CHESTER AND HOLYHEAD RAILWAY--MENAI AND CONWAY BRIDGES.
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
Edinburg. 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
Dynallen, as proposed by the Irish Railway Commissioners. His advice
was strongly in favor 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, observed that it clearly appeared that the rival line
through Shrewsbury was quite impracticable. Mr. Stephenson, he added,
was present in the room, ready to answer any questions which might be
put to him on the subject; and "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 railroad bridge across the Straits had not yet been completed.
The Dublin Chamber of Commerce passed resolutions in favor of
Stephenson's line after hearing his explanations 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. A tunnel
of 10-1/2 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 a
mile and a quarter. 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, which formed a deep shingle-bank in front of the
wall.
[Illustration: PENMAEN MAWR. [By Percival Skelton, after his original
Drawing.]]
The part of the work which lies to 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 northwesterly gale which blew in October,
1846, accompanied 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 42 feet long. This
accident farther 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 farther havoc with the
work, entailing heavy expenses and a complete reorganization of the
contract. Increased solidity was then given to the masonry, and the
face of the wall underwent farther 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 labor; 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 would have
been effected. He also said he had arrived at the conclusion that in
railway works engineers should endeavor as far as possible to avoid the
necessity of contending with the sea;[89] 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 endeavoring to
surmount them, than he had done from easily-won successes. While 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 of 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 being there
about 1100 feet.
[Illustration: MAP OF MENAI STRAIT; BRITANNIA BRIDGE]
The engineer's first idea was to construct the bridge of two cast-iron
arches of 350 feet span each. 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 erected on the principle of suspension being considered an
indispensable condition of the proposed structure.
Mr. Stephenson next considered the expediency of erecting a bridge by
means of suspended centering, after the ingenious method proposed by
Telford in 1810,[90] by which the arching was to be carried out by
placing equal and corresponding voussoirs on opposite sides of the pier
at the same time, tying them together by horizontal tie-bolts. The
arching, thus extended outward from each pier and held in equilibrium,
would have been connected at the crown with the extremity of the arch
advanced in like manner from the adjoining pier. It was, however, found
that this method of construction was not applicable at the crossing
of the Conway, and it was eventually abandoned. 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 severe 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 devised, 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.[91] Recurring to his first idea of this bridge, the
engineer 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, after a method adopted by Mr. Rendel in stiffening the
suspension bridge at Montrose with wooden trellis-work a few years
before; 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 to
him, 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 upon 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."[92]
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 steam-ship, 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 cleat 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 tube 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 steam-ships, and
invited me to his works at Millwall to examine the construction of
an iron steam-ship which was then in progress."[93]
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 intrusted 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."[94]
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 of May following, it appears that the idea
which prevailed in his mind was that of a bridge with openings of
450 feet (afterward 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 favorable opinion entertained by Mr. Stephenson
of tubes in that shape, extending them subsequently to those of an
elliptical form.[95] 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 of June, 1845, the directors
of the company, with great liberality, voted a sum for the purpose of
enabling the experiments to be prosecuted, and upward of £6000 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 separate 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
afterward reduced them to the required formulæ, though Mr. Fairbairn
states that they did not appear in time to be of any practical service
in proportioning the parts of the largest tubes.[96]
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, which
he felt were safe in the hands of Mr. Fairbairn--"a gentleman," as he
stated to the Committee of the Commons, "whose experience was greater
than that of any other man in England." 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 sufficient 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.[97] 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 his foot
down upon it--that he should, early in 1846, have appointed his able
assistant, Mr. Edwin Clark, to scrutinize carefully the results of
every experiment, whether made by Mr. Fairbairn or Mr. Hodgkinson,
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. That great progress
had been made by the two chief experimenters before the end of 1846
appears from the papers on the subject read by Messrs. Fairbairn and
Hodgkinson before the British Association at Southampton in September
of that year. In the course of the following month Mr. Stephenson had
become 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.[98] While these important discussions
were in progress, measures were taken to proceed with the masonry
of the bridges simultaneously at Conway and the Menai Strait. The
foundation-stone of the Britannia Bridge was laid by Mr. Frank Forster,
the resident engineer, on the 10th of April, 1846; and on the 12th
of May following that of the Conway Bridge was laid by Mr. A. M.
Ross, resident engineer at that part of the works. 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 neighborhood of the Conway and Britannia Bridges
presented scenes of extraordinary bustle and industry. On the 11th of
July, 1847, Mr. Clark informed Mr. Stephenson that "the masonry gets on
rapidly. The abutments on the Anglesea side resemble the foundations
of a great city rather than of a single structure, and nothing appears
to stand still here." 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, Anglesea 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 to 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 abutments 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 longer spans, at the points
where the tubes repose on the masonry, is not less than 1587 tons.
On the centre tower the tubes lie 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 Church-yard, it would reach nearly 100 feet above the
cross.
[Illustration: CONSTRUCTION OF THE MAIN BRITANNIA TUBE ON THE STAGING.]
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, was 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--in like manner as Rennie had floated into their
places the centerings of his Waterloo and other bridges--and then
raised into their proper places by means of hydraulic power, after a
method originally suggested by Mr. Edwin Clark. The tubes of the Conway
Bridge also were to be constructed on shore, and floated to their
places on pontoons, as in the case of the main centre tubes of the
Britannia Bridge.
[Illustration: CONWAY BRIDGE. [By Percival Skelton.]]
The floating of these 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 places they were permanently to
occupy, was an anxious and exciting operation. The first proceeding of
this nature was at Conway, where Mr. Stephenson directed it in person,
assisted by Captain Claxton, Mr. Brunel, and other engineering friends.
On the 6th of 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 of 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 within
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 of punching the holes for the rivets by Mr. Roberts, of
Manchester. The tube was finally fixed in its permanent bed on the 2d
of January, 1849.
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 of
June, 1849. On the land attachments being cut, the pontoons began to
float off; but one of the capstans having given way from the too great
strain put upon it, 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 overrode
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. Charles 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, arrested 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 corbeling 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."[99]
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 effecting this perilous 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. Afterward 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 gray of the morning, when I looked
across the Square,[100] 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 newborn child. He had strolled
down from the neighboring 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 afterward 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, so 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 cross-head 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 crunched 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 upward 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 toward completion."[101]
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.
[Illustration: MENAI BRIDGE. [By Percival Skelton, after his original
Drawing.]]
The floating and raising of the remaining tubes need not be described
in detail. The second was floated on the 3d of December, and set in
its permanent place on the 7th of January, 1850. The others[102] were
floated and raised in due course; on the 5th of March Mr. Stephenson
put the last rivet in the tube, and passed through the completed
bridge, accompanied by about a thousand persons, drawn by three
locomotives. The bridge was found almost entirely rigid, scarcely
showing the slightest deflection. When, in the course of the day, a
train of 200 tons of coal was allowed to rest with all its weight,
for two hours, in the centre of the eastern land tube, the deflection
was only four tenths of an inch, or less than that produced upon the
structure by half an hour's sunshine;[103] while the whole bridge might
with safety, and without injury to itself, be deflected to the extent
of 13 inches. The bridge was opened for public traffic on the 18th of
March. The cost of the whole work was £234,450.
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 organized and
directed, through his assistants, the vast band of skilled workmen
and laborers who were for so many years occupied in carrying his
magnificent original conception to a successful practical issue.
But it was not accomplished without the greatest anxiety and mental
pressure. Mr. Clark has well observed that few persons who merely
witness the results of the engineer's labors can form any conception
of the real difficulties overcome, and the intense anxiety involved
in their elaboration. "If the stranger," he says, "who contemplates
the finished reality, requires so much thought to appreciate its
principles and comprehend its detail, what weary hours must he have
undergone who first conceived its bold proportions--who, combating,
almost alone, every prejudice that assailed him, and with untiring
labor discussing every objection, listening to every opinion, and
embodying every inquiry, at length matured, step by step, this noble
monument?" On the occasion of raising the last tube into its place, Mr.
Stephenson declared, in reply to the felicitations of a large company
who had witnessed the proceedings with intense interest, that not all
the triumph which attended this great work, and the solution of the
difficult problem of carrying a rigid roadway across an arm of the sea
at such a height as to allow the largest vessels to pass with all their
sails set beneath it, could repay him for the anxieties he had gone
through, the friendships he had compromised, and the unworthy motives
which had been attributed to him; and that, were another work of the
same magnitude offered to him with like consequences, he would not for
worlds undertake it!
The Britannia Bridge was indeed 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.
[Illustration: CONWAY BRIDGE--FLOATING THE FIRST TUBE.]
FOOTNOTES:
[89] The simple fact that in a heavy storm the force of impact of
the waves is from one and a half to two tons per square foot, must
necessarily dictate the greatest possible caution in approaching so
formidable an element. Mr. R. Stevenson (Edinburg) registered a force
of three tons per square foot at Skerryvore during a gale in the
Atlantic, when the waves were supposed to run twenty feet high.
[90] See "Lives of the Engineers," vol. ii., p. 445. It appears that
Mr. Fairbairn suggested this idea in his letter to Mr. Stephenson,
dated the 3d of June, 1845, accompanied by a drawing. See his "Account
of the Construction of the Britannia and Conway Tubular Bridges," etc.
London, 1849.
[91] Robert Stephenson's narrative of the early history of the design,
in Edwin Clark's "Britannia and Conway Tubular Bridges," vol. i., p.
25, London, 1850.
[92] Robert Stephenson's narrative in Clark's "Britannia and Conway
Tubular Bridges," vol. i., p. 27.
[93] Robert Stephenson's narrative in Clark's "Britannia and Conway
Tubular Bridges," vol. i., p. 27.
[94] "Account of the Construction of the Britannia and Conway Tubular
Bridges." By W. Fairbairn, C.E., London, 1849.
[95] Mr. Stephenson continued to hold that the elliptical tube was
the right idea, and that sufficient justice had not been done to it.
A year or two before his death, Mr. Stephenson remarked to the author
that, had the same arrangement for stiffening been adopted to which the
oblong rectangular tubes owe a great part of their strength, a very
different result would have been obtained.
[96] "Mr. Fairbairn's Account," p. 22.
[97] The following passage occurs in Robert Stephenson's report to
the directors of the Chester and Holyhead Railway, dated the 9th of
February, 1846: "You will observe in Mr. Fairbairn's remarks that
he contemplates the feasibility of stripping the tube entirely of
all the chains that may be required in the erection of the bridge;
whereas, on the other hand, Mr. Hodgkinson thinks the chains will be an
essential, or, at all events, a useful auxiliary, to give the tube the
requisite strength and rigidity. This, however, will be determined by
the proposed additional experiments, and does not interfere with the
construction of the masonry, which is designed so as to admit of the
tube, with or without chains. The application of chains as an auxiliary
has occupied much of my attention, and I am satisfied that the ordinary
mode of applying them to suspension bridges is wholly inadmissible in
the present instance; if, therefore, it be hereafter found necessary or
desirable to employ them in conjunction with the tube, another mode of
employing them must be devised, as it is absolutely essential to attach
them in such a manner as to preclude the possibility of the smallest
oscillation."
[98] In a letter of Mr. Fairbairn to Mr. Stephenson, dated July 18th,
1846, he says: "To get rid of the chains will be a desideratum; and I
have made the tube of such strength, and intend putting it together
upon such a principle, as will insure its carrying a dead weight,
equally distributed over its hollow surface, of 4000 tons. With a
bridge of such powers, what have we to fear? and why, in the name of
truth and in the face of conclusive facts, should we hesitate to adopt
measures calculated not only to establish the principle as a triumph of
art, but, what is of infinitely more importance to the shareholders,
the saving of a large sum of money, nearly equal to half the cost
of the bridge? I have been ably assisted by Mr. Clark in all these
contrivances; but in a matter of such importance we must have your
sanction and support."--"Mr. Fairbairn's Account," p. 93.
[99] "The Britannia and Conway Tubular Bridges." By Edwin Clark. Vol.
ii., p. 683-4.
[100] No. 34 Gloucester Square, Hyde Park, where he lived.
[101] The hydraulic presses were of an extraordinary character. The
cylinders of those first constructed were of wrought iron (cast iron
being found altogether useless), not less than 8 inches thick. They
were tested by being subjected to an internal pressure of 3 or 3-1/2
tons to the circular inch. The pressure was such that it squeezed
the fibres of the iron together; so that, after a few tests of this
character, the piston, which at first fitted it quite closely, was
found considerably too small. "A new piston," says Mr. Clark, "was then
made to suit the enlarged cylinder; and a farther enlargement occurring
again and again with subsequent use, the new pistons became as
formidable an obstacle as the cylinders. The wrought-iron cylinder was
on the point of being abandoned, when Mr. Amos (the iron manufacturer),
having carefully gauged the cylinder inside and out, found to
his surprise that, although the internal diameter had increased
considerably, the external diameter had retained precisely its original
dimensions. He consequently persevered in the construction of new
pistons, and ultimately found that the cylinder enlarged no longer,
and to this day it continues in constant use. Layer after layer having
attained additional permanent set, sufficient material was at length
brought into play, with sufficient tenacity to withstand the pressure;
and thus an obstacle, apparently insurmountable, and which threatened
at one time to render much valuable machinery useless, was entirely
overcome. The workman may be excused for calling the stretched cylinder
stronger than the new one, though it is only stronger as regards the
amount of its yielding to a given force."--Clark, vol. i., p. 306. The
hydraulic presses used in raising the tubes of the Britannia Bridge, it
may be remembered, were afterward used in starting the _Great Eastern_
from her berth on the shore at Milwall, where she had been built.
[102] While the preparations were in progress for floating the third
tube, Mr. Stephenson received a pressing invitation to a public railway
celebration at Darlington, in honor of his old friend, Edward Pease.
His reply, dated the 15th of May, 1850, was as follows: "I am prevented
having the pleasure of a visit to Darlington on the 22d, owing to that
or the following day having been fixed upon for floating the next tube
at the Menai Straits; and as this movement depends on the tide, it is,
of course, impossible for me to alter the arrangements. I sincerely
regret this circumstance, for every early association connected with
my profession would have tended to render my visit a gratifying one.
It would, moreover, have given me an opportunity of saying publicly
how much the wonderful progress of railways was dependent upon the
successful issue of the first great experiment, and how much that issue
was influenced by your great discernment, and your confidence in my
late revered father. In my remembrance you stand among the foremost of
his patrons and early advisers; and I know that throughout his life
he regarded you as one of his very best friends. One of the things in
which he took especial delight was in frequently and very graphically
describing his first visit to Darlington, on foot, to confer with you
on the subject of the Stockton and Darlington Railway."
[103] The effect of sunshine in deflecting the bridge is very curious.
When the first main tube was tested, ballast-wagons loaded with iron
were drawn into the centre and left standing there. The first 20 tons
increased the deflection an eighth of an inch, and with 50 tons the
deflection was 9 inches. After standing all night, the deflection in
the morning was found to be only 8-3/8 inches. How was this to be
accounted for? Mr. Clark says: "This was attributed at the time to
an error made in the reading; but this, and many other anomalies in
the deflection, were afterward fully accounted for by local changes
of temperature. _A gleam of sunshine_ on the top of the tube raised
it on one occasion nearly an inch in half an hour with 200 tons at
the centre, the top plates being expanded by increase of temperature,
while the lower plates remained constant from radiation to the water
immediately beneath them. In a similar manner, the tube was drawn
sidewise to the extent of an inch from _the sun shining on one side_,
and returned immediately as clouds passed over the sun, being, in fact,
a most delicate thermometer in constant motion, both vertically and
laterally."
[Illustration: VIEW IN TAPTON GARDENS. [By Percival Skelton.]]