Page:EB1911 - Volume 27.djvu/983

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. 1); 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 Serbjstan and Firuzabad built domes of similar form to those shown in the Nimrud sculptures, the, chief differr ence being that, constructed in rubble stone and cemented with mortar, they still exist, though probably abandoned on the Mahommedan invasion in the 7th century.

Fig. 1.

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 18 or 20in. to 10 in., being generally square and about 4 to 2 in. thick,, and they were not shaped as voussoirs, the con- necting 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. 650 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 B.C.) to drain the marshy ground between the Palatine and the Capitoline Hills, was according to Commendatore Boni vaulted over in the

Fig. 2.

1st century B.C., the vault being over 800 ft. long, 10 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 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 80 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 bâtir 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 10 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 2 in. 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 vault; 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 were of disagreeable effect : though every attempt was made to mask this in the decoration of the vault by panels and reliefs modelled in stucco.