92

HISTORY OF ENGINEERING.

Book I.

The triumphal character of the entrance to a portion of the city of Autun , called St.Andre, also deserves our notice. Most of the cities in Provence , and throughout the south

Fig. 109. GATE OF ACTl’N.

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of France , were embellished by the emperors at various times; and so similar in design andconstruction are these structures to those of the imperial city, that they cannot for a momentbe considered but as erected by Roman engineers.

Architecture on some is more lavishly introduced than on others, and in that at Autun the Attic is beautifully proportioned and executed.

Distribution and Situation of the Buildings within the Walls. — The circuit being com-pleted, the next work was to set out the streets, and mark the sites for the public as well asprivate dwellings. The Roman engineers are described by Vitruvius to have laid downfirst a marble pavement in the centre of the ground comprised within the walls; after theyhad made this smooth and polished, they raised upon it a brass gnomon ; at the fifth ante-meridional hour, the shadow that it cast was noticed, and its extreme point accuratelydetermined. Around the gnomon was then described a circle, the radius of which wasmade equal to the length of the shadow. When the sun had passed the meridian, it wasagain remarked when the shadow of the gnomon reached the circle. From these twopoints on the circumference, two arcs were described, intersecting each other, throughwhich intersection, and the centre of the gnomon, a line was drawn which indicated thenorth and south points. On the circumference to the right and left of the north and southpoints, one sixteenth part of the circumference was set out, and lines through the centre ofthe gnomon drawn to connect them; one eighth part of the entire area of the circle repre-sented the region of the north, another the region of the south. The remaining portionwas divided into three equal parts on each side.

The directions of these several lines mark out the wide as well as narrow streets ; for itwas considered that if they were set out parallel to the direction of the winds, the latterwould rush through with greater violence, and that during a gale or strong wind, the anglesof the different divisions of the city dissipated it, and prevented it doing any mischief toeither the buildings or inhabitants.

When the city was near the sea, the Forum was placed close to the harbour;when inland, in the centre. The temples of the gods were mounted on an eminence thatcommanded the city : that of Mercury was established in the Forum ; of Isis and Scrapis, inthe great public square ; Hercules, near the Circus ; Mars, out of the walls; and of Venus,near the gate.

Materials used in the Ancient Edifices of Borne. — The materials which the Romans em-ployed were either for the purposes of construction or for ornament. The first, as lime,pozzolana, clay, and stone, the immediate neighbourhood furnished; those which luxury