The Express.
London, England, 1863-12-14 00:00:00

HICII LEVEL BRIDGES OVER THE THAIIES.

When the Select Committee of the blouse of
Lords reported In favour of a railway which should
form an "outer circle" round London, with " a
'bridge on the eastern aide of the metropolis to con-
nect the railways north and south of the Thames,"
they must have very imperfectly considered the
difficulties attending, such a work. Certainly they
had no evidence before them to justify such recom-
mendations The evidence went to show the pos-
sibility and the difficulty of passing through Lon-
don, and connecting the different railways within
the metropolitan circle, and it also illustrated the
possibility and the difficulty of using the Thames
Tunnel as a means of communication from one aide
of the Thames to the other. No one, however, was
hold enough to propose an " outer circle" railway
with " a bridge " across the navigable part of the
river ; nor can it be supposed for a moment that
the Lords themselves knew what such recommenda-
tions would involve.

First, let us take the bridge. A bridge across
the Thanies is, of course, the key of inch &railway.
Without it an outer circle railway would only work
back traffic from Blackwell on the north side of the
river, round by Kensington, to Rotherhithe, on the
south of the river, and rersd. This it is
possible to do at present by the North London and
other lines with even greater facility than an
"outer circle" line could effect it. A bridge at the
east of London, then, is the key of the outer circle
system, without which an outer circle line must fail.
Now, for the benefit of the House of Lords and of
bbs public generally, let it be considered what such
a bridge implies.

"A bridge at the eastern side of the metropolis"
implies a bridge on a high level, so as not to inter-
fere with the shipping of the Thames. It was to
avoid the construction of such a bridge that Sir
Isambard Brunel proposed the Thames Tunnel. So
far back, then, as the time of that enterprising en-
gineer any sort of communication, however difficult
to effect, was considered preferable to a bridge.
One reason for this is that the high level required
(not leis than 150 feet above high water mark)
must be obtained from perfectly flat shores. How
is this to be accomplished 1 Hitherto such a work
has certainly never been attempted anywhere. The
high level railway bridge atNewcastle—which being
above the navigable part of the Tyne was not required
to be on a high level for the convenience of ship-
ping, but only for the convenience of railway com-
munication—starts from the summit of the high
cliffs through which the river flows. The Britannia-
bridge across the Menai Straits also starts from
banks very much elevated by nature on either shore.
Bat in the case of the bridge across the Thames
imagined by the House of Lords an artificial access
to the high level must be obtained from both shores
of the river, requiring the construction: of a hun-
dred bridges, on high levels, across thoroughfares,
railways, canals, rivers, and even across docks,
before the level of the bridge across the Thames can
be approached. It is needless to point out how
much the construction of such approaches must
add to the expenses of the bridge itself.
But this is far from being all. The bridge must
'tot only be on a high level, but, to aimit of the
free passage of shipping, it must be of a great span.
The smallest span spoken of in connexion with
these high level bridges is a span of 800 feet. No
fixed bridge has ever been attempted by the boldest
engineer with any such span as 800 feet. The only
bridge in the world over which railway carriages
are passed, which has a span of 800 feet, is the
bridge at the Falls of Niagara, constructed by Mr.
Boebliug, an American engineer, on the suspension
principle. But the suspension principle would be
utterly inapplicable to a bridge across the Thames.
A suspension bridge oscillates with the weight of
tie load, and with the momentum of the carriages
raring over it. No load much exceeding 300 tons
Las ever been conveyed, at any one time, over the
Niagara suspension bride, and no train is allowed
to pass over that bridge at a greater speed than
three miles an hour. Even with these limitations
the span of the Niagara suspension bridge has
been thought so dangerous that it has been practi-
cally diminished to 700 feet, by building up sup-
ports at each of its extremities.

No engineer would propose a suspension bridge
for railway purposes across the Thames ; and
therefore we may take it at once that the proposed
bridge would be a fixed bridge, built upon the
tubular principle, or on the lattice principle,
which is a modification of it. Now the
central tubes of the Britannia-bridge are each
472 feet long. The height of these tubes is 31
feet ; their width 14 feet 8 inches ; their weightlittle
less than 2,000 tons. It will be remembered that
a great facility presented itself to Mr. Robert Ste-
phenson for the erection of the Britannia Bridge.
In the very centre of the Menai Straits there was a
reek, called the Britannia Rock, of which he took
advantage to build a pier, upon which the tubes
were rested. No such favourable opportunity pre-
sents itself in the channel of the Thames. There
is no favouring rock in the middle of the Thames,
and to build a pier or staging from the bed of the
river, in the centre of the tideway, of sufficient
height to carry the works over the centre of the
stream 150 feet above high water mark, may be
considered an impossibility. The bridge, there-
fore, must have a span from shore to shore, which
implies a span of 800 feet at the very leash; and
in order to be in proportion to 800 feet in length
the bridge will require to be nearly 60 feet in height
and of corresponding breadth. The weight of such
a bridge could not be estimated at less than 5,000
tons ; and whether such a weight could stand at a
span of 800 feet, or whether a bridge with such a
span would not break down of its own weight, is a
question which experience at present has not
detenuined.

One thing, however, has been determined re-
specting such a bridge—and that is, the difficulty
of Netting it up. The iron tubes of the Britannia,
470 feet in length, were floated into the Menai
Straits, and with all the advantages of that posi-
tion were got into their places with extreme
difficulty, and were only raised to the required
levels, after six or seven weeks' hard labour, with
the aid of the most powerful hydraulic machines
ever employed. Now let us suppose • tube, ',early
double the size of those used for the Britannia_
bridge, to be constructed on the Thames, it may
be at Milwall ; let us suppose it safely floated to
the point at which the piers for the high level
bridge are to erected on either side the Thames.
How is this tube, or beam, or whatever it may be,
of 800 foot in length, and nobody knows how many
thousand tons weight, to be erected 150 feet
above high water mark ? Can such a work be
executed without stopping the navigation of
the river far a period of time which may depend
*a a variety of contingencies and accidents ? Con-
tractors are bold men ; but is there any contractor
looki *sough to encounter the liability which would
acres from the stoppage of the navigation of the
River 'Thames for a month, a week, or even for a
day, during the progress of an attempt to lift •
Ws, ors nintliplicity of beams, equal in measure-