backwards against the one behind it, like a row of bricks on end which have been pushed at one end and have all tilted in the same direction. This device enabled each arch to afford a certain support to that which came next to it, in addition to the mortar hold. The effect of this to the eye would, of course, have been very bad if the brickwork had been intended to be seen; but as the interior surfaces of all Byzantine domes and vaults were intended to be covered with cement and mosaic, or some analogous decoration, the arrangement of the bricks, so long as it constructively answered its purpose, was of no consequence. The device is one of the most curious and ingenious in the history of building construction, and is characteristic of the whole spirit of early Byzantine building.

While there is all this interest and constructive ingenuity in the Byzantine method of dome building, and all this grand interior effect produced by the domed treatment of St. Sophia, it must be admitted that externally the building is not a beautiful one in lines and composition. To produce a splendid internal effect was the object of the architect, and the external design seems to have been left to take care of itself in great measure, to take that shape which the structure of the domes and of their counterforts necessitated, without any particular thought being bestowed on its effect as seen from without. The very form of dome employed, with its flat exterior curves, is one which can hardly be made very effective as an exterior feature. The great difficulty with the dome, in fact, as an architectural feature, is that of rendering it equally effective externally and internally. If its

section is lofty in proportion, its interior effect is a good deal weakened by its apparent size being lessened by perspective diminution. If the section is less in proportion it has a squat effect externally; and as a large dome must always be a central feature, on account of its requiring sufficient masses of wall round it to resist its thrust, this squat appearance externally is intensified by the effect of perspective, the outer masses of the building on a near view being close up to the eye, and the dome being partially lost behind them. Even at St. Peter's, where the dome is lofty both in interior and exterior section, it is nearly lost as the west front is approached, and would have had a far finer effect if the nave had not been subsequently lengthened in a manner not contemplated by Michelangelo. In St. Paul's Wren solved the problem by having two domes, an interior and an exterior one; but he did so at the expense of architectural truth, and his visible exterior dome, like those of St. Mark's, Venice, is merely a timber framework erected outside of the real construction. The flat form of the true Byzantine dome, again, with its thin crown, leaves no possibility of erecting upon it a lantern or crowning feature, such as those which add so much to the soaring effect of St. Peter's and St. Paul's domes; and even were it structurally possible the lines of the dome do not appear to lead up to or call for such a finish. It was no doubt the perception of this which led the builders of St. Mark's, which internally is a thoroughly Byzantine design, to endeavour to add that picturesqueness to the exterior which the normal conditions of Byzantine architecture did not admit of, by the addition of those rather gewgaw cupolas which give the building its characteristic exterior appearance, but which sadly detract from its monumental character. St. Sophia is thoroughly monumental, both externally and internally, but externally it is a heavy and prosaic-looking building; the magic is in the interior. In later Byzantine architecture, in the endeavour to escape from this squatness of external effect, it became the custom, instead of building a large dome directly on the main substructure, to elevate smaller domes on a high octagon structure, which in a sense realised the desired aim, but greatly detracted from the monumental appearance of the dome as well as from its interior effect; it was no longer the natural covering in of the main building, but a separate erection arbitrarily mounted upon it.

Before quitting the subject of the construction of St. Sophia's dome it should be noted that the dome as now

existing is not that which was at first built. The first dome was partially destroyed by an earthquake in 558, a catastrophe which it is said the architects attributed to some extent to the fact that the great piers had been built with too many hollow spaces in them, to lessen the immense mass of material swallowed up by them. Whether this would really have had the effect of rendering the structure more likely to be affected by earthquake it is impossible to say unless we knew exactly how the piers were originally built. An important point is that the dome was rebuilt, apparently, about twenty to twenty-five feet higher than before. It is obvious from the study of the building as now existing that this does not mean that the whole dome was raised, as it were, bodily: the springing was where it formerly was, but the height of the crown of the dome in relation to its springing was raised; in other words, it formed a larger segment of a circle with a greater curvature. Whether this was done from the ambition to build somewhat higher (as after any accident to a building there is a natural desire to reinstate it better than before), or whether it was thought that the former section was too flat either for safety or effect, is doubtful; but we should be inclined to think the latter was the reason, judging especially from the remark of Agathias, another contemporary writer, that the dome, after the earthquake, was built 'not 'so wide but higher, so that it did not frighten the spec'tators as formerly.' There seems every reason to believe that the first part of the statement was a mistake, that the dome was the same width as at first, but it would convey the impression of not being so wide when its curve was raised, from the effect of perspective (already alluded to) in diminishing the apparent size of a lofty dome to the spectator looking up at it from below, and for this reason it may be a question whether the interior effect of the dome was improved by increasing its curvature, and whether the very flat dome which was apparently at first built would not have had a greater effect of size and grandeur, though from its greater lateral thrust and flatness at the crown it would certainly have been less safe constructively, and might possibly have fallen in again in course of time and from the effect of any slight settlement in the abutments.

The magic of St. Sophia, as we have said, lay in its interior, which, even if built of plain stone and unadorned, must have had a grand and striking effect. But this was increased immeasurably by the wealth of decoration lavished