moval of the obstruction caused
by the old bridge caused a lowering of the low-water level by 5 ft., and a
considerable deepening of the river-bed. (See Smiles, _Lives of the
Engineers_, "Rennie.")

[Illustration: FIG. 6.--Old London Bridge, A.D. 1600. From a Drawing in the
Pepysian Library Magdalene College, Cambridge.

From J. R Green's _A Short History of the English People_, by permission of
Macmillan & Co., Ltd.]

The architects of the Renaissance showed great boldness in their designs. A
granite arch built in 1377 over the Adda at Trezzo had a span at low water
of 251 ft. This noble bridge was destroyed for military reasons by
Carmagnola in 1416. The Rialto bridge at Venice, with a span of 91 ft., was
built in 1588 by Antonio da Ponte. Fig. 7 shows the beautiful Ponte della
Trinita erected at Florence in 1566 from the design of B. Ammanati.

6. _Modern Bridges._--(a) _Timber._--In England timber bridges of
considerable span, either braced trusses or laminated arches (_i.e._ arches
of planks bolted together), were built for some of the earlier railways,
particularly the Great Western and the Manchester, Sheffield &
Lincolnshire. They have mostly been replaced, decay having taken place at
the joints. Timber bridges of large span were constructed in America
between the end of the 18th and the middle of the 19th century. The
Amoskeag bridge over the Merrimac at Manchester, N.H., U.S.A., built in
1792, had 6 spans of 92 ft. The Bellows Falls bridge over the Connecticut
(built 1785-1792) had 2 spans of 184 ft. The singular Colossus bridge,
built in 1812 over the Schuylkill, a kind of flat arched truss, had a span
of 340 ft. Some of these timber bridges are said to have lasted ninety
years with ordinary repairs, but they were road bridges not heavily loaded.
From 1840, trusses, chiefly of timber but with wrought-iron tension-rods
and cast-iron shoes, were adopted in America. The Howe truss of 1830 and
the Pratt truss of 1844 are examples. The Howe truss had timber chords and
a lattice of timber struts, with vertical iron ties. In the Pratt truss the
struts were vertical and the ties inclined. Down to 1850 such bridges were
generally limited to 150 ft. span. The timber was white pine. As railway
loads increased and greater spans were demanded, the Howe truss was
stiffened by timber arches on each side of each girder. Such a composite
structure is, however, fundamentally defective, the distribution of loading
to the two