BRIDGE. In its broadest sense, the term signifies any kind of ah independent connecting structure. Specifically, it designates a struc ture erected for the purpose of continuing a roadway over a stream, valley or any other natural or artificial obstruction without closing the way beneath as with an embankment.
The origin of the term is difficult to trace, and the art of bridge-building itself is so ancient, that its, beginning lies in the efforts of primitive man who felled trees, or swung jungle vines across the streams to facilitate his movements on his hunting trips, or when en gaged in trade or travel from village to village.
The construction of a bridge, however, as an engineering structure, although quite elemental in its engineering features, may be traced back more or less accurately to the Chinese who ap pear to have been the first people to employ the masonry arch for the purpose of continuing roadways across streams.
The art of bridge-building, however, as developed in the various countries was not evolved from the earlier Chinese practice, but independent effort characterizes its develop ment in each country, and by the very nature of the structures built in the past, the present day investigator is enabled to formulate very clear and accurate ideas as to the character, customs and importance of the peoples who built them.
Whether the Egyptians were bridge-builders is conjectural for no bridges have survived among the structures of their ancient civiliza tion. Some brick arches of crude construction have been found in ruins along the Nile, ascribed to the period 2900 B.C. It is not known that they were bridge arches. Similar condi tions appear to have obtained among the Babylonians and the Assyrians, and the earliest examples that remain, other than those of the Chinese, appear to belong to the "Hittite" and "Pelasgic" tribes, who inhabited the shores of the Mediterranean Sea during a prehistoric period antedating that of the building of Troy. Their engineering methods appear to have been imitated by the Cretans, whose cities in Asia Minor, Greece and Italy, and in the countries bordering on the Mediterranean, were con nected by macadamized roads which required bridges that were constructed by a high order of engineering skill. They were somewhat similar to the one built about 425 a.c. across the Euripus in Euboea, a province of Greece, and consisted of massive abutments and piers of masonry with a connecting superstructure of planks. The typical Greek method is shown by the bridge at Assos, in which parallel stone lintels doweled together are employed to con nect the piers and abutments.
On the other hand, the engineering genius of the "Pelasgic" tribes was inherited and de veloped by the Etruscans and the Romans in the application of the basic principles of the arch.
The bridges at Vulci, Bieda and Cora, in Italy, were not true arches, the stone courses having been corbeled out to meet at the centres of the spans. These were built during the 7th and 6th centuries B.C. The true Roman semi-circular arch did not appear for an other century. From that time onward the Roman bridge-building methods were carried through stages of greater and greater excellence until the 4th century ac., when the Roman policy of constructing great military roads to bind their possessions together permitted the highest development of their engineering skill, in structures of this kind.
The brilliant and unrivalled career of bridge and viaduct construction thus inaugurated by the building of the famous Via Appia, and cul minating in the erection of the eight great bridges of Rome, has furnished models for all the succeeding centuries, and is applicable to day to the construction of stone structures.
Some of the bridges of the Republican period still remain. Of these, the Ponte Lupo and the viaduct for the Anio Vetus aqueduct were built about 143 ac., and consists of great stone arches of tufa and travertine; while the viaduct near Gabii, built about 122 ac., con sisting of seven arches about 292 feet long, is still in use. Within Rome itself, the /Emilian bridge, built in 179-142 B.C. across the Tiber, is supposed to have been the first stone bridge to span that stream, all the others having been constructed of wood, a material which con tinued in use until quite a late period for bridges across wide rivers. Another, the Sublician Bridge built 621 a.c. by Ancus Martius across the Tiber between the Janiculum and the Aventine Mountain was famous for its defense by Horatius Codes against the great army under Lars Porsenna. The only one of the urban bridges of Rome that remains intact is the Fabrician Bridge, or "Ponte Fabricio," built in 62 B.C., but the greater part of the most magnificent of them all, the "Elian Bridge, or "Ponte Sant' Angelo," built by Hadrian in 136 A.n., also remains. The former consists of two round Roman arches of 80 feet span built of peperino and tufa, faced with massive blocks of travertine; the latter of eight arches, arranged to grade upward toward the centre of the structure, thus adding greatly to its archi tectural effect.