independent systems being indeterminate. Remarkably high timber
piers were built. The Genesee viaduct, 800 ft. in length, built in
1851-1852 in 10 spans, had timber trestle piers 190 ft. in height. (See
Mosse, "American Timber Bridges," _Proc. Inst. C.E._ xxii. p. 305, and for
more modern examples, cxlii. p. 409; and clv. p. 382; Cooper, "American
Railroad Bridges," _Trans. Am. Soc. C.E._ vol. xxi pp. 1-28.) These timber
framed structures served as models for the earlier metal trusses which
began to be used soon after 1850, and which, except in a few localities
where iron is costly, have quite superseded them.

[Illustration: FIG. 7.--Ponte della Trinita, Florence.]

7. (b) _Masonry._--The present London Bridge, begun in 1824 and completed
in 1831, is as fine an example of a masonry arch structure as can be found
(figs. 8 and 9). The design was made by John Rennie the elder, and the
acting engineer was his son, Sir John Rennie. The semi-elliptical shape of
the arches the variation of span, the slight curvature of the roadway, and
the simple yet bold architectural details, combine to make it a singularly
beautiful bridge. The centre arch has a span of 152 ft., and rises 29 ft. 6
in above Trinity high-water mark; the arches on each side of the centre
have a span of 140 ft. and the abutment arches 130 ft. The total length of
the bridge is 1005 ft., its width from outside to outside 56 ft., and
height above low [v.04 p.0536] water 60 ft. The two centre piers are 24 ft.
thick, the exterior stones are granite, the interior, half Bramley Fall and
half from Painshaw, Derbyshire. The voussoirs of the centre arch (all of
granite) are 4 ft. 9 in. deep at the crown, and increase to not less than 9
ft. at the springing. The general depth at which the foundations are laid
is about 29 ft. 6 in. below low water. The total cost was L1,458,311, but
the contractor's tender for the bridge alone was L425,081.

[Illustration: FIG. 8.--London New Bridge.]

Since 1867 it had been recognized that London Bridge was inadequate to
carry the traffic passing over it, and a scheme for widening it was adopted
in 1900. This was carried out in 1902-1904, the footways being carried on
granite corbels, on which are mounted cornices and open parapets. The width
between parapets is now 65 ft., giving a roadway of 35 ft. and two footways
of 15 ft. each. The architect was Andrew Murray and the engineer, G. E. W.
Cruttwell. (Cole, _Proc. Inst. C.E._ clxi. p.