156
HISTORY Or ENGINEERING.
Book I.
commons and the patricians, and by this fable won over the people from their attempts to putthe consuls to death. The stone arch, a part of the ancient bridge, now sustains a tower,constructed probably in the fifth or sixth century, which has undergone several changesduring the middle ages, when it was used as a fortress: in its present state it is a verypicturesque object. Many of the Roman bridges had towers either on them, or at theirextremities, encircled with battlements, which served to defend the passage, as also tocollect their dues.
It is to be regretted that Vitruvius has not left an account of the manner of buildingbridges in his time, or that he should not in the slightest degree have alluded to thesubject: all that we know of them is by studying the numerous remains which span therivers of Europe , where the great roads required them. The vaults are worked muchin the same manner as those of the triumphal arches: large blocks were generally selected,and truly cut; one course of deep voussoirs supported a mass of rubble, on w’hich the road-way was laid; precautions were also taken to defend the piers, and to carry off the waterfrom the road, by means of pipes and drains. All the refinement adopted by modernconstructors is traceable to the examples left us by the Romans, and we cannot too highlyprize their design, and economical use of material: solid stone was employed wherenecessary, with a filling in of concrete equally durable, resisting in many instances all theefTorts of time and weather.
Pons Janicidum or Ponte Sisto has been several times rebuilt; the present was executedin the time of Sixtus IV. , in 1478. It is composed of four arches, from 53 feet to 70 feetspan : in the distance is seen the Farnese palace.
11 j *
J J 1 i
1111
Fig.172.
POME SISTO.
Pons Fabricius , and Pons Cestius _These bridges, now called Ponte Quatro Capi and
Ponte Ferrato, are situate at Rome, on the two arms of the Tiber , which surround the
;* PH
Fig.173.