80
HISTORY OF ENGINEERING.
Book I.
Servius Tullius added the Quirinal and Viminal hills, and extended the limits of theEsquilia?, where he established his own residence. lie also surrounded the city with arampart, trenches, and a wall. By this means, the poiruerium on each side of the wall wasextended, which space it was the custom of the Etrurian augurs to consecrate. On theinside of the wall no buildings were admitted, the space being required for defence.
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Fig. 100.
The w'all built by Servius Tullius surrounded the entire city, the Colline and Esquiline region being connected by a mound 7 furlongs in length, formed with the earth throwm outof a ditch 30 feet in depth, and above 100 feet in breadth : here was raised a wall 50 feetwide, above 60 feet high, and, although chiefly built of puzzolana, faced, towardsthe moat or ditch, with hard stone, and defended with towers. The Colline gate wassituated where the Quirinal was nearly flat, and from thence, up the acclivity of the hill, asimilar wall was constructed.
The Viminal hill , so called from the osiers it produced, was then taken into the city,w hich, at this time, measured about 6 miles in circumference. The city of Rome remainedin this state for centuries; but the ramparts which defended its citizens were not suf-ficient in the time of the Emperor Aurelian to satisfy them. Beyond the wall around theseven hills, numerous buildings which had been constructed in the suburbs were now in-closed ; the city and its inhabitants covered the field of Mars , and also extended their dwellingsfor a considerable distance on the various roads which led from it. The new walls, builtby this emperor, and finished by Probus, were in circuit about 21 miles, and comprisednearly the whole of the suburbs. Probus also built a stone wall from the Rhine to the
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: 7V : 7V
Fig. 101.