On each side of the river, both above and below the bridge, the quays are ordinarily lined with vessels berthed alongside each of the quays, and as the river is rather narrow at this point, the line of fairway for vessels passing through the bridge is confined nearly to the center of the river.  This consideration, together with some others connected with the proposed future deepening of the fairway, rendered it very desirable to locate the opening span nearly in the center of the river, as shown in the general plan of the situation, which we publish herewith.  At the same time it was necessary to avoid any encroachment upon the width of the existing quays, which form important lines of communication for vehicular and passenger traffic along each side of the river, and to and from the railway stations.  Again, it was necessary to preserve the full existing width of waterway in the river itself, which is sometimes subjected to heavy floods.

These considerations evidently precluded the construction of a central pier and double-armed swing bridge, and on the other hand they also precluded the construction of any solid masonry substructure for the turntable, either upon the quay or projected into the river.  To meet these several conditions the bridge has been designed in the form of a three-span bridge, that is to say, it is only supported by the two abutments and two intermediate piers, each consisting of a pair of cast-iron cylinders or columns, as shown by the dotted circles upon the general plan.

The central opening is that which serves for the passage of vessels.  The swing bridge extends over two openings, or from the north abutment to the southern pier, its center of revolution being situated over the center of the northern span, and revolves upon a turntable, which is carried upon a lower platform or frame of girders extending across the northern span of the bridge.  The southern opening is spanned by an ordinary pair of lattice girders in line with the girders and superstructure of the swing bridge.

We propose at an early date to publish further details of this bridge, and the hydraulic machinery by which it is worked.

We present a perspective view of the bridge as seen from the entrance to the exhibition building, which is situated in close proximity to the southern end of the bridge.—­Engineering.

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PORTABLE RAILWAYS.

[Footnote:  Paper read before the Institution of Mechanical Engineers.]

By M. Decauville, Aine, of Petit-Bourg (Seine and Oise), France.

Narrow gauge railways have been known for a very long time in Great Britain.  The most familiar lines of this description are in Wales, and it is enough to instance the Festiniog Railway (2 feet gauge), which has been used for the carriage of passengers and goods for nearly half a century.  The prosperous condition of this railway, which has been so successfully improved by Mr. James Spooner and his son, Mr. Charles Spooner, affords sufficient proof that narrow gauge railways are not only of great utility, but may be also very remunerative.