The earlier architecture of the Romans was doubtless chiefly derived from that of the Etruscans, who, like the Greeks, followed the trabeated principle. This origin is distinctly traceable in the plans of the Roman temples, which are never truly peripteral, or surrounded with a detached colonnade, like those of the Greeks, but have a deep portico at one end only, in front of the cella. Of this arrangement we have seen a beautiful example in the Maison Carrée at Nimes. But the Central Italians must have early received some impressions from the Hellenic art of Magna Grecia, and the way would thus be opened for the introduction at a later period of the finer developments of Greek architecture which were so universally followed during the Empire. Meanwhile the arch, the antagonistic element to the trabeated principle, was gradually progressing; and from its primitive obscure use in substructures, conduits, and similar engineering situations, it had forced itself into notice above ground, and had gained recognition in the elevations as a proper architectural element. Hence arose the combination, so conspicuous in the architecture of the Romans, of trabeated features, such as pilasters and entablatures, with the arched method of construction which they had adopted from an early period, and of which they ultimately shewed themselves such masters. The amphitheatres and the triumphal arches of the empire well illustrate this mixture of arched construction, as shewn in the round-headed wall openings, combined with trabeated decoration, in the form of horizontal entablatures supported on engaged columns or pilasters. This mixed style long prevailed, and examples of it are to be found in every part of the Roman world. But in later times, when purity of taste had begun to decay, the Romans gradually gave fuller scope to their noble constructive powers, and allowed them to find a worthier expression in their designs. This took place chiefly in their engineering works, such as the Pont du Gard, and in their interior architecture, as, for instance, in the great halls of the Baths of Diocletian and Caracalla, the Basilica of Constantine (or Maxentius) and other similar works, in some of which immense intersecting vaults were successfully executed. The simple barrel or tunnel vault is of very ancient origin, and was adopted by the Romans from the earliest times. They also freely employed round intersecting vaults for covering spaces of all sizes up to the great examples above referred to. But the most astonishing feat of the Romans in connection with vault construction is their adoption and application of the dome. In the Pantheon at Rome we have an example of that species of vault introduced at once in its perfect form in the largest example in the world. The portico of this temple belongs to the age of Augustus, and it is therefore thought by many that the rotunda and dome are of the same date. It is very remarkable that no smaller Roman domes of earlier date are to be found, and that this style should, as it were, be born in perfect manhood without having passed through the stages of infancy and growth. These no doubt existed, although we have as yet been unable to trace them. Possibly, as Professor Baldwin Brown suggests, the dome is of eastern origin, and its enlarged construction may have been worked out in some of the Hellenistic cities, such as Alexandria, where the earlier examples have now perished.