ction, 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.

_The Roofs._

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 va