THE Plan (or floor-disposition).—The plans of Roman buildings are striking from their variety and the vast extent which in some cases they display, as well as from a certain freedom, mastery, and facility of handling which are not seen in earlier work. Their variety is partly due to the very various purposes which the buildings of the Romans were designed to serve: these comprised all to which Greek buildings had been appropriated, and many others, the product of the complex and luxurious civilisation of the Empire. But independent of this circumstance, the employment of such various forms in the plans of buildings as the ellipse, the circle, and the octagon, and their facile use, seem to denote a people who could build rapidly, and who looked carefully to the general masses and outlines of what they built, however carelessly they handled the minute details. The freedom with which these new forms were employed  arises partly also from the fact that the Romans were in possession of a system of construction which rendered them practically independent of most of the restrictions which had fettered the genius of the Egyptians, Assyrians, and Greeks. Their vaulted roofs could be supported by a comparatively small number of piers of great solidity, placed far apart; and accordingly in the great halls of the Thermæ and elsewhere we find planning in which, a few stable points of support being secured, the outline of the spaces between them is varied at the pleasure of the architect in the most picturesque and pleasing manner.
The actual floor received a good deal of attention from the Romans. It was generally covered with tesselated pavement, often with mosaic, and its treatment entered into the scheme of the design for most interiors.
The construction of these was essentially different from that adopted by most earlier nations. The Romans rather avoided than cultivated the use of large blocks of stone; they invented methods by which very small materials could be aggregated together into massive and solid walls. They used mortar of great cementing power, so much so that many specimens of Roman walling exist in this country as well as in Italy or France, where the mortar is as hard as the stones which it unites. They also employed a system of binding together the small materials so employed by introducing, at short distances apart, courses of flat stones or bricks, called “bond courses,” and they further fortified such walls by bands of flat materials placed edgeways after the manner popularly known as herring-bone work. The result of these methods of construction was  that the Roman architect could build anywhere, no matter how unpromising the materials which the locality afforded; that he could put the walls of his building together without its being requisite to employ exclusively the skilled labour of the mason, and that both time and expense were thus saved. This economy and speed were not pushed so far as to render the work anything but durable; they had, however, a bad effect in another direction, for these rough rubble walls were habitually encased in some more sightly material, in order to make them look as though they were something finer than they really were; and accordingly, the exterior was often faced with a thin skin of masonry, and not infrequently plastered. The interior was also almost invariably plastered, but to this little exception can be taken. This casing of the exteriors was, however, the beginning of a system of what may be called false architecture, and one which led to much that was degrading to the art.
The walls were in many cases, it has been already observed, gathered into strong masses, such as it is customary to term piers, in order to support the vaulted roofs at the proper points. They were often carried to a much greater height than in Greek buildings, and they played altogether a far more important part in the design of Roman buildings than they had done in that of the Greeks.
As has been already stated, the Romans, in their possession of a new system of construction, enjoyed a degree of freedom which was unknown before. This system was based upon the use of the arch, and arched roofs and domes, and it enabled the Romans to produce interiors  unapproached before for size and splendour, and such as have hardly been surpassed since, except by the vaulted churches of the Middle Ages,—buildings which are themselves descended from Roman originals. The art of vaulting was, in short, the key to the whole system of Roman architecture, just as the Orders were to that of the Greeks.
The well-known arch over the Cloaca Maxima at Rome (Fig. 123, p. 142) may be taken as an illustration of the most ancient and most simple kind of vault, the one which goes by the significant name of “barrel or waggon-head vault.” This is simply a continuous arched vault springing from the top of two parallel walls; in fact, like the arch of a railway tunnel. Such a vault may be constructed of very great span, and affords a means of putting a permanent roof over a floor the outline of which is a parallelogram; but it is heavy and uninteresting in appearance. It was soon found to be possible to introduce a cross vault running at right angles to the original one; and where such an intersecting vault occurs the side walls of the original vault may be dispensed with, for so much of their length as the newly-added vault spans.
The next step was to introduce a succession of such cross vaults close to one another, so that large portions of the original main wall might be dispensed with. What remained of the side walls was now only a series of oblong masses or piers, suitably fortified so as to carry the great weight resting upon them, but leaving the architect free to occupy the space between them as his fancy might dictate, or to leave it quite open. In this way were constructed the great halls of the Thermæ; and the finest halls of modern classic architecture—such, for example, as the Madeleine at Paris, or St. George’s Hall at Liverpool—are only a  reproduction of the splendid structures which such a system of vaulting rendered possible.
When the floor of the space to be vaulted was circular, the result of covering it with an arched roof was the dome—a familiar feature of Roman architecture, and the noblest of all forms of roof. We possess in the dome of the Pantheon a specimen, in fairly good preservation, of this kind of roof on the grandest scale.
We shall find that in later ages the dome and the vault were adopted by the Eastern and the Western schools of Christian architecture respectively. In Rome we have the origin of both.
These were both square-headed and arched; but the arched ones occur far more frequently than the others, and, when occasion required, could be far bolder. The openings became of much greater importance than in earlier styles, and soon disputed with the columns the dignity of being the feature of the building: this eventually led, as will be related under the next head, to various devices for the fusion of the two.
The adoption of the arch by the Romans led to a great modification in classic architecture; for its influence was to be traced in every part of the structure where an opening of any sort had to be spanned. Formerly the width of such openings was very limited, owing to the difficulty of obtaining lintels of great length. Now their width and height were pure matters of choice, and doorways, windows, and arcades naturally became very prominent, and were often very spacious.
These necessarily took an altered place as soon as buildings were carried to such a height that one order could not, as in Greek temples, occupy the whole space from pavement to roof. The Greek orders were modified by the Romans in order to fit these altered circumstances, but columnar construction was by no means disused when the arch came to play so important a part in building. The Roman Doric order, and a very simple variety of it called Tuscan, were but rarely used. The chief alteration from the Greek Doric, in addition to a general degradation of all the mouldings and proportions, was the addition of a base, which sometimes consists of a square plinth and large torus, sometimes is a slightly modified Attic base; the capital has a small moulding round the top of the abacus, and under the ovolo are two or three small fillets with a necking below; the shaft was from 6 to 7 diameters in height, and was not fluted; the frieze was ornamented with triglyphs, and the metopes between these were frequently enriched with sculptured heads of bulls: the metopes were exact squares, and the triglyphs at the angles of buildings were placed precisely over the centre of the column.
The Ionic order was but slightly modified by the Romans, the chief alteration being made in the capital. Instead of forming the angular volutes so that they exhibited a flat surface on the two opposite sides of the capital, the Romans appear to have desired to make the latter uniform on all the four sides; they therefore made the sides of the abacus concave on plan, and arranged the volutes so that they seemed to spring out of the mouldings under the abacus and faced anglewise. The capital altogether seems  compressed and crowded up, and by no means elegant; in fact, both this and the Doric order were decidedly deteriorations from the fine forms of Greek architecture.
The Corinthian order was much more in accordance with the later Roman taste for magnificence and display, and hence we find its use very general both in Rome and in other cities of the Empire. Its proportions did not greatly differ from those of the Greek Corinthian, but the mouldings in general were more elaborate. Numerous variations of the capital exist (Figs. 145, 145a), but the principal one was an amalgamation of the large Ionic volutes in the upper with the acanthus leaves of the lower portion of the capital: this is known as the Composite order, and the capital thus treated has a strength and vigour which was wanting to the Greek order (see Fig. 145a).  The shafts of the columns were more often fluted than not, though sometimes the lower portion was left plain and the upper only fluted. The Attic base was generally used, but an example has been found of an adaptation of the graceful Persepolitan base to the Corinthian column. This was the happiest innovation that the Romans made; it seems, however, to have been but an individual attempt, and, as it was introduced very shortly before the fall of the Empire, the idea was not worked out.
The orders thus changed were employed for the most part as mere decorative additions to the walls. In many cases they did not even carry the eaves of the roof, as they always did in a Greek temple; and it was not uncommon for two, three, or more orders to be used one above another, marking the different stories of a lofty building.
The columns, or pilasters which took their place, being reduced to the humble function of ornaments added to the wall of a building, it became very usual to combine them with arched openings, and to put an arch in the interspace between two columns, or, in other words, to add a column to the pier between two arches (Fig. 146). These arched openings being often wide, a good deal of disproportion between the height of the columns and their distance apart was liable to occur; and, partly to correct this, the column was often mounted upon a pedestal, to which the name of “stylobate” has been given.
It was also sometimes customary to place above the order, or the highest order where more than one was employed, what was termed an attic—a low story ornamented with piers or pilasters. The exterior of the Colosseum (Fig. 5), the triumphal arches of Constantine (Fig. 139) and Titus, and the fragments of the upper part  of the Forum of Nerva (Fig. 147) may be consulted as illustrations of the combination of an order and an arched opening, and of the use of pedestals and attics.
Another peculiarity, of which we give an illustration from the baths of Diocletian (Fig. 148), was the surmounting a column or pilaster with a square pillar of stone, moulded in the same way as an entablature, i.e. with the regular division into architrave, frieze, and cornice. This was a  decided perversion of the use of the order; it occurs in examples of late date. So also do various other arrangements for making an arch spring from the capital of a column; one of these, from the palace of Diocletian at Spalatro, we are able to illustrate (Fig. 149).
In conclusion, it may be worth while to say that the Roman writers and architects recognised five orders: the Tuscan, Doric, Ionic, Corinthian, and Composite, the first and last in this list being, however, really only  variations; and that when they placed the orders above one another, they invariably used those of them which they selected in the succession in which they have been named; that is to say, the Tuscan or Doric lowest, and so on in succession.
The mouldings with which Roman buildings are ornamented are all derived from Greek originals, but are often extremely rough and coarse. It is true that in some old Roman work, especially in those of the tombs which are executed in marble, mouldings of considerable delicacy and refinement of outline occur, but these are exceptional. The profiles of the mouldings are, as a rule, segments of  circles, instead of being more subtle curves, and the result is that violent contrasts of light and shade are obtained, telling enough at a distance, but devoid of interest if the spectator come near.
Carving is executed exactly on the same principles as those which govern the mouldings—that is to say, with much more coarseness than in Greek work; not lacking in vigour, or in a sort of ostentatious opulence of ornament, but often sadly deficient in refinement and grace.
Statues, many of them copies of Greek originals, generally executed with a heavy hand, but sometimes clearly of Greek work, were employed, as well as bronzes, inlaid marbles, mosaics, and various devices to ornament the interiors of Greco-Roman buildings; and free use was made of ornamental plaster-work, both on walls and vaults.
Coloured decoration was much in vogue, and, to judge  from what has come down to us, must have been executed with great taste and much spirit. The walls of a Roman dwelling-house of importance seem to have been all painted, partly with that light kind of decoration to which the somewhat inappropriate name of arabesque has been given, and partly with groups or single figures, relieved by dark or black backgrounds. The remains of the Palace of the Cæsars in Rome, much of it not now accessible, and the decorations visible at Pompeii, give a high idea of the skill with which this mural ornamentation was executed; our illustration (Fig. 154) may be taken as affording a good example of the combined decorations in relief and colour often applied to vaulted ceilings.
It is, however, characteristic of the lower level at which Roman art stood as compared with Greek that, though statues abounded, we find no traces of groups of sculpture designed to occupy the pediments of temples, or of bas-reliefs fitted to special localities in the buildings, such as were all but universal in the best Greek works.
The nature of this will have been to a large extent gathered from the observations already made. Daring, energy, readiness, structural skill, and a not too fastidious taste were characteristic of the Roman architect and his works. We find traces of vast spaces covered, bold construction successfully and solidly carried out, convenience studied, and a great deal of magnificence attained in those buildings the remains of which have come down to us; but we do not discover refinement or elegance, a fine feeling for proportion, or a close attention to details, to a degree at all approaching the extent to which these qualities are to be met with in Greek buildings. We  are thus sometimes tempted to regret that it was not possible to combine a higher degree of refinement with the great excellence in construction and contrivance exhibited by Roman architecture.