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. _The Openings._ 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.