STONE BRIDGES.
Ancient Bridges: first stone bridges were doubtless stone slabs spanning small spaces. In Egypt the annual overflow of the Nile was a hindrance to bridge-construction, and no relics of ancient bridges there exist. That the Egyptians were acquainted with the arch, however, is demonstrated by the finding of an arched ceiling in the sar cophagus-chamber of one of the pyramids.
Greel.- and Roman Greeks were familiar with the con struction of the arch from the age of Perikles, but no arch bridges of that period have survived to the present time. Of arch bridges built by the Romans, about twenty remain, which have been in part restored ins later times. One of the most celebrated of these is the superb structure span ning the Tagus at Alcantara. It was built about A. D. I00, in honor of the emperor Trajan. It is of granite, and consisted originally of six arches of various spans. It had a total length of 67o feet and a height of zio feet. It has several times been partially destroyed and restored. At present a single arch only, remains. The bridges of Fabricius (now Ouattro-Capi) and Cestius Gallus (now Ponte-Ferrato) are old Roman structures with semicircular arches. The mastery which the Romans had acquired in this form of construction is shown by the nuinerous remains of their splendid aqueducts. The great sewer (Cloaca Maxima) built by one of the Tarquins five hundred years B. C., and performing its functions to-day in modern Rome after nearly twenty-five centuries of service, affords an instructive example of the substantial character of the work of these ancient masons.
Chinese Chinese have long- been familiar with the art of bridge-building; their bridges, like those among the Persians, are remarkable for their extreme length. In China there are arch bridges of great magnitude and of unknown but high antiquity (1/. 4o, Jig. 5). In his " Treatise on Bridge Architecture " II), Thomas Pope makes mention of a stone bridge in China, called the "flying bridge," built from one mountain to another. This is said to consist of a single semicircular arch of 600 feet span, 75o feet high.
Bridges: Croyland 3 exhibits the unique triangular bridge at Croyland, England, which stands at the conflu ence of three streams. It is formed of three pointed arches, the abut ments of which are placed at the angles of an equilateral triangle. These arches meet in a connnon centre. This structure has three roadways. The present bridge—of which an illustration is given—is believed to have been erected about the beginning of the fourteenth century; but it has Leen proved from certain documents that a similar triangular bridge existed on the same site as early as the year 943.
SainIes Bridge. —Figure 4 (pi. 40) is an ideal representation of a famous medi:eval bridge at Saintes, in France. The pictnre is intended to give the characteristics of this structure as it may have appeared toward the close of the fourteenth century. This restoration is dile to M. Viollet-le Dile, in whose Die/to/moire roisonne l' Architect/ire it appears. The great tower, the crenellated parapets, and the wooden shore-ends between the first gate and the Roman arch on one side, and the tower and the town gates, which could readily be removed, on the other side were all provisions for defence.
The Bridge al SI. Chamas,in France, described by Cresy in his Ency of Civil Engineering, is a typical specimen of a bridge built by the Romans. It affords an example of their disposition to adorn such struc tures with triumphal arches. This bridge, which is pictured in Figure 2, is a small single-arch span of 42 feet.
Old Lona'ou first stone bridge over the Thames is be lieved to have been built toward the close of the twelfth century. It was covered with wooden buildings, which were frequently destroyed by fire. The main portion of the bridge, however, appears to have remained intact until the beginning- of this century. A roadway was left between the houses. The waterway was 336 feet, and two-thirds of the stream was occupied by the piers. Figure 6 is a view of this historic structure as it appeared in the year 17oo.