In very many cases of renewals, the description and arrangement of the old structure will materially influence or control the design for the new one, and the details of the latter must be schemed out so as to disturb as little as possible the stability of the old work remaining as the working road.

The following list gives the lengths of the main spans of some railway bridges, and may be found useful for reference:—

Lengths of Main Spans of some Large Railway Bridges.

Name. Span. Description.
feet.
Forth Bridge1,710Cantilever.
Niagara821Suspension.
Sukkur820Cantilever.
Poughkeepsie, U.S.A.548Cantilever.
Douro525Arch.
St. Louis520Arch.
Cincinnati515Linville truss.
Haarlem510Arch.
Kuilemburg492Lattice bow.
St. John’s River477Cantilever.
Niagara470Cantilever.
Britannia460Tube.
Ohio River, Pennsylvania442Pratt through truss.
Saltash434Tube and girder.
Hawkesberry Viaduct410Compound truss.
Conway400Tube.
Vistula397Lattice.
Spey River, Garmouth, N.B.350Bowstring.
St. Laurence330Tube.
Hamburg316Double bow.
Cologne313Lattice.
Runcorn305Lattice.
Sunderland300Bowstring.
Rondout Bridge, Buffalo264Pratt through truss.
Newark Dyke (New)259Lattice bow.
Tay Bridge (New)245Lattice bow.
Ohio River, Louisville245Fink truss.
Beaver Bridge, Pennsylvania230Pratt deck truss.
Craigellachie Bridge200Lattice.
Rohrbach Bridge, St. Gothard River197Wrought-iron arch.
Windsor Bridge187Bowstring.
Victoria Bridge over Thames175Wrought-iron arch.
Shannon River Bridge165Bowstring.
Carron Bridge over Spey150Cast-iron arch.
Preston Viaduct102Cast-iron arch.
Trent River Bridge100Cast-iron arch.

Retaining Walls.—Instances frequently occur during the construction of a railway where it is advisable, if not absolutely necessary, to substitute retaining walls in preference to forming the slopes of cuttings and embankments.

The excavation of a cutting may be greatly reduced in quantity by introducing low retaining walls, as in [Fig. 195], and the saving in the material to be removed will be all the more important in those cases where cutting is in excess of embankment.

The amount of filling for an embankment and the land on which it has to be formed may both be considerably diminished by building a low retaining wall, say 6 or 7 feet high, at the foot of the slope, as shown in [Fig. 196]. Such a retaining wall makes a most efficient fence and well defined boundary of property.

The policy of adopting low retaining walls in cases like the