,
built in the 6th century (fig. 3), carried the Via Salaria over the Anio.
It was destroyed in 1867, during the approach of Garibaldi to Rome. It had
a fortification such as became usual in later bridges for defence or for
the enforcement of tolls. The great lines of aqueducts built by Roman
engineers, and dating from 300 B.C. onwards, where they are carried above
ground, are arched bridge structures of remarkable magnitude (see
AQUEDUCTS, Sec. _Roman_). They are generally of brick and concrete.

[Illustration: FIG. 4.--First Span of Schaffhausen Bridge.]

5. _Medieval and other Early Bridges_.--Bridges with stone piers and timber
superstructures were no doubt constructed from Roman times onward, but they
have perished. Fig. 4 shows a timber bridge erected by the brothers
Grubenmann at Schaffhausen about the middle of the 18th century. It had
spans of 172 and 193 ft., and may be taken as a representative type of
bridges of this kind. The Wittingen bridge by the same engineers had a span
of 390 ft., probably the longest timber [v.04 p.0535] span ever
constructed. Of stone bridges in Great Britain, the earliest were the
cyclopean bridges still existing on Dartmoor, consisting of stone piers
bridged by stone slabs. The bridge over the East Dart near Tavistock had
three piers, with slabs 15 ft. by 6 ft. (Smiles, _Lives of the Engineers,_
ii. 43). It is reputed to have lasted for 2000 years.

[Illustration: FIG. 5.--Crowland Bridge.]

The curious bridge at Crowland near Peterborough (fig. 5) which now spans
roadways, the streams which formerly flowed under it having been diverted,
is one of the earliest known stone bridges in England. It is referred to in
a charter of the year 943. It was probably built by the abbots. The first
bridges over the Thames at London were no doubt of timber. William of
Malmesbury mentions the existence of a bridge in 994. J. Stow (_Survey of
the Cities of London and Westminster_) describes the building of the first
stone bridge commonly called Old London Bridge: "About the year 1176, the
stone bridge was begun to be founded by Peter of Colechurch, near unto the
bridge of timber, but more towards the west." It carried timber houses
(fig. 6) which were frequently burned down, yet the main structure existed
till the beginning of the 19th century. The span of the arches ranged from
10 to 33 ft., and the total waterway was only 337 ft. The waterway of the
present London Bridge is 690 ft., and the re