152
HISTORY OF ENGINEERING.
Book I.
and hills, its bridges, and foT the canal which accompanies it through the Pontine marshes,for the double object of draining the land and conveying the material of war from Latium to Terracina; this was important to a state not master of the sea.
The Setian was the military road to Campania from Velitra? to Terracina. To reachTerracina in one march from Cisterna would be impossible in summer; to encampbetween the two places in the latter season and autumn would be fatal, in the rainyseasons equally impossible; in the hot months, one single night spent by an army in theneighbourhood of Cisterna would produce fever.
Forum Appii on the canal was also built by Appius Claudius .
Tlie Pontine marshes were in all probability originally a bay behind the downs on thesea-coast; when this became filled with mud, by the river flowing into it, a marsh wasslowly but gradually raised.
Pranestine Way. Where this magnificent road crosses the low marshy ground, con-structions of the most solid kind were made, with arches turned in a symmetrical and perfectmanner, rivalling the aqueducts for their beauty. The spandrills were filled in with rubble,
Vf AS TERBENAS ON THE PR.SNBSTINB WAY.
and walls were carried on it, for the support of the level road above; the direction of thisroad is given in most of the Itineraries, and a description of the several statues which weresituated upon it.
The Bridges of (he Bomans are remarkable for their solidity, and for the almost universaladoption of the semicircular arch; the stone used in their construction is the hardest thatthe neighbourhood afforded; many have stood the force of violent torrents, and at the pre-sent day exhibit their original design, whilst others have undergone such changes, thattheir primitive features can scarcely be discerned.
One peculiar feature of Roman arches of great dimensions, and particularly of their bridges,is the leaving a projecting stone on each side, at about thirty degrees above the spring-ing, upon which their centres were strutted, consisting of a longitudinal piece of timber,with inclined and perpendicular supports. Vitruvius , in lib. vi. cap. 11., observes that care