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_.

* * * * *

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.

In Wales the first narrow gauge railway dates from 1832. It was
constructed merely for the carriage of slates from Festiniog to
Port-Madoc, and some years later another was built from the slate
quarries at Penrhyn to the port of Bangor. As the tract of country