162

HISTORY OF ENGINEERING.

Rook I.

originally its height equalled its entire width : it has been said by some writers, that thetwo arches were not so coupled in the situation from whence they were brought, but that a

Fig. 183.

MEDIOLANUM.

single opening terminated each end of the original bridge, as was the practice of the Romanengineers. In Italy we often have the foundations of piers of triumphal arches, andpedestals for statues on the abutments of bridges; these not only added weight, where itcould give additional strength, but contributed much to the beauty of the structure ; massesof stone built over the springing of an arch assisted in preventing any spreading or slippingupon the haunches ; a triumphal arch placed at each extremity would have the same use,and contribute much to the effect. A vast catalogue of Roman bridges might be made,and it is somewhat remarkable that a selection has not been measured by the modernengineers, and classed according to the span of their arches and boldness of construction.The great rivers of Italy, France, Germany, Spain, and Portugal , all afford examples, someerected where the water is broad, rapid, or deep, or on foundations which presented con-siderable difficulties. Timber platforms on piles were universally adopted on soft ground,and a concrete, formed of hydraulic mortar, is found to have been made use of very gene-rally throughout Italy , wherever it could be applied.

In Italy , the Roman bridges have generally served for foundations for the modern;many have had their semicircular arches altered during the middle ages, and in someinstances, timber constructions are formed upon the massive piers, which timeand thefloods have spared. One great cause of their destruction was the elevation of the beds ofthe rivers, in some instances so great, that the openings have entirely silted up, and beenclosed by the deposit; in consequence the structure, not affording sufficient water-way, hasbeen carried away by the flood.

Pavia over the Tessin _A covered bridge, of Gothic construction, built of brick, con-

sists of seven arches, each 70 feet span, and 64 feet in height. The piers, whose breadthis 16 feet, have a rounded form, but longer in the direction of the stream than acrossit. The tympanums of the arches are pierced, resembling a curvilinear triangle twosides of which are parallel to the entrados of the vaults; thus the weight in a great mea-sure rests against the keystones, the thickness of which is 5 feet 6 inches. The bricks withwhich these vaults are constructed have the form given to them suited to their positionas voussoirs, and are pierced in the middle to diminish the weight. The piers arecovered with white marble, the arches have an archivolt, and the whole is crowned with aGothic parapet of the same material, worked out to give it the greatest possible light-ness ; each footway has a covering supported by two rows of small coloured marblecolumns, 9 inches in diameter, 14 feet 4 inches apart, whose bases and capitals are ofwhite marble. The vaulted coverings are ornamented with gilt arabesques, on an azureground, and sustain two terraces, which are ascended by steps, placed at the extremity ofthe bridge. The thrust of the vaults is opposed, as in many other Italian buildings, byiron rods placed on a level with the springing. This beautiful work was executed byGaleazzo Visconti, Duke of Milan , to which prince the city owes the Charter House,Hospital, and Lazaretto .

Bridge of the Goldsmiths , at Florence, over the Arno, called Ponte Vecchio , was rebuilt in