In like manner it was good architecture to use flying buttresses, even where they were not essential to stability. They explained externally that the building was vaulted, and that its thrusts were abutted and stability secured. The mistake in their employment was where they became so essential to security, that the constructive necessities controlled the artistic propriety of the design, and the architect found himself compelled to employ either a greater number, or buttresses of greater strength than he would have desired had he been able to dispense with them. The architecture of the Greeks was so simple, that they required few artifices to explain their construction; but in their triglyphs their mutules, the form of their cornices and other devices, they took pains to explain, not only that these parts had originally been of wood but that the temple still retained its wooden roof. Had they ever adopted a vault, they would have employed a totally different system of decoration. Having no constructive use whatever, these parts were wholly under the control of the architects, and they consequently became the beautiful things we now so much admire. With their more complicated style the Romans introduced many new modes of constructive decoration. They were the first to employ vaulting shafts. In all the great halls of their Baths, or of their vaulted Basilicas, they applied a Corinthian pillar as a vaulting shaft to the front of the pier from which the arch appears to spring, though the latter really supported the vault. All the pillars have now been removed, but without at all interfering with the stability of the vaults; they were mere decorative features to explain the construction, but indispensable for that purpose. The Romans also suggested most of the other decorative inventions of the Middle Ages, but their architecture never reached beyond the stage of transition. It was left for the Gothic architects freely to elaborate this mode of architectural effect, and they carried it to an extent never dreamt of before; but it is to this that their buildings owe at least half the beauty they possess. The same system of course applies to dwelling-houses, and to the meanest objects of architectural art. The string-course that marks externally the floor-line of the different storeys is as legitimate and indispensable an ornament as a vaulting shaft, and it would also be well that the windows should be grouped so as to indicate the size of the rooms, and at least a plain space left where a partition wall abuts, or better still a pilaster or buttress, or line of some sort, ought to mark externally that feature of internal construction.