The problem was, to throw a bridge across this
wide chasm--a bridge of unusual span and dimensions--of such strength as
to be capable of bearing the heaviest loads at high speeds, and at such a
uniform height throughout as not in any way to interfere with the
navigation of the Strait. From an early period, Mr. Stephenson had fixed
upon the spot where the Britannia Rock occurs, nearly in the middle of
the channel, as the most eligible point for crossing; the water-width
from shore to shore at high water there being about 1100 feet. His first
idea was to construct the bridge of two cast-iron arches, each of 350
feet span. There was no novelty in this idea; for, as early as the year
1801, Mr. Rennie prepared a design of a cast-iron bridge across the
Strait at the Swilly rocks, the great centre arch of which was to be 450
feet span; and at a later period, in 1810, Telford submitted a design of
a similar bridge at Inys-y-Moch, with a single cast-iron arch of 500
feet. But the same objections which led to the rejection of Rennie's and
Telford's designs, proved fatal to Robert Stephenson's, and his
iron-arched railway bridge was rejected by the Admiralty. The navigation
of the Strait was under no circumstances to be interfered with; and even
the erection of scaffolding from below, to support the bridge during
construction, was not to be permitted. The idea of a suspension bridge
was dismissed as inapplicable; a degree of rigidity and strength, greater
than could be secured by any bridge constructed on the principle of
suspension, being considered an indispensable condition of the proposed
structure.

[Picture: Britannia Bridge]

Various other plans were suggested; but the whole question remained
unsettled even down to the time when the Company went before Parliament,
in 1844, for power to construct the proposed bridges. No existing kind
of structure seemed to be capable of bearing the fearful extension to
which rigid bridges of the necessary spans would be subjected; and some
new expedient of engineering therefore became necessary.

Mr. Stephenson was then led to reconsider a design which he had made in
1841 for a road bridge over the river Lea at Ware, with a span of 50
feet,--the conditions only admitting of a platform 18 or 20 inches thick.
For this purpose a wrought-iron platform was designed, consisting of a
series of simple cells, formed of boiler-plates riveted together with
angle-iron.