THE MENAI AND CONWAY BRIDGES.
[Image] Map of Menai Strait [Ordnance Survey]
So long as the dangerous Straits of Menai had to be crossed in an
open ferry-boat, the communication between London and Holyhead was
necessarily considered incomplete. While the roads through North
Wales were so dangerous as to deter travellers between England and
Ireland from using that route, the completion of the remaining link
of communication across the Straits was of comparatively little
importance. But when those roads had, by the application of much
capital, skill, and labour, been rendered so safe and convenient
that the mail and stage coaches could run over them at the rate of
from eight to ten miles an hour, the bridging of the Straits became
a measure of urgent public necessity. The increased traffic by this
route so much increased the quantity of passengers and luggage,
that the open boats were often dangerously overloaded; and serious
accidents, attended with loss of life and property, came to be of
frequent occurrence.
The erection of a bridge over the Straits had long been matter of
speculation amongst engineers. As early as 1776, Mr. Golborne
proposed his plan of an embankment with a bridge in the middle of it;
and a few years later, in 1785, Mr. Nichols proposed a wooden
viaduct, furnished with drawbridges at Cadnant Island. Later still,
Mr. Rennie proposed his design of a cast iron bridge. But none of
these plans were carried out, and the whole subject remained in
abeyance until the year 1810, when a commission was appointed to
inquire and report as to the state of the roads between Shrewsbury,
Chester, and Holyhead. The result was, that Mr. Telford was called
upon to report as to the most effectual method of bridging the
Menai Strait, and thus completing the communication with the port
of embarkation for Ireland.
[Image] Telford's proposed Cast Iron Bridge
Mr. Telford submitted alternative plans for a bridge over the
Strait: one at the Swilly Rock, consisting of three cast iron
arches of 260 feet span, with a stone arch of 100 feet span between
each two iron ones, to resist their lateral thrust; and another at
Ynys-y-moch, to which he himself attached the preference,
consisting of a single cast iron arch of 500 feet span, the crown
of the arch to be 100 feet above high water of spring tides, and
the breadth of the roadway to be 40 feet.
The principal objection taken to this plan by engineers generally,
was the supposed difficulty of erecting a proper centering to
support the arch during construction; and the mode by which
Mr. Telford proposed to overcome this may be cited in illustration
of his ready ingenuity in overcoming difficulties. He proposed to
suspend the centering from above instead of supporting it from
below in the usual manner--a contrivance afterwards revived by
another very skilful engineer, the late Mr. Brunel. Frames, 50 feet
high, were to be erected on the top of the abutments, and on these,
strong blocks, or rollers and chains, were to be fixed, by means of
which, and by the aid of windlasses and other mechanical powers,
each separate piece of centering was to be raised into, and
suspended in, its proper place. Mr. Telford regarded this method of
constructing centres as applicable to stone as well as to iron
arches; and indeed it is applicable, as Mr. Brunel held, to the
building of the arch itself.*[1]
[Image] Proposed Plan of Suspended Centering
Mr. Telford anticipated that, if the method recommended by him were
successfully adopted on the large scale proposed at Menai, all
difficulties with regard to carrying bridges over deep ravines
would be done away with, and a new era in bridge-building begun.
For this and other reasons--but chiefly because of the much greater
durability of a cast iron bridge compared with the suspension
bridge afterwards adopted--it is matter of regret that he was not
permitted to carry out this novel and grand design. It was,
however, again objected by mariners that the bridge would seriously
affect, if not destroy, the navigation of the Strait; and this
plan, like Mr. Rennie's, was eventually rejected.
Several years passed, and during the interval Mr. Telford was
consulted as to the construction of a bridge over Runcorn Gap on
the Mersey, above Liverpool. As the river was there about 1200 feet
wide, and much used for purposes of navigation, a bridge of the
ordinary construction was found inapplicable. But as he was
required to furnish a plan of the most suitable structure, he
proceeded to consider how the difficulties of the case were to be met.
The only practicable plan, he thought, was a bridge constructed on
the principle of suspension. Expedients of this kind had long been
employed in India and America, where wide rivers were crossed by
means of bridges formed of ropes and chains; and even in this
country a suspension bridge, though of a very rude kind, had long
been in use near Middleton on the Tees, where, by means of two
common chains stretched across the river, upon which a footway of
boards was laid, the colliers were enabled to pass from their
cottages to the colliery on the opposite bank.
Captain (afterwards Sir Samuel) Brown took out a patent for forming
suspension bridges in 1817; but it appears that Telford's attention
had been directed to the subject before this time, as he was first
consulted respecting the Runcorn Bridge in the year 1814, when he
proceeded to make an elaborate series of experiments on the
tenacity of wrought iron bars, with the object of employing this
material in his proposed structure. After he had made upwards of
two hundred tests of malleable iron of various qualities, he
proceeded to prepare his design of a bridge, which consisted of a
central opening of 1000 feet span, and two side openings of 500
feet each, supported by pyramids of masonry placed near the
low-water lines. The roadway was to be 30 feet wide, divided into
one central footway and two distinct carriageways of 12 feet each.
At the same time he prepared and submitted a model of the central
opening, which satisfactorily stood the various strains which were
applied to it. This Runcorn design of 1814 was of a very
magnificent character, perhaps superior even to that of the Menai
Suspension Bridge, afterwards erected; but unhappily the means were
not forthcoming to carry it into effect. The publication of his
plan and report had, however, the effect of directing public
attention to the construction of bridges on the suspension
principle; and many were shortly after designed and erected by
Telford and other engineers in different parts of the kingdom.
Mr. Telford continued to be consulted by the Commissioners of the
Holyhead Roads as to the completion of the last and most important
link in the line of communication between London and Holyhead,
by bridging the Straits of Menai; and at one of their meetings in
1815, shortly after the publication of his Runcorn design, the
inquiry was made whether a bridge upon the same principle was not
applicable in this particular case. The engineer was instructed
again to examine the Straits and submit a suitable plan and
estimate, which he proceeded to do in the early part of 1818.
The site selected by him as the most favourable was that which had
been previously fixed upon for the projected cast iron bridge,
namely at Ynys-y-moch--the shores there being bold and rocky,
affording easy access and excellent foundations, while by spanning
the entire channel between the low-water lines, and the roadway
being kept uniformly 100 feet above the highest water at spring tide,
the whole of the navigable waterway would be left entirely
uninterrupted. The distance between the centres of the supporting
pyramids was proposed to be of the then unprecedented width of 550
feet, and the height of the pyramids 53 feet above the level of the
roadway. The main chains were to be sixteen in number, with a
deflection of 37 feet, each composed of thirty-six bars of
half-inch-square iron, so placed as to give a square of six on each
side, making the whole chain about four inches in diameter, welded
together for their whole length, secured by bucklings, and braced
round with iron wire; while the ends of these great chains were to
be secured by a mass of masonry built over stone arches between
each end of the supporting piers and the adjoining shore. Four of
the arches were to be on the Anglesea, and three on the
Caernarvonshire side, each of them of 52 feet 6 inches span.
The roadway was to be divided, as in the Runcorn design with a
carriage way 12 feet wide on each side, and a footpath of 4 feet in
the middle. Mr. Telford's plan was supported by Mr. Rennie and other
engineers of eminence; and the Select Committee of the House of Commons,
being satisfied as to its practicability, recommended Parliament to
pass a Bill and to make a grant of money to enable the work to be
carried into effect.
[Image] Outline of Menai Bridge
The necessary Act passed in the session of 1819, and Mr. Telford
immediately proceeded to Bangor to make preparations for beginning
the works. The first proceeding was to blast off the inequalities
of the surface of the rock called Ynys-y-moch, situated on the
western or Holyhead side of the Strait, at that time accessible
only at low water. The object was to form an even surface upon it
for the foundation of the west main pier. It used to be at this
point, where the Strait was narrowest, that horned cattle were
driven down, preparatory to swimming them across the channel to the
Caernarvon side, when the tide was weak and at its lowest ebb. The
cattle were, nevertheless, often carried away, the current being
too strong for the animals to contend against it.
At the same time, a landing-quay was erected on Ynys-y-moch, which
was connected with the shore by an embankment carrying lines of
railway. Along these, horses drew the sledges laden with stone
required for the work; the material being brought in barges from
the quarries opened at Penmon Point, on the north-eastern extremity
of the Isle of Anglesea, a little to the westward of the northern
opening of the Strait. When the surface of the rock had been
levelled and the causeway completed, the first stone of the main
pier was laid by Mr. W.A. Provis, the resident engineer, on the
10th of August, 1819; but not the slightest ceremony was observed
on the occasion.
Later in the autumn, preparations were made for proceeding with the
foundations of the eastern main pier on the Bangor side of the
Strait. After excavating the beach to a depth of 7 feet, a solid
mass of rock was reached, which served the purpose of an immoveable
foundation for the pier. At the same, time workshops were erected;
builders, artisans, and labourers were brought together from
distant quarters; vessels and barges were purchased or built for
the special purpose of the work; a quay was constructed at Penmon
Point for loading the stones for the piers; and all the requisite
preliminary arrangements were made for proceeding with the building
operations in the ensuing spring.
A careful specification of the masonry work was drawn up, and the
contract was let to Messrs. Stapleton and Hall; but as they did not
proceed satisfactorily, and desired to be released from the contract,
it was relet on the same terms to Mr. John Wilson, one of Mr. Telford's
principal contractors for mason work on the Caledonian Canal.
The building operations were begun with great vigour early in 1820.
The three arches on the Caernarvonshire side and the four on the
Anglesea side were first proceeded with. They are of immense
magnitude, and occupied four years in construction, having been
finished late in the autumn of 1824. These piers are 65 feet in
height from high-water line to the springing of the arches, the
span of each being 52 feet 6 inches. The work of the main piers
also made satisfactory progress, and the masonry proceeded so
rapidly that stones could scarcely be got from the quarries in
sufficient quantity to keep the builders at work. By the end of
June about three hundred men were employed.
The two principal piers, each 153 feet in height, upon which the
main chains of the bridge were to be suspended, were built with
great care and under rigorous inspection. In these, as indeed in
most of the masonry of the bridge, Mr. Telford adopted the same
practice which he had employed in his previous bridge structures,
that of leaving large void spaces, commencing above high water mark
and continuing them up perpendicularly nearly to the level of the
roadway. "I have elsewhere expressed my conviction," he says, when
referring to the mode of constructing these piers, "that one of the
most important improvements which I have been able to introduce
into masonry consists in the preference of cross-walls to rubble,
in the structure of a pier, or any other edifice requiring strength.
Every stone and joint in such walls is open to inspection in the
progress of the work, and even afterwards, if necessary; but a
solid filling of rubble conceals itself, and may be little better
than a heap of rubbish confined by side walls." The walls of these
main piers were built from within as well as from without all the
way up, and the inside was as carefully and closely cemented with
mortar as the external face. Thus the whole pier was bound firmly
together, and the utmost strength given, while the weight of the
superstructure upon the lower parts of the work was reduced to its
minimum.
[Image] Section of Main Pier
Over the main piers, the small arches intended for the roadways
were constructed, each being 15 feet to the springing of the arch,
and 9 feet wide. Upon these arches the masonry was carried
upwards, in a tapering form, to a height of 53 feet above the
level of the road. As these piers were to carry the immense weight
of the suspension chains, great pains were taken with their
construction, and all the stones, from top to bottom, were firmly
bound together with iron dowels to prevent the possibility of their
being separated or bulged by the immense pressure they had to
withstand.
The most important point in the execution of the details of the
bridge, where the engineer had no past experience to guide him, was
in the designing and fixing of the wrought iron work. Mr. Telford
had continued his experiments as to the tenacity of bar iron, until
he had obtained several hundred distinct tests; and at length,
after the most mature delilberation, the patterns and dimensions
were finally arranged by him, and the contract for the manufacture
of the whole was let to Mr. Hazeldean, of Shrewsbury, in the year
1820. The iron was to be of the best Shropshire, drawn at Upton
forge, and finished and proved at the works, under the inspection
of a person appointed by the engineer.
[Image] Cut showing fixing of the chains in the rock
The mode by which the land ends of these enormous suspension chains
were rooted to the solid ground on either side of the Strait, was
remarkably ingenious and effective. Three oblique tunnels were made
by blasting the rock on the Anglesea side; they were each about six
feet in diameter, the excavations being carried down an inclined
plane to the depth of about twenty yards. A considerable width of
rock lay between each tunnel, but at the bottom they were all
united by a connecting horizontal avenue or cavern, sufficiently
capacious to enable the workmen to fix the strong iron frames,
composed principally of thick flat cast iron plates, which were
engrafted deeply into the rock, and strongly bound together by the
iron work passing along the horizontal avenue; so that, if the iron
held, the chains could only yield by tearing up the whole mass of
solid rock under which they were thus firmly bound.
A similar method of anchoring the main chains was adopted on the
Caernarvonshire side. A thick bank of earth had there to be cut
through, and a solid mass of masonry built in its place, the rock
being situated at a greater distance from the main pier; involving
a greater length of suspending chain, and a disproportion in the
catenary or chord line on that side of the bridge. The excavation
and masonry thereby rendered necessary proved a work of vast
labour, and its execution occupied a considerable time; but by the
beginning of the year 1825 the suspension pyramids, the land piers
and arches, and the rock tunnels, had all been completed, and the
main chains were firmly secured in them; the work being
sufficiently advanced to enable the suspending of the chains to be
proceeded with. This was by far the most difficult and anxious part
of the undertaking.
With the same careful forethought and provision for every
contingency which had distinguished the engineer's procedure in the
course of the work, he had made frequent experiments to ascertain
the actual power which would be required to raise the main chains
to their proper curvature. A valley lay convenient for the purpose,
a little to the west of the bridge on the Anglesea side.
Fifty-seven of the intended vertical suspending rods, each nearly
ten feet long and an inch square, having been fastened together, a
piece of chain was attached to one end to make the chord line 570
feet in length; and experiments having been made and comparisons
drawn, Mr. Telford ascertained that the absolute weight of one of
the main chains of the bridge between the points of suspension was
23 1/2 tons, requiring a strain of 39 1/2 tons to raise it to its
proper curvature. On this calculation the necessary apparatus
required for the hoisting was prepared. The mode of action finally
determined on for lifting the main chains, and fixing them into
their places, was to build the central portion of each upon a raft
450 feet long and 6 feet wide, then to float it to the site of the
bridge, and lift it into its place by capstans and proper tackle.
At length all was ready for hoisting the first great chain, and
about the middle of April, 1825, Mr. Telford left London for Bangor
to superintend the operations. An immense assemblage collected to
witness the sight; greater in number than any that had been
collected in the same place since the men of Anglesea, in their
war-paint, rushing down to the beach, had shrieked defiance across
the Straits at their Roman invaders on the Caernarvon shore.
Numerous boats arrayed in gay colours glided along the waters; the
day--the 26th of April--being bright, calm, and in every way
propitious.
At half-past two, about an hour before high water, the raft bearing
the main chain was cast off from near Treborth Mill, on the
Caernarvon side. Towed by four boats, it began gradually to move
from the shore, and with the assistance of the tide, which caught
it at its further end, it swung slowly and majestically round to
its position between the main piers, where it was moored. One end
of the chain was then bolted to that which hung down the face of
the Caernarvon pier; whilst the other was attached to ropes
connected with strong capstans fixed on the Anglesea side, the
ropes passing by means of blocks over the top of the pyramid of the
Anglesea pier. The capstans for hauling in the ropes bearing the
main chain, were two in number, manned by about 150 labourers. When
all was ready, the signal was given to "Go along!" A Band of fifers
struck up a lively tune; the capstans were instantly in motion, and
the men stepped round in a steady trot. All went well. The ropes
gradually coiled in. As the strain increased, the pace slackened a
little; but "Heave away, now she comes!" was sung out. Round went
the men, and steadily and safely rose the ponderous chain.
[Image] Cut of Bridge, showing state of Suspension Chain
The tide had by this time turned, and bearing upon the side of the
raft, now getting freer of its load, the current floated it away
from under the middle of the chain still resting on it, and it
swung easily off into the water. Until this moment a breath less
silence pervaded the watching multitude; and nothing was heard
among the working party on the Anglesea side but the steady tramp
of the men at the capstans, the shrill music of the fife, and the
occasional order to "Hold on!" or "Go along!" But no sooner was the
raft seen floating away, and the great chain safely swinging in the
air, than a tremendous cheer burst forth along both sides of the
Straits.
The rest of the work was only a matter of time. The most anxious
moment had passed. In an hour and thirty-five minutes after the
commencement of the hoisting, the chain was raised to its proper
curvature, and fastened to the land portion of it which had been
previously placed over the top of the Anglesea pyramid. Mr. Telford
ascended to the point of fastening, and satisfied himself that a
continuous and safe connection had been formed from the Caernarvon
fastening on the rock to that on Anglesea. The announcement of the
fact was followed by loud and prolonged cheering from the workmen,
echoed by the spectators, and extending along the Straits on both
sides, until it seemed to die away along the shores in the distance.
Three foolhardy workmen, excited by the day's proceedings, had the
temerity to scramble along the upper surface of the chain--which
was only nine inches wide and formed a curvature of 590 feet--from
one side of the Strait to the other!*[2] Far different were the
feelings of the engineer who had planned this magnificent work.
Its failure had been predicted; and, like Brindley's Barton Viaduct,
it had been freely spoken of as a "castle in the air." Telford had,
it is true, most carefully tested every part by repeated experiment,
and so conclusively proved the sufficiency of the iron chains to
bear the immense weight they would have to support, that he was
thoroughly convinced as to the soundness of his principles of
construction, and satisfied that, if rightly manufactured and
properly put together, the chains would hold, and that the piers
would sustain them. Still there was necessarily an element of
uncertainty in the undertaking. It was the largest structure of
the kind that had ever been attempted. There was the contingency
of a flaw in the iron; some possible scamping in the manufacture;
some little point which, in the multiplicity of details to be
attended to, he might have overlooked, or which his subordinates
might have neglected. It was, indeed, impossible but that he
should feel intensely anxious as to the result of the day's
operations. Mr. Telford afterwards stated to a friend, only a few
months before his death, that for some time previous to the opening
of the bridge, his anxiety was so great that he could scarcely
sleep; and that a continuance of that condition must have very soon
completely undermined his health. We are not, therefore, surprised
to learn that when his friends rushed to congratulate him on the
result of the first day's experiment, which decisively proved the
strength and solidity of the bridge, they should have found the
engineer on his knees engaged in prayer. A vast load had been
taken off his mind; the perilous enterprise of the day had been
accomplished without loss of life; and his spontaneous act was
thankfulness and gratitude.
[Image] Menai Bridge
The suspension of the remaining fifteen chains was accomplished
without difficulty. The last was raised and fixed on the 9th of
July, 1825, when the entire line was completed. On fixing the final
bolt, a band of music descended from the top of the suspension pier
on the Anglesea side to a scaffolding erected over the centre of
the curved part of the chains, and played the National Anthem
amidst the cheering of many thousand persons assembled along the
shores of the Strait: while the workmen marched in procession along
the bridge, on which a temporary platform had been laid, and the
St. David steam-packet of Chester passed under the chains towards
the Smithy Rocks and back again, thus re-opening the navigation of
the Strait.
In August the road platform was commenced, and in September the
trussed bearing bars were all suspended. The road was constructed
of timber in a substantial manner, the planking being spiked
together, with layers of patent felt between the planks, and the
carriage way being protected by oak guards placed seven feet and a
half apart. Side railings were added; the toll-houses and
approach-roads were completed by the end of the year; and the
bridge was opened for public traffic on Monday, the 30th of January,
1826, when the London and Holyhead mailcoach passed over it for the
first time, followed by the Commissioners of the Holyhead roads,
the engineer, several stage-coaches, and a multitude of private
persons too numerous to mention.
We may briefly add a few facts as to the quantities of materials
used, and the dimensions of this remarkable structure. The total
weight of iron was 2187 tons, in 33,265 pieces. The total length of
the bridge is 1710 feet, or nearly a third of a mile; the distance
between the points of suspension of the main bridge being 579 feet.
The total sum expended by Government in its erection, including the
embankment and about half a mile of new line of road on the
Caernarvon side, together with the toll-houses, was 120,000L.
Notwithstanding the wonders of the Britannia Bridge subsequently
erected by Robert Stephenson for the passage across the same strait
of the Chester and Holyhead Railway, the Menai Bridge of Telford is
by far the most picturesque object. "Seen as I approached it," says
Mr. Roscoe, "in the clear light of an autumnal sunset, which threw
an autumnal splendour on the wide range of hills beyond, and the
sweep of richly variegated groves and plantations which covered
their base--the bright sun, the rocky picturesque foreground,
villas, spires, and towers here and there enlivening the prospect--
the Menai Bridge appeared more like the work of some great magician
than the mere result of man's skill and industry."
[Image] Conway Suspension Bridge
Shortly after the Menai Bridge was begun, it was determined by the
Commissioners of the Holyhead road that a bridge of similar design
should be built over the estuary of the Conway, immediately
opposite the old castle at that place, and which had formerly been
crossed by an open ferry boat. The first stone was laid on the
3rd of April, 1822, and the works having proceeded satisfactorily,
the bridge and embankment approaching it were completed by the summer
of 1826. But the operations being of the same kind as those
connected with the larger structure above described, though of a
much less difficult character, it is unnecessary to enter into any
details as to the several stages of its construction. In this
bridge the width between the centres of the supporting towers is
327 feet, and the height of the under side of the roadway above
high water of spring tides only 15 feet. The heaviest work was an
embankment as its eastern approach, 2015 feet in length and about
300 feet in width at its highest part.
It will be seen, from the view of the bridge given on the opposite
page, that it is a highly picturesque structure, and combines,
with the estuary which it crosses, and the ancient castle of Conway,
in forming a landscape that is rarely equalled.
Footnotes for Chapter XII.
*[1] In an article in the 'Edinburgh Review,' No. exli., from the
pen of Sir David Brewster, the writer observes:--"Mr. Telford's
principle of suspending and laying down from above the centering of
stone and iron bridges is, we think, a much more fertile one than
even he himself supposed. With modifications, by no means
considerable, and certainly practicable, it appears to us that the
voussoirs or archstones might themselves be laid down from above,
and suspended by an appropriate mechanism till the keystone was
inserted. If we suppose the centering in Mr. Telford's plan to be
of iron, this centering itself becomes an iron bridge, each rib of
which is composed of ten pieces of fifty feet each; and by
increasing the number of suspending chains, these separate pieces
or voussoirs having been previously joined together, either
temporarily or permanently, by cement or by clamps, might be laid
into their place, and kept there by a single chain till the road
was completed. The voussoirs, when united, might be suspended from
a general chain across the archway, and a platform could be added
to facilitate the operations." This is as nearly as possible the
plan afterwards revived by Mr. Brunel, and for the originality of
which, we believe, he has generally the credit, though it clearly
belongs to Telford.
*[2] A correspondent informs us of a still more foolhardy exploit
performed on the occasion. He says, "Having been present, as a boy
from Bangor grammar school, on the 26th of April, when the first
chain was carried across, an incident occurred which made no small
impression on my mind at the time. After the chain had reached its
position, a cobbler of the neighbourhood crawled to the centre of
the curve, and there finished a pair of shoes; when, having
completed his task, he returned in safety to the Caernarvon side!
I need not say that we schoolboys appreciated his feat of
foolhardiness far more than Telford's master work."