ists of
fourteen rafts, 105 ft. in length, each supported by two pontoons placed 64
ft. apart. The pairs of rafts are joined by three baulks 15 ft. long laid
in parallel grooves in the framing. Two spans are arranged for opening
easily. The total length is 1720 ft. and the width 46 ft. The pontoons are
of iron, 851/2 ft. in length, and their section is elliptical, 101/2 ft.
horizontal and 12 ft. vertical. The displacement of each pontoon is 180
tons and its weight 22 tons. The mooring chains, weighing 22 lb per ft.,
are taken from the upstream end of each pontoon to a downstream screw pile
mooring and from the downstream end to an upstream screw pile.

13. _Transporter Bridges._--This new type of bridge consists of a high
level bridge from which is suspended a car at a low level. The car receives
the traffic and conveys it across the river, being caused to travel by
electric machinery on the high level bridge. Bridges of this type have been
erected at Portugalete, Bizerta, Rouen, Rochefort and more recently across
the Mersey between the towns of Widnes and Runcorn.

[Illustration: FIG. 34.--Widnes and Runcorn Transporter Bridge.]

The Runcorn bridge crosses the Manchester Ship Canal and the Mersey in one
span of 1000 ft., and four approach spans of 551/2 ft. on one side and one
span on the other. The low-level approach roadways are 35 ft. wide with
footpaths 6 ft. wide on each side. The supporting structure is a cable
suspension bridge with stiffening girders. A car is suspended from the
bridge, carried by a trolley running on the underside of the stiffening
girders, the car being [v.04 p.0545] propelled electrically from one side
to the other. The underside of the stiffening girder is 82 ft. above the
river. The car is 55 ft. long by 241/2 ft. wide. The electric motors are
under the control of the driver in a cabin on the car. The trolley is an
articulated frame 77 ft. long in five sections coupled together with pins.
To this are fixed the bearings of the running wheels, fourteen on each
side. There are two steel-clad series-wound motors of 36 B.H.P. For a test
load of 120 tons the tractive force is 70 lb per ton, which is sufficient
for acceleration, and maintaining speed against wind pressure. The brakes
are magnetic, with auxiliary handbrakes. Electricity is obtained by two gas
engines (one spare) each of 75 B.H.P.

On the opening day passengers were taken across at the rate of more than
2000 per hour in addition