ROBERT STEPHENSON'S TUBULAR BRIDGES AT MENAI AND CONWAY.
We have now to describe briefly another great undertaking, begun by
George Stephenson, and taken up and completed by his son, in the course
of which the latter carried out some of his greatest works--we mean the
Chester and Holyhead Railway, completing the railway connection with
Dublin, as the Newcastle and Berwick line completed the connection with
Edinburgh. It will thus be seen how closely Telford was followed by the
Stephensons in perfecting the highways of their respective epochs; the
former by means of turnpike-roads, and the latter by means of railways.
George Stephenson surveyed a line from Chester to Holyhead in 1838, and
at the same time reported on the line through North Wales to Port
Dynllaen, proposed by the Irish Railway Commissioners. His advice was
strongly in favour of adopting the line to Holyhead, as less costly and
presenting better gradients. A public meeting was held at Chester, in
January, 1839, in support of the latter measure, at which he was present
to give explanations. Mr. Uniacke, the Mayor, in opening the
proceedings, said that Mr. Stephenson was present, ready to answer any
questions which might be put to him on the subject; and it was
judiciously remarked that "it would be better that he should be asked
questions than required to make a speech; for, though a very good
engineer, he was a bad speaker." One of the questions then put to Mr.
Stephenson related to the mode by which he proposed to haul the passenger
carriages over the Menai Suspension Bridge by horse power; and he was
asked whether he knew the pressure the bridge was capable of sustaining.
His answer was, that "he had not yet made any calculations; but he
proposed getting data which would enable him to arrive at an accurate
calculation of the actual strain upon the bridge during the late gale.
He had, however, no hesitation in saying that it was more than twenty
times as much as the strain of a train of carriages and a locomotive
engine. The only reason why he proposed to convey the carriages over by
horses, was in order that he might, by distributing the weight, not
increase the wavy motion. All the train would be on at once; but
distributed. This he thought better than passing them, linked together,
by a locomotive engine." It will thus be observed that the
practicability of throwing a rigid railway bridge across the Straits had
not yet been contemplated.
The Dublin Chamber of Commerce passed resolutions in favour of
Stephenson's line, after hearing his explanation of its essential
features. The project, after undergoing much discussion, was at length
embodied in an Act passed in 1844; and the work was brought to a
successful completion by his son, with several important modifications,
including the grand original feature of the tubular bridges across the
Menai Straits and the estuary of the Conway. Excepting these great
works, the construction of this line presented no unusual features;
though the remarkable terrace cut for the accommodation of the railway
under the steep slope of Penmaen Mawr is worthy of a passing notice.
About midway between Conway and Bangor, Penmaen Mawr forms a bold and
almost precipitous headland, at the base of which, in rough weather, the
ocean dashes with great fury. There was not space enough between the
mountain and the strand for the passage of the railway; hence in some
places the rock had to be blasted to form a terrace, and in others
sea-walls had to be built up to the proper level, on which to form an
embankment of sufficient width to enable the road to be laid. [Picture:
Penmaen Mawr. (By Percival Skelton.)] A tunnel 10.5 chains in length was
cut through the headland itself; and on its east and west sides the line
was formed by a terrace cut out of the cliff, and by embankments
protected by sea walls; the terrace being three times interrupted by
embankments in its course of about 1.25 mile. The road lies so close
under the steep mountain face, that it was even found necessary at
certain places to protect it against possible accidents from falling
stones, by means of a covered way. The terrace on the east side of the
headland was, however, in some measure protected against the roll of the
sea by the mass of stone run out from the tunnel, and forming a deep
shingle bank in front of the wall.
The part of the work which lies on the westward of the headland
penetrated by the tunnel, was exposed to the full force of the sea; and
the formation of the road at that point was attended with great
difficulty. While the sea wall was still in progress, its strength was
severely tried by a strong north-westerly gale, which blew in October,
1846, with a spring tide of 17 feet. On the following morning it was
found that a large portion of the rubble was irreparably injured, and 200
yards of the wall were then replaced by an open viaduct, with the piers
placed edgeways to the sea, the openings between them being spanned by
ten cast-iron girders each 42 feet long. This accident induced the
engineer to alter the contour of the sea wall, so that it should present
a diminished resistance to the force of the waves. But the sea repeated
its assaults, and made further havoc with the work; entailing heavy
expenses and a complete reorganisation of the contract. Increased
solidity was then given to the masonry, and the face of the wall
underwent further change. At some points outworks were constructed, and
piles were driven into the beach about 15 feet from the base of the wall,
for the purpose of protecting its foundations and breaking the force of
the waves. The work was at length finished after about three years'
anxious labour; but Mr. Stephenson confessed that if a long tunnel had
been made in the first instance through the solid rock of Penmaen Mawr, a
saving of from 25,000 to 30,000 pounds would have been effected. He also
said he had arrived at the conclusion that in railway works engineers
should endeavour as far as possible to avoid the necessity of contending
with the sea; {324} but if he were ever again compelled to go within its
reach, he would adopt, instead of retaining walls, an open viaduct,
placing all the piers edgeways to the force of the sea, and allowing the
waves to break upon a natural slope of beach. He was ready enough to
admit the errors he had committed in the original design of this work;
but he said he had always gained more information from studying the
causes of failures and endeavouring to surmount them than he had done
from easily-won successes. Whilst many of the latter had been forgotten,
the former were indelibly fixed in his memory.
But by far the greatest difficulty which Robert Stephenson had to
encounter in executing this railway, was in carrying it across the
Straits of Menai and the estuary of the Conway, where, like his
predecessor Telford when forming his high road through North Wales, he
was under the necessity of resorting to new and altogether untried
methods of bridge construction. At Menai the waters of the Irish Sea are
perpetually vibrating along the precipitous shores of the strait; rising
and falling from 20 to 25 feet at each successive tide; the width and
depth of the channel being such as to render it available for navigation
by the largest ships. 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. The bridge was not, however, carried out after this design,
but was made of separate wrought-iron girders composed of riveted plates.
Recurring to his first idea of this bridge, Mr. Stephenson thought that a
stiff platform might be constructed, with sides of strongly trussed
frame-work of wrought-iron, braced together at top and bottom with plates
of like material riveted together with angle-iron; and that such platform
might be suspended by strong chains on either side to give it increased
security. "It was now," says Mr. Stephenson, "that I came to regard the
tubular platform as a beam, and that the chains should be looked upon as
auxiliaries." It appeared, nevertheless, that without a system of
diagonal struts inside, which of course would have prevented the passage
of trains _through_ it, this kind of structure was ill-suited for
maintaining its form, and would be very liable to become lozenge-shaped.
Besides, the rectangular figure was deemed objectionable, from the large
surface which it presented to the wind.
It then occurred to him that circular or elliptical tubes might better
answer the intended purpose; and in March, 1845, he gave instructions to
two of his assistants to prepare drawings of such a structure, the tubes
being made with a double thickness of plate at top and bottom. The
results of the calculations made as to the strength of such a tube, were
considered so satisfactory, that Mr. Stephenson says he determined to
fall back on a bridge of this description, on the rejection of his design
of the two cast-iron arches by the Parliamentary Committee. Indeed, it
became evident that a tubular wrought-iron beam was the only structure
which combined the necessary strength and stability for a railway, with
the conditions deemed essential for the protection of the navigation. "I
stood," says Mr. Stephenson, "on the verge of a responsibility from
which, I confess, I had nearly shrunk. The construction of a tubular
beam of such gigantic dimensions, on a platform elevated and supported by
chains at such a height, did at first present itself as a difficulty of a
very formidable nature. Reflection, however, satisfied me that the
principles upon which the idea was founded were nothing more than an
extension of those daily in use in the profession of the engineer. The
method, moreover, of calculating the strength of the structure which I
had adopted, was of the simplest and most elementary character; and
whatever might be the form of the tube, the principle on which the
calculations were founded was equally applicable, and could not fail to
lead to equally accurate results." {327} Mr. Stephenson accordingly
announced to the directors of the railway that he was prepared to carry
out a bridge of this general description, and they adopted his views,
though not without considerable misgivings.
While the engineer's mind was still occupied with the subject, an
accident occurred to the _Prince of Wales_ iron steamship, at Blackwall,
which singularly corroborated his views as to the strength of
wrought-iron beams of large dimensions. When this vessel was being
launched, the cleet on the bow gave way, in consequence of the bolts
breaking, and let the vessel down so that the bilge came in contact with
the wharf, and she remained suspended between the water and the wharf for
a length of about 110 feet, but without any injury to the plates of the
ship; satisfactorily proving the great strength of this form of
construction. Thus, Mr. Stephenson became gradually confirmed in his
opinion that the most feasible method of bridging the strait at Menai and
the river at Conway was by means of a hollow beam of wrought-iron. As
the time was approaching for giving evidence before Parliament on the
subject, it was necessary for him to settle some definite plan for
submission to the committee. "My late revered father," says he, "having
always taken a deep interest in the various proposals which had been
considered for carrying a railway across the Menai Straits, requested me
to explain fully to him the views which led me to suggest the use of a
tube, and also the nature of the calculations I had made in reference to
it. It was during this personal conference that Mr. William Fairbairn
accidentally called upon me, to whom I also explained the principles of
the structure I had proposed. He at once acquiesced in their truth, and
expressed confidence in the feasibility of my project, giving me at the
same time some facts relative to the remarkable strength of iron
steamships, and invited me to his works at Millwall, to examine the
construction of an iron steamship which was then in progress." The date
of this consultation was early in April, 1845, and Mr. Fairbairn states
that, on that occasion, "Mr. Stephenson asked whether such a design was
practicable, and whether I could accomplish it: and it was ultimately
arranged that the subject should be investigated experimentally, to
determine not only the value of Mr. Stephenson's original conception (of
a circular or egg-shaped wrought-iron tube, supported by chains), but
that of any other tubular form of bridge which might present itself in
the prosecution of my researches. The matter was placed unreservedly in
my hands; the entire conduct of the investigation was entrusted to me;
and, as an experimenter, I was to be left free to exercise my own
discretion in the investigation of whatever forms or conditions of the
structure might appear to me best calculated to secure a safe passage
across the Straits." {329a} Mr. Fairbairn then proceeded to construct a
number of experimental models for the purpose of testing the strength of
tubes of different forms. The short period which elapsed, however,
before the bill was in committee, did not admit of much progress being
made with those experiments; but from the evidence in chief given by Mr.
Stephenson on the subject, on the 5th May following, it appears that the
idea which prevailed in his mind was that of a bridge with openings of
450 feet (afterwards increased to 460 feet); with a roadway formed of a
hollow wrought-iron beam, about 25 feet in diameter, presenting a rigid
platform, suspended by chains. At the same time, he expressed the
confident opinion that a tube of wrought iron would possess sufficient
strength and rigidity to support a railway train running inside of it
without the help of the chains.
While the bill was still in progress, Mr. Fairbairn proceeded with his
experiments. He first tested tubes of a cylindrical form, in consequence
of the favourable opinion entertained by Mr. Stephenson of the tubes in
that shape, extending them subsequently to those of an elliptical form.
{329b} He found tubes thus shaped more or less defective, and proceeded
to test those of a rectangular kind. After the bill had received the
royal assent on the 30th June, 1845, the directors of the company, with
great liberality, voted a sum for the purpose of enabling the experiments
to be prosecuted, and upwards of 6000 pounds were thus expended to make
the assurance of their engineer doubly sure. Mr. Fairbairn's tests were
of the most elaborate and eventually conclusive character, bringing to
light many new and important facts of great practical value. The due
proportions and thicknesses of the top, bottom, and sides of the tubes
were arrived at after a vast number of trials; one of the results of the
experiments being the adoption of Mr. Fairbairn's invention of
rectangular hollow cells in the top of the beam for the purpose of giving
it the requisite degree of strength. About the end of August it was
thought desirable to obtain the assistance of a mathematician, who should
prepare a formula by which the strength of a full-sized tube might be
calculated from the results of the experiments made with tubes of smaller
dimensions. Professor Hodgkinson was accordingly called in, and he
proceeded to verify and confirm the experiments which Mr. Fairbairn had
made, and afterwards reduced them to the required formula.
Mr. Stephenson's time was so much engrossed with his extensive
engineering business that he was in a great measure precluded from
devoting himself to the consideration of the practical details. The
results of the experiments were communicated to him from time to time,
and were regarded by him as exceedingly satisfactory. It would appear,
however, that while Mr. Fairbairn urged the rigidity and strength of the
tubes without the aid of chains, Mr. Stephenson had not quite made up his
mind upon the point. Mr. Hodgkinson, also, was strongly inclined to
retain them. Mr. Fairbairn held that it was quite practicable to make
the tubes "sufficiently strong to sustain not only their own weight, but,
in addition to that load, 2000 tons equally distributed over the surface
of the platform,--a load ten times greater than they will ever be called
upon to support."
It was thoroughly characteristic of Mr. Stephenson, and of the caution
with which he proceeded in every step of this great undertaking--probing
every inch of the ground before he set down his foot upon it--that he
should, early in 1856, (_sic_) have appointed his able assistant, Mr.
Edwin Clark, to scrutinise carefully the results of every experiment, and
subject them to a separate and independent analysis before finally
deciding upon the form or dimensions of the structure, or upon any mode
of procedure connected with it. At length Mr. Stephenson became
satisfied that the use of auxiliary chains was unnecessary, and that the
tubular bridge might be made of such strength as to be entirely
self-supporting.
While these important discussions were in progress, measures were taken
to proceed with the masonry of the bridges simultaneously at Conway and
the Menai Straits. The foundation-stone of the Britannia Bridge was laid
on the 10th April, 1846; and on the 12th May following that of the Conway
Bridge was laid. Suitable platforms and workshops were also erected for
proceeding with the punching, fitting, and riveting of the tubes; and
when these operations were in full progress, the neighbourhood of the
Conway and Britannia Bridges presented scenes of extraordinary bustle and
industry. About 1500 men were employed on the Britannia Bridge alone,
and they mostly lived upon the ground in wooden cottages erected for the
occasion. The iron plates were brought in ship-loads from Liverpool,
Anglesey marble from Penmon, and red sandstone from Runcorn, in Cheshire,
as wind and tide, and shipping and convenience, might determine. There
was an unremitting clank of hammers, grinding of machinery, and blasting
of rock, going on from morning till night. In fitting the Britannia
tubes together, not less than 2,000,000 of bolts were riveted, weighing
some 900 tons.
The Britannia Bridge consists of two independent continuous tubular
beams, each 1511 feet in length, and each weighing 4680 tons, independent
of the cast-iron frames inserted at their bearings on the masonry of the
towers. These immense beams are supported at five places, namely, on the
abutments and on three towers, the central of which is known as the Great
Britannia Tower, 230 feet high, built on a rock in the middle of the
Strait. The side towers are 18 feet less in height than the central one,
and the abutment 35 feet lower than the side towers. The design of the
masonry is such as to accord with the form of the tubes, being somewhat
of an Egyptian character, massive and gigantic rather than beautiful, but
bearing the unmistakable impress of power.
The bridge has four spans,--two of 460 feet over the water, and two of
230 feet over the land. The weight of the larger spans, at the points
where the tubes repose on the masonry, is not less than 1587 tons. On
the centre tower the tubes rest solid; but on the land towers and
abutments they lie on roller-beds, so as to allow of expansion and
contraction. The road within each tube is 15 feet wide, and the height
varies from 23 feet at the ends to 30 feet at the centre. To give an
idea of the vast size of the tubes by comparison with other structures,
it may be mentioned that each length constituting the main spans is twice
as long as London Monument is high; and if it could be set on end in St.
Paul's Churchyard, it would reach nearly 100 feet above the cross.
The Conway Bridge is, in most respects, similar to the Britannia,
consisting of two tubes, of 400 feet span, placed side by side, each
weighing 1180 tons. The principle adopted in the construction of the
tubes, and the mode of floating and raising them, were nearly the same as
at the Britannia Bridge, though the general arrangement of the plates is
in many respects different.
It was determined to construct the shorter outer tubes of the Britannia
Bridge on scaffoldings in the positions in which they were permanently to
remain, and to erect the larger tubes upon wooden platforms at
high-water-mark on the Caernarvon shore, from whence they were to be
floated in pontoons.
The floating of the tubes on pontoons, from the places where they had
been constructed, to the recesses in the masonry of the towers, up which
they were to be hoisted to the positions they were permanently to occupy,
was an anxious and exciting operation. The first part of this process
was performed at Conway, where Mr. Stephenson directed it in person,
assisted by Captain Claxton, Mr. Brunel, and other engineering friends.
On the 6th March, 1848, the pontoons bearing the first great tube of the
up-line were floated round quietly and majestically into their place
between the towers in about twenty minutes. Unfortunately, one of the
sets of pontoons had become slightly slued by the stream, by which the
Conway end of the tube was prevented from being brought home; and five
anxious days to all concerned intervened before it could be set in its
place. In the mean time, the presses and raising machinery had been
fitted in the towers above, and the lifting process was begun on the 8th
April, when the immense mass was raised 8 feet, at the rate of about 2
inches a minute. On the 16th, the tube had been raised and finally
lowered into its permanent bed; the rails were laid along it; and, on the
18th, Mr. Stephenson passed through with the first locomotive. The
second tube was proceeded with on the removal of the first from the
platform, and was completed and floated in seven months. The rapidity
with which this second tube was constructed was in no small degree owing
to the Jacquard punching-machine, contrived for the purpose by Mr.
Roberts of Manchester. This tube was finally fixed in its permanent bed
on the 2nd of January, 1849.
[Picture: Conway Tubular Bridge]
The floating and fixing of the great Britannia tubes was a still more
formidable enterprise, though the experience gained at Conway rendered it
easy compared with what it otherwise would have been. Mr. Stephenson
superintended the operation of floating the first in person, giving the
arranged signals from the top of the tube on which he was mounted, the
active part of the business being performed by a numerous corps of
sailors, under the immediate direction of Captain Claxton. Thousands of
spectators lined the shores of the Strait on the evening of the 19th
June, 1849. On the land attachments being cut, the pontoons began to
float off; but one of the capstans having given way from excessive
strain, the tube was brought home again for the night. By next morning
the defective capstan was restored, and all was in readiness for another
trial. At half-past seven in the evening the tube was afloat, and the
pontoons swung out into the current like a monster pendulum, held steady
by the shore guide-lines, but increasing in speed to almost a fearful
extent as they neared their destined place between the piers. "The
success of this operation," says Mr. Clark, "depended mainly on properly
striking the 'butt' beneath the Anglesey tower, on which, as upon a
centre, the tube was to be veered round into its position across the
opening. This position was determined by a 12-inch line, which was to be
paid out to a fixed mark from the Llanfair capstan. The coils of the
rope unfortunately over-rode each other upon this capstan, so that it
could not be paid out. In resisting the motion of the tube, the capstan
was bodily dragged out of the platform by the action of the palls, and
the tube was in imminent danger of being carried away by the stream, or
the pontoons crushed upon the rocks. The men at the capstan were all
knocked down, and some of them thrown into the water, though they made
every exertion to arrest the motion of the capstan-bars. In this dilemma
Mr. Rolfe, who had charge of the capstan, with great presence of mind,
called the visitors on shore to his assistance; and handing out the spare
coil of the 12-inch line into the field at the back of the capstan, it
was carried with great rapidity up the field, and a crowd of people, men,
women, and children, holding on to this huge cable, arresting the
progress of the tube, which was at length brought safely against the butt
and veered round. The Britannia end was then drawn into the recess of
the masonry by a chain passing through the tower to a crab on the far
side. The violence of the tide abated, though the wind increased, and
the Anglesey end was drawn into its place beneath the corbelling in the
masonry; and as the tide went down, the pontoons deposited their valuable
cargo on the welcome shelf at each end. The successful issue was greeted
by cannon from the shore and the hearty cheers of many thousands of
spectators, whose sympathy and anxiety were but too clearly indicated by
the unbroken silence with which the whole operation had been
accompanied." {335} By midnight all the pontoons had been got clear of
the tube, which now hung suspended over the waters of the Strait by its
two ends, which rested upon the edges cut in the rock for the purpose at
the base of the Britannia and Anglesey towers respectively, up which the
tube had now to be lifted by hydraulic power to its permanent place near
the summit. The accuracy with which the gigantic beam had been
constructed may be inferred from the fact that, after passing into its
place, a clear space remained between the iron plating and the rock
outside of it of only about three-quarters of an inch!
Mr. Stephenson's anxiety was, of course, very great up to the time of
performing this trying operation. When he had got the first tube floated
at Conway, and saw all safe, he said to Captain Moorsom, "Now I shall go
to bed." But the Britannia Bridge was a still more difficult enterprise,
and cost him many a sleepless night. Afterwards describing his feelings
to his friend Mr. Gooch, he said: "It was a most anxious and harassing
time with me. Often at night I would lie tossing about, seeking sleep in
vain. The tubes filled my head. I went to bed with them and got up with
them. In the grey of the morning, when I looked across the Square, {336}
it seemed an immense distance across to the houses on the opposite side.
It was nearly the same length as the span of my tubular bridge!" When
the first tube had been floated, a friend observed to him, "This great
work has made you ten years older." "I have not slept sound," he
replied, "for three weeks." Sir F. Head, however relates, that when he
revisited the spot on the following morning, he observed, sitting on a
platform overlooking the suspended tube, a gentleman, reclining entirely
by himself, smoking a cigar, and gazing, as if indolently, at the aerial
gallery beneath him. It was the engineer himself, contemplating his new
born child. He had strolled down from the neighbouring village, after
his first sound and refreshing sleep for weeks, to behold in sunshine and
solitude, that which during a weary period of gestation had been either
mysteriously moving in his brain, or, like a vision--sometimes of good
omen, and sometimes of evil--had, by night as well as by day, been
flitting across his mind.
The next process was the lifting of the tube into its place, which was
performed very deliberately and cautiously. It was raised by powerful
hydraulic presses, only a few feet at a time, and carefully under-built,
before being raised to a farther height. When it had been got up by
successive stages of this kind to about 24 feet, an extraordinary
accident occurred, during Mr. Stephenson's absence in London, which he
afterwards described to the author in as nearly as possible the following
words:--"In a work of such novelty and magnitude, you may readily imagine
how anxious I was that every possible contingency should be provided for.
Where one chain or rope was required, I provided two. I was not
satisfied with 'enough:' I must have absolute security, as far as that
was possible. I knew the consequences of failure would be most
disastrous to the Company, and that the wisest economy was to provide for
all contingencies at whatever cost. When the first tube at the Britannia
had been successfully floated between the piers, ready for being raised,
my young engineers were very much elated; and when the hoisting apparatus
had been fixed, they wrote to me saying,--'We are now all ready for
raising her: we could do it in a day, or in two at the most. But my
reply was, 'No: you must only raise the tube inch by inch, and you must
build up under it as you rise. Every inch must be made good. Nothing
must be left to chance or good luck.' And fortunate it was that I
insisted upon this cautious course being pursued; for, one day, while the
hydraulic presses were at work, the bottom of one of them burst clean
away! The crosshead and the chains, weighing more than 50 tons,
descended with a fearful crash upon the press, and the tube itself fell
down upon the packing beneath. Though the fall of the tube was not more
than nine inches, it crushed solid castings, weighing tons, as if they
had been nuts. The tube itself was slightly strained and deflected,
though it still remained sufficiently serviceable. But it was a
tremendous test to which it was put, for a weight of upwards of 5000 tons
falling even a few inches must be admitted to be a very serious matter.
That it stood so well was extraordinary. Clark immediately wrote me an
account of the circumstance, in which he said, 'Thank God, you have been
so obstinate. For if this accident had occurred without a bed for the
end of the tube to fall on, the whole would now have been lying across
the bottom of the Straits.' Five thousand pounds extra expense was
caused by this accident, slight though it might seem. But careful
provision was made against future failure; a new and improved cylinder
was provided: and the work was very soon advancing satisfactorily towards
completion."
When the Queen first visited the Britannia Bridge, on her return from the
North in 1852, Robert Stephenson accompanied Her Majesty and Prince
Albert over the works, explaining the principles on which the bridge had
been built, and the difficulties which had attended its erection. He
conducted the Royal party to near the margin of the sea, and, after
describing to them the incident of the fall of the tube, and the reason
of its preservation, he pointed with pardonable pride to a pile of stones
which the workmen had there raised to commemorate the event. While
nearly all the other marks of the work during its progress had been
obliterated, that cairn had been left standing in commemoration of the
caution and foresight of their chief.
The floating and raising of the remaining tubes need not be described in
detail. The second was floated on the 3rd December, and set in its
permanent place on the 7th January, 1850. The others were floated and
raised in due course. On the 5th March, Mr. Stephenson put the last
rivet in the last tube, and passed through the completed bridge,
accompanied by about a thousand persons, drawn by three locomotives. The
bridge was opened for public traffic on the 18th March. The cost of the
whole work was 234,450 pounds.
[Picture: The Britannia Bridge. (By Percival Skelton)]
The Britannia Bridge is one of the most remarkable monuments of the
enterprise and skill of the present century. Robert Stephenson was the
master spirit of the undertaking. To him belongs the merit of first
seizing the ideal conception of the structure best adapted to meet the
necessities of the case; and of selecting the best men to work out his
idea, himself watching, controlling, and testing every result, by
independent check and counter-check. And finally, he organised and
directed, through his assistants, the vast band of skilled workmen and
labourers who were for so many years occupied in carrying his magnificent
original conception to a successful practical issue. As he himself said
of the work,--"The true and accurate calculation of all the conditions
and elements essential to the safety of the bridge had been a source not
only of mental but of bodily toil; including, as it did, a combination of
abstract thought and well-considered experiment adequate to the magnitude
of the project."
The Britannia Bridge was the result of a vast combination of skill and
industry. But for the perfection of our tools and the ability of our
mechanics to use them to the greatest advantage; but for the matured
powers of the steam-engine; but for the improvements in the iron
manufacture, which enabled blooms to be puddled of sizes before deemed
impracticable, and plates and bars of immense size to be rolled and
forged; but for these, the Britannia Bridge would have been designed in
vain. Thus, it was not the product of the genius of the railway engineer
alone, but of the collective mechanical genius of the English nation.
[Picture: Conway Bridge.--Floating the First Tube]
[Picture: View in Tapton Gardens]