“Yes; I see the mistakes now. This bridge fell because it had too much

weight on its haunches.”

“Haunches! you mean the two side-curves of the arch were too heavy.”

“Ay; you’ve heard the proverb no doubt that ‘An arch never sleeps.’

That is, should too great a weight fall on the crown or top part, the

arch will fall at the sides outwardly, and the crown will sink; while,

curiously enough, if it be built with too little weight on the crown,

as this was, the crown will be forced upwards, and the sides will fall

inwards.”

“Then you mean to build your third bridge with less weight

proportionately on its haunches?”

“Exactly so.”

“Well, I wish you good luck, friend Edwards, for we need a bridge

sorely over the brawling Taff.”

“You shall have it, neighbour. I shall succeed this time. I have

gripped the right principle at last.”

He had indeed, for the bridge he then built lasts to this day. It was

the famous Pontypridd bridge over the Taff on the Llantrissant and

Merthyr road, and was called the Pont y du Prydd, or the bridge by the

earthen house, for a mud hut stood near.

[Illustration:

PONTYPRIDD BRIDGE. _From Encyclopædia Britannica._ ]

About the year 1745 it was determined to build a bridge over the

rushing Taff, and William Edwards, a self-taught mason of the country,

undertook the task. The first bridge he built was of three arches,

which, in less than three years, was dashed away by a great flood. The

water rose so high as to surge over the parapet.

It must have been a sore disappointment to the hard worker to see his

structure suddenly swept to ruins. But he was a shrewd, common-sense,

observing man, and, nothing daunted, he tried again. This time he

determined to build one bold arch of 140 feet. The object was to

obviate the necessity of raising piers for more arches, and so

obstructing the water; these former piers having caused, or assisted

in causing, the destruction of his first bridge.

But the second gave way from the proportionally heavy weights on the

haunches, as Edwards, we imagine, told his friend, and once more he had

to face ruins. Yet a third time he tried, and the third time he was

successful. Generations have come and gone, the children who played

about its abutments have grown grey and have passed away, but still the

country mason’s bridge of 140 feet span stands its ground and serves

the community.

He reduced the heavy weight on the sides by making openings in the

spandrels—that is, the part above the curve of the arch; while, instead

of filling up the interior space with rubble, he used charcoal. But the

arch is very steep, and a chain and drag is kept to assist any horse

when descending.

These bridges illustrate the principle of the arch. Passing by the

fact that it is evidently safer to span a swelling river by a bridge

of wide, rather than of several narrow arches, three powers or forces

act on the row of stones or bricks forming the arch. There is first the

force that would carry the stone downward—that is, the force of its own

weight and of anything that might be placed upon it. But then there are

stones or bricks pressing against it on either side, and in its turn it

presses upon them. When, therefore, every part presses equally, one not

heavier or weaker than the others, a support for all is gained by the

contiguous pressure and by the balance of forces.

Long bridges were sometimes built in this way, and the longest in

England in the Middle Ages was at Burton, over the Trent. It was 1545

feet long, and had 36 arches. It was not superseded till 1864, when a

new bridge was built.

In an arched bridge, the higher it rises in proportion to the width of

the arch, the easier is its construction, and the less is the stress

upon its parts; moreover, any inaccuracy in design or in building is

likely to be less harmful. We are not surprised, therefore, that

Edwards, in his third attempt, decided upon that form.

One of the widest arches in the world is that of the famous Grosvenor

Bridge at Chester. It has a span of 200 feet, with a rise of 42 feet.

An arch, however, in the Washington Aqueduct extends to 220 feet span,

while the central span in the Southwark Bridge, designed by Rennie, is

240 feet. This last, however, is of cast-iron.

The principle of the arch, however, does not appear first in the

history of bridge building. Bridges are as old as mankind; that is, no

one knows when first men began to cross streams and chasms by placing

the trunk of a tree from one side to the other, and thus bridging the

gulf.

Then, possibly, the next step was to build up a pile of stones in the

centre of the stream—taking the stones there by coracle or canoe—and

placing a tree trunk from the side to the central heap.

Yet another development would most likely be a simple cantilever

bridge—though these early builders would not have known that

Frenchified word. But they knew that after embedding a tree trunk

firmly on each side of the bank so that a considerable portion should

project over the stream, they could place a third log from one end to

the other, and thus get a bridge much longer than when made of one tree

trunk alone.

This principle, known so long ago, was used and immensely developed in

the construction of the famous Forth Bridge, one of the most remarkable

structures of the nineteenth century. This cantilever principle is

very important in bridge building, and it is said that there exists an

ancient bridge on this principle across the Sutlej in India with a span

of 200 feet.

[Illustration: THE POST BRIDGE, DARTMOOR.

(_An example of an early bridge, of “slab” construction._)]

A further variety of early bridges was the “slab” bridge, consisting

of slabs of granite placed from side to side, or from the sides of

the bank to heaps of stones piled up in the stream. A good example

of such a bridge may be seen at “Post Bridge” over the Dart on

Dartmoor. Ages ago this bridge was built, and as we study it and

compare it with the modern structure not far distant, we wonder how the

ancient Britons—if those sturdy individuals are really responsible for

it—could raise and place those huge slabs of stone without engineering

apparatus. Probably it was done with levers and rollers, and there must

have been many shoulders to the wheel in the process. Certainly they

had plenty of granite at hand on wild Dartmoor.

But passing by all these early forms of bridges—which it will be

noticed are built of a few large pieces of material—it was left to

the Romans, at all events in Europe, to largely adopt the arch as a

principle of construction.

Now, here we are dealing with an altogether different principle. The

arch is made up of a number of comparatively small pieces of material

bound together by mortar, or cement, or even clamps, and by the power

of gravitation.

We doubt if that idea is realised by half the people using the

multitudinous arches abounding to-day; yet it is true. Or to put it in

another way, the various parts are arranged so that they keep up each

other by pressure.

If you take two cards, or bricks, or slabs of stone and lean them

together at the top, while the other ends may be far apart, you will

find they will bear a certain amount of weight. Here you have the

principle of the arch in its simplest form; and it may be that out of

that primitive performance the arch has grown. This kind of triangular

arch is to be met with in ancient structures in Great Britain. The

flanks or haunches of an arch are its sides, from the first stone to

the keystone; and the crown is its highest part; while the central

wedge-shaped piece of stone or brick is called the keystone.

The stones or bricks are cemented together when being built over a

framework of timber, called the centering, and when the keystone is

placed and the arch is complete it ought to remain firm.

But should too great a weight fall on the crown the bridge will fall

outwardly at the sides, and the crown will sink; while, curiously

enough, if it be built with too little weight on the crown, it will be,

as it were, forced upwards, and the sides will fall inwards, as in the

case of the second of the famous Pontypridd bridges, which actually did

this. The material in the middle of the arch was less in proportion

than that over the sides or “haunches,” and these heavier weights on

the sides caused the crown to be forced upwards.

Two causes combined to make changes in bridge building. These were the

needs of railways and the introduction of iron as a building material.

The first iron bridge was constructed over the Severn, near an

appropriately named place, Ironbridge, in 1779. It had an arch of near

upon a hundred feet span.

When, however, very wide span bridges were required, the question

arose of the superiority of wrought-iron over cast-iron for such

structures. The Menai Strait had to be crossed for the Chester and

Holyhead Railway, and the greatest existing cast-iron span was Rennie’s

Southwark Bridge, where 240 feet had been reached. But over the Conway

and the Menai Strait, spans of 400 feet were involved. How were these

yawning gulfs to be bridged?

CHAPTER II.

A NEW IDEA—THE BRITANNIA TUBULAR.

“We must cross the Strait at the Britannia Rock—that is settled.”

“And where is the Britannia Rock?”

“Nearly in mid-channel. It seems placed there for the purpose.”

And the great engineer smiled.

“What are the distances?”

“From coast to coast the span of the Strait is some 1100 feet, with

that rock in the centre. Now the problem is, to build a bridge across

that gulf of surging water strong enough to bear heavy trains at high

speeds, and sufficiently above the water to prevent any interference

with navigation.”

“And how will you manage it?”

“First I thought of large cast-iron arches, but they will not do. I

doubt if they would stand the strain; and moreover we should impede

navigation by raising scaffolding during the building. At length I came

to the idea of a tube bridge.”

“What! a tube bridge! I’ve never heard of it!”

“No, it is a new idea. By reconsidering a design I had made for a small

bridge over the Lea at Ware in 1841, and thinking over the matter, I

came to the idea that a bridge consisting of a hollow beam or tube

might solve the difficulty.”

“A huge hollow girder, so to speak!” exclaimed his friend.

“Exactly so. Accordingly,” the engineer continued, “I had drawings

prepared and calculations made, by which to ascertain the strength

of such a bridge, and they were so satisfactory that I decided on

attempting one.”

“It is like constructing one huge hollow beam of iron by rivetting

plates together. Can it be done?” remarked his friend.

“The making of the high-level bridge over the Tyne, in which I had

a part—the bridge between Newcastle and Gateshead, you know—was a

transition between an arched bridge and a girder bridge. A girder of

course is a beam, it may be of iron or wood, and the little bridge at

Ware has been built of girders made of plates of wrought-iron rivetted

together. Therefore, you see, I am not unused to wrought-iron girders,

and what they will bear.”

“Why, it is like a huge extension of the primitive log-bridge of our

ancestors.”

“If you like,” replied the engineer, laughing.

Robert Stephenson—for he it is whom we suppose to be speaking to his

friend on this gigantic engineering enterprise—became satisfied by

reflection that the principles involved in constructing an immense

tubular beam were but a development of those commonly in use; and Sir

William Fairbairn was entrusted with the duty of experimenting as to

the strength of tubes, the directors of the Railway Company voting a

sum of money for the purpose.

Sir William, then Mr., Fairbairn concluded that rectangular tubes were

the strongest, and a model was made of the suggested bridge. It proved

successful, and indicated that the tube would be able to stand the

strain of a heavy train passing rapidly over it.

In September, 1846, Mr. Fairbairn read a paper on the subject at

the meeting of the British Association at Southampton, as also did

Professor Hodgkinson, a mathematician, who had verified Fairbairn’s

experiments. Not long afterwards Stephenson became satisfied that

chains were not needed to assist in supporting the bridge, and that his

tubes would be strong enough to support themselves entirely between the

piers.

Work therefore went forward. Some 1500 men were engaged on the

Britannia Bridge, and the quiet shores of the Menai Straits resounded

with the busy hum of hammers and machinery. Cottages of wood were built

for the men, and workshops for the punching and rivetting of the plates

for the gigantic tubes.

The design included two abutments of masonry on either side of the

Strait, and three towers or huge piers, one of which, the centre pier,

was to rise from the Britannia Rock, 230 feet high. There are four

spans, two over the water of 460 feet each, and two of 230 feet each

over the land. Two tubes, quite independent of each other, but lying

side by side, form the bridge across. Each tube or beam is 1510 feet

long, and weighs 4680 tons. Its weight at one of the long spans is

1587 tons.

Now how could these gigantic tubes be put together and raised to their

positions? Here was a problem almost as great as the original one of

the bridge itself, and it troubled the engineer sorely.

[Illustration: ROBERT STEPHENSON.]

“Often at night,” he declared, “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

Gloucester Square, 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.”

The principle adopted was to construct the shorter tubes on scaffolds

in the places which they were to occupy. This could be done, for such

scaffolding would not impede navigation. But scaffolding could not be

built for the large tubes across the great spans of water. What then

was to be done?

It was decided to build them on platforms on the shore quite close

to the water, and float them when ready on pontoons to their places

between the piers, raising them to their position by hydraulic power.

Such a task would be hazardous enough. It was first tried at Conway,

where a similar bridge was being built by Robert Stephenson, being

indeed part of the same railway. The Britannia was, however, a much

greater enterprise, though the span of the Conway is 400 feet. The

Conway bridge, indeed, is but of one span, and contains two tubes.

The experience at Conway was of great benefit to the gigantic

undertaking at the Menai Strait. The floating of the first tube was to

take place on the 19th of June, 1849, in the evening; but owing to some

of the machinery having given way, the great event was put off to the

next night. The shores were crowded with spectators. When the tube was

finished it could be transferred to the pontoons; for the tubes had

been built at high-water mark. When the pontoons were fairly afloat on

this fateful evening, they were held and guided by leading strings of

mighty strength. Stephenson himself directed in person, from a point

of vantage at the roof of the tube. Thence he gave the signals which

had been agreed upon, whilst a crew of sailors, directed by Captain

Claxton, manned the strange barque.

A pontoon is a light, buoyant boat, and the tube was supported on sets

of these, their speed increasing terribly as they approached their

place by the towers. The idea was, as related by Mr. Edwin Clark,

Stephenson’s assistant, that they should strike a “butt” properly,

underneath 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 twelve-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.”

Destruction seemed imminent. The capstan was actually dragged from the

platform, and the tube seemed likely to be swept away. Then Mr. Rolfe,

the captain of the capstan, shouted to the spectators, and threw out a

spare twelve-inch rope. Seizing this, the crowd, with right good-will,

rushed it up the field, and clung tightly to it, checking the voyage of

the mighty tube. It was brought to the “butt,” and duly turned round.

A recess had been left in the masonry of the tower, and the end near

the Britannia pier was drawn into it by means of a chain. The Anglesey

end followed. Then the tide gradually sank, the pontoons sank with it,

and the tube subsided also to a shelf which had been made at either

end. The first stage was accomplished; the mighty tube was in position

to be raised.

Shouts of rejoicing burst from the sympathetic crowds, and the boom of

cannon joined its congratulatory note at the grand success. But the

further stages remained. At midnight the pontoons were all cleared

away, and the huge, hollow beam hung silent over the surging water.

It rested on the shelves or beds prepared for it at either end. The

second great operation, of course, was to haul it up the towers to its

permanent position. This was to be performed by hydraulic machinery of

great power, and Mr. Stephenson’s instructions were to raise it a short

distance at a time, and then build under it.

He took every imaginable precaution against accident or failure; and

well was it that he did so, for an accident happened which, but for the

careful building under the tube in the towers as it was raised, would

have been most calamitous. The accident occurred while Mr. Stephenson

was absent in London. One day, suddenly, while the machinery was at

work raising the tube, the bottom burst from one of the hydraulic

presses, and down fell the tube on to the bed provided for it.

Though the fall was but nine inches, tons weight of metal castings were

crushed, and the mighty tube itself was strained and slightly bent. But

it was serviceable still, and the fact that it stood the strain so well

showed its great strength. It weighed some five thousand tons, and for

such an immense weight to fall even three-quarters of a foot was a very

severe test.

But for Stephenson’s wise precaution in lifting it slowly, and building

underneath it as it was raised, the tube would have crashed to the

bottom of the water. As it was, the accident cost £5000; but the tube

was soon being hauled upward again. In due course the others followed,

and on the 5th of March, 1850, Robert Stephenson inserted the final

rivet in the last tube, and the bridge was complete. He crossed over

with about a thousand persons, three locomotives whirling them along.

The tubes of the bridge are made of iron plates, and at the top and

bottom are a number of small cells or tubes—instead of thick iron

plating—which assist in giving strength to the whole gigantic tube.

Thus it may be said the floor and roof are tubular, as well as the

body. These hollow cells appear to have been Fairbairn’s invention. The

size of the tube grows slightly larger at the middle by the Britannia

tower, where externally the tubes are 30 feet high, and 26 internally,

while they are 22¾ feet and 18¾ feet at the abutments. The width is 14

feet, 8 inches externally, and 13 feet 5 inches inside.

At the Britannia tower the tubes are placed solidly on their bed,

but at the abutments, and at the land towers, the tubes rest on

roller-beds. This arrangement was adopted to permit of expansion and

contraction. Iron, of course, solid and unyielding as it appears, is

yet very susceptible to warmth, and the effect of the sun’s rays on

this massive iron structure is very marked. A rise of temperature

causes it to expand in a comparatively short time, and it is said that

the tubes occasionally move two and a-half inches as the sun gleams

upon them. Mr. Edwin Clark observed the effect of the sun on the iron,

which appears in a small degree to be always moving as the temperature

varies. Well, therefore, that the able engineer planned an arrangement

allowing for this constant expansion and contraction of the iron mass.

[Illustration: THE BRITANNIA TUBULAR BRIDGE.]

The Britannia Bridge was a great triumph for Robert Stephenson. He

appears first to have seized the idea, and, assisted no doubt by

Fairbairn’s experiments and by able coadjutors, he carried it through

to a successful completion. He was of course the son of George

Stephenson, who had done so much for the locomotive, and according to

Smiles, “he almost worshipped his father’s memory, and was ever ready

to attribute to him the chief merit of his own achievements as an

engineer.”

“It was his thorough training,” Mr. Smiles once heard him remark, “his

example, and his character, which made me the man I am.” Further, in an

address as President of the Institution of Civil Engineers, in January,

1856, he said: “All I know, and all I have done is primarily due to the

parent whose memory I cherish and revere.”

That father had died before the Britannia Bridge was completed, though

he had been present at the floating of the first tube at Conway. The

great engineer passed away on the 12th of August, 1848, at the age of

sixty-seven, and his distinguished son Robert, who had no children,

only survived him by eleven years.

But before he died he had designed, and Mr. A. M. Ross, who had

assisted at the Conway Bridge, had assisted in carrying out the

celebrated Victoria Tubular Bridge over the great St. Lawrence River at

Montreal.

This bridge was for the Grand Trunk Railway of Canada, and for immense

length and vastness of proportions, combined with magnificent strength,

is one of the wonders of the world. It is five times as long as the

Britannia Bridge, being not far short of two miles. It has a big

central span of 330 feet, and twenty-four spans of 242 feet. The iron

tubes are suspended sixty feet above the water beneath.

[Illustration: VICTORIA TUBULAR BRIDGE, MONTREAL.]

One great difficulty in the problem was the ice. Immense quantities

come down in the spring, and to resist this enormous pressure the piers

are most massive, containing thousands of tons each of solid masonry.

These piers are based on the solid rock, the two central towers being

eighteen feet in width and the others fifteen feet. To protect them

from the ice, huge guards made of stone blocks clamped with rivets

built up in the form of an incline were placed before the piers on the

up-stream side. The bridge was begun in July, 1854, and occupied four

and a-half years in construction, it being completed in December,

1859, about two months after its designer had died.

Gigantic though this structure is, and great as is the honour which it

reflects on Robert Stephenson and the resident and joint engineer Mr.

Ross, yet with the exception of the remarkable and massive ice-guards

to the piers, it does not differ materially from the Britannia and

Conway Tubular Bridges. These were the first famous examples of the new

principle.

Why, then, are massive tubular bridges not more generally built?

Because they led to another and very natural development in

bridge-building, a development whereby great strength for long spans is

gained, with, however, a marked saving both in labour and in material.

That development was the lattice bridge.

CHAPTER III.

LATTICE AND SUSPENSION BRIDGES.

“The expense of a tubular bridge would be too great.”

“But if we could get the strength without the expense.”

“What mean you?”

“By iron lattice work we could, I think, gain the stiffness and support

needed, without such great cost of labour and material. In other words,

I propose a lattice or trellis work girder, instead of a solid sided,

or a tubular girder.”

“That is, you would have the sides of lattice or trellis work, instead

of solid plates?”

“Exactly. I would use bars of iron placed diagonally. These lattice or

trellis bridges are developed from the tubular bridges, also from the

loose wooden lattice bridges of America. We make a web of iron instead

of a solid sheet. The same kind of structures are largely used over the

wide rivers of India. Sir John MacNeill designed the first in iron, and

it was built in 1843 on the Dublin and Drogheda Railway with a span

of eighty-four feet. I consider they will be among the most popular

bridges of the future for longish spans.”

The engineer’s prediction has come true; for lattice bridges have

undoubtedly been very widely adopted. We may suppose that he was

advising the directors of a proposed railway, and we doubt not but that

he carried the day.

A fine specimen of a lattice bridge is that across the Thames near

Charing Cross, for the South-Eastern Railway. It has a total length

of more than a quarter of a mile—viz., 1365 feet, and six of its nine

spans are 154 feet wide. Two principal girders, fourteen feet deep,

are connected transversely by other girders which carry the rails and

project on the other side to support a footpath. The two main girders

are nearly fifty feet apart and one weighs 190 tons.

The sides have upper and lower booms made of plate iron connected by

perpendicular bars, between which are a couple of bars crossing each

other diagonally at an angle of forty-five degrees, and fixed to the

booms by bolts of five and seven inches in diameter.

The old Hungerford Bridge stood here previously, and its two piers of

brickwork were used for the new bridge. Other piers are huge cylinders

of cast iron ten feet across, but fourteen feet in diameter in the

ground. Thus they are broadly based. These piers are filled with

concrete and also brickwork, and are topped with bearing-blocks of

granite. They are formed of plates of cast iron bolted together, and

they were sunk into the ground many feet below high-water by combined

forces; divers scooped out the mud and gravel and clay from within the

cylinders; water was pumped out and heavy weights pressed them down.

The piers became fixed on the London clay, but when filled were heavily

weighted to drive them down again, and finally they were forced to a

depth of over sixty-two feet below high-water mark.

But before lattice girder bridges had become so popular, another class

had come into use, and afford some splendid specimens of engineering

skill. These are suspension bridges, and, perhaps of all kinds, they

are the most picturesque. Their graceful sweeps and curves yield

perhaps a more pleasing sight for the eye than the solid, rigid,

straight lines of the girder bridges.

It was the genius of Thomas Telford which gave a great impetus to

this class of bridge. Like Stephenson after him, he had to bridge the

surging Menai Straits, but for a carriage road, not a line of rails;

and at length, after various plans had been suggested and abandoned, he

proposed the Suspension Bridge.

Now, in its simplest form, a suspension bridge has been known for ages.

It is merely a pathway, or even a small movable car, suspended from a

rope or ropes across a chasm. Ulloa describes suspension bridges built

by the Peruvians in South America. Four stout cables span a river, and

on these four is placed the platform of sticks and branches, while two

other ropes connected with the platform are useful as hand rails. Such

bridges sway with the wind and move with the passenger, but for light

loads they appear to be perfectly safe.

In Telford’s Menai Bridge the carriage-way is hung from four huge

chains or cables, each chain made up of four others, and passing over

high piers. The chains are anchored on the landward side, sixty feet

in pits, and grafted by iron frames to the rocks. The chains are so

complex and so strong, that parts may be removed for repair without

imperilling the safety of the structure. The length of the span thus

gained is 560 feet, and it is 150 feet above high-water. The remainder

of the bridge is composed of arches of stone, of 52½ feet span.

The piers from which the great span is suspended rise above the

carriage-way fifty-two feet, and are topped by blocks of cast-iron,

which can move on rollers to permit the chains passing over them

to expand and contract freely with the temperature. There are two

carriage-roads, and also a footpath. The roads are separated by iron

lattice work, which also gives them stability and decreases vibration.

[Illustration: THE CLIFTON BRIDGE.]

In its day, this stupendous bridge was as great a wonder as its later

companion over the same Straits—the Britannia Tubular. Six years were

occupied in building, and it was opened in 1825. Why, then, did not

Stephenson construct a similar bridge when, twenty years or so later,

he had to solve a similar problem?

The answer is, that suspension bridges are not—or were not—considered

sufficiently strong and rigid for railway work. In America, however,

they have been used for this purpose; witness the famous Niagara

Suspension Bridge, 2⅓ miles below the Falls, and with a superb span

of 822 feet; but American engineers appear to stiffen the roadway

considerably, so as to distribute the stress of the rushing train over

a large portion of the cable. The Niagara Bridge is not supported by

plate-link chains, but by four immense wire cables, stretching from

cliff to cliff over the roaring rapids. Four thousand distinct wires

make up each cable, which pass over lofty piers, and from them hangs

the railway by numerous rods.

[Illustration: THE BROOKLYN BRIDGE.]

Probably the famous Brooklyn Bridge is the largest suspension bridge in

the world, even as the Clifton Suspension Bridge, in England, is one

of the most interesting. The Brooklyn Bridge has a magnificent central

span of 1595½ feet over the East River between Brooklyn and New York;

further, there are two land spans of 930 feet, which, together with the

approaches, make up a total of about a mile and a furlong. The cables,

four in number, are each composed of 5000 steel wires, and measure

15¾ inches in diameter. They are anchored to solid stone structures

at either end, measuring 119 feet by 132 feet, and weighing 60,000

tons; while the towers from which the main span is suspended rise to

the height of 276 feet, and are embedded in the ground 80 feet below

high-water. It has been estimated that the weight hung between these

towers is nearly 7000 tons.

The roadway of the bridge is divided into five thoroughfares. Those on

the outer sides are for vehicles, and are 19 feet wide; the centre is

for foot passengers, and is 15½ feet in width; while the two others are

for tramway traffic. The bridge was opened in 1883, and affords a great

triumph of engineering skill.

Much smaller, but none the less interesting, is the Suspension Bridge

at Clifton. As far back as 1753, Alderman William Vick, of Bristol,

left a sum of £1000 to build a bridge at Clifton. The sum was to lie

at compound interest until £10,000 was reached. However, the money

was increased by subscriptions, and in 1830 an Act of Parliament was

obtained for its construction.

The work coming into the hands of Mr. I. K. Brunel, he designed a

bridge of 702 feet span, and 250 feet above high-water. The piers and

abutments were built, but lack of cash, which forms an obstacle to so

many brilliant enterprises, stopped the progress of the bridge for

nearly fourteen years.

Then it occurred that the Hungerford Suspension Bridge was to be

removed to make way for the Charing Cross Railway Bridge, so the chains

were purchased at a comparatively small cost, and the work at Clifton

proceeded, and was finally completed.

Three chains on either side suspend long wrought-iron girders, which

help to stiffen the platform; and cross girders between support the

floor. The chains pass over rollers on the piers, and are ultimately

anchored to plates bedded in brickwork abutting on rock. The platform

is hung by upright rods from the chains, and hand-railing is used with

lattice-work, to assist in rendering it rigid. The roadway, twenty

feet wide, is made of creosoted wood, five inches thick, while the

pathways on either side are made with wood half as thick. Between the

piers the weight of the structure, including the chains, amounts to

nearly a thousand tons.

In all these suspension bridges, however large, the principles are much

the same. The platform, or roadway, is hung from chains or cables,

which pass over piers and are anchored fast at the ends. Some are

stiffened with girders and bracing to prevent undue undulation. The

chains take a graceful and definite curve, that of the Menai Bridge

dipping fifty-seven feet. The strain is the greatest at the lower part,

and is increased, should the chain be drawn flatter over the same

space. These bridges became widely adopted.

But there came a time when none of the bridges in vogue seemed to give

what was required. A new principle was wanted. Where was it to be found?

CHAPTER IV.

THE GREATEST BRIDGE IN THE WORLD.

“Have you heard the news? The Tay Bridge is blown down!”

“Yes. A terrible disaster. I should think they would give up their

scheme of bridging the Firth of Forth after that.”

“Not they! The scheme may be altered, but bridge it they will.

Engineers never give in.”

The comments of these newspaper readers were right. The Tay Bridge,

the longest in the world, had been blown down one wild December night

in 1879, and girders, towers, and the train which was rushing over it,

were suddenly hurled into the surging flood.

At that time a scheme was in hand to bridge the Forth for the North

British Railway system, and Sir Thomas Bouch had proposed two

suspension bridges hung by steel chains. But ultimately a new design

altogether was adopted, the plan being by Sir Benjamin Baker and Sir

John Fowler.

It was the new principle—or, rather, a remarkable development of an old

principle—for which the bridge-making world was waiting: the principle,

namely, of the cantilever.

A cantilever is, in fact, a bracket; and Sir Benjamin Baker has

described it as such. It is a strong support, built out from a firm

base, and is like a powerful and magnified bracket upholding a shelf.

In the Forth Bridge there are two huge spans, 1700 feet wide, crossed

by these cantilevers; bridging channels of some 200 feet deep.

The longest spans on the Tay Bridge were 245 feet; it was over two

miles long, and had ninety spans. It was an iron girder bridge, and was

opened on the 31st of May, 1878. Not to be beaten, however, after the

panic had subsided, another and more stable bridge was constructed,

also a girder, but not so high in elevation, and sixty feet further

up the river. It was opened in 1887, and is 10,779 feet long, with 85

piers, the navigable channel being under four of the spans, the centre

spans being 245 feet wide.

It will be seen at once that the cantilevers at the Forth Bridge cover

very much wider spans; and the channel being so deep, the impossibility

of building piers will also be obvious. The best place for the bridge

was marked by the projection of the Inverkeithing peninsula on the

north shore, and also the Inchgarvie rock in the channel itself. The

peninsula brought the two shores together, reducing the space to be

bridged, and the rock gave firm support for a pier. Still there were

the two immense spans of 1700 feet to be crossed, and the engineers

decided on the cantilever principle. Thus, though the Tay Bridge was

the longest in the world, the Forth presented by far the greatest

spans—viz., the two main spans of 1700 feet each, in addition to which

there are two of 675 feet each, and fifteen of 168 feet each.

The total length of this magnificent bridge, which Sir Benjamin Baker

rightly claimed was the most wonderful in the world, is somewhat over

1½ miles in length, or 8296 feet, including the piers, while almost a

mile is bridged by the huge and superb cantilevers. This is, perhaps,

the great marvel. The clear space under the centre is no less than 152

feet at high-water, while the highest portion is 361 feet above the

same mark.

And now, how was this great bridge constructed? Workshops were erected

at South Queensferry, and the mammoth cantilevers were put up there

piece by piece. They were fitted together and then taken plate by plate

to the bridge itself. The shops were lit by electricity, and furnished

with appliances for bending, cutting, moulding, holing, and planing

plates. The workshops were surrounded by quite a maze of railways.

But what of the piers, without which all these preparations would be

unavailing? Now the foundations of piers are usually laid by means of

cofferdams; that is, piles of timber are driven down through the water

into the bed of the river close together, and the interstices filled

with clay; or a casing of iron may be used instead. The water in the

enclosure thus formed can be pumped out and excavation proceeded with,

and the foundations laid. Cofferdams are sometimes made of iron boxes

or caissons with interstices fitted with felt, and caissons of this

kind about 12½ feet long and 7 feet wide were used in constructing the

Victoria Embankment on the Thames.

But with certain of the piers for the Forth Bridge the water was

too deep for timber cofferdams, and the usual diving-bell was not

sufficiently large. The piers were to be of immense size, no less than

55 feet in diameter, and the diving-bell of ordinary size would not

cover that great width.

Huge caissons were therefore made, 70 feet wide, constructed of iron

plates and rising in height, according to the depth of water, up to 150

feet. The lower part of the immense caisson or tank was fitted as a

water-tight division and filled with compressed air, the object being

to resist the pressure of the water. Two shafts communicated with this

air-tight division or mining chamber, one for the removal of the earth

excavated, and the other for the men to pass up and down. The escape

of the air through the shafts was prevented by the use of an air-lock,

working on the same principle as a water-lock on rivers or canals.

There were two doors in the lock, one communicating with the shaft and

the other with the outside air. When the latter was closed and the lock

filled with compressed air by opening a valve or tap, the door of the

shaft could be opened and the man could descend to his work below.

That work consisted chiefly of excavation in the bed of the river.

Drills, hydraulic cutters, and dynamite blasting were all utilised

until huge holes, many feet below the river bed, were hollowed out. As

the caisson was filled with concrete above the air-tight chamber where

the men worked it was exceedingly heavy, and sank by its own weight

into the space prepared.

The mining chamber was lit by electricity, and was about seven feet

high. The mud of the river bed was mixed with water and blown away by

the compressed air which seems to have been about 33 lbs. to the square

inch. The caissons were sunk down to rock or boulder clay, and when

they had reached the required distance the mining chamber was filled

with concrete, and the same material used to the level of the water;

the piers were then built up with huge stones placed in cement, the

whole forming a magnificent mass of concrete and masonry, carried down

in some cases to about 40 feet below the bed of the river.

[Illustration: THE FORTH BRIDGE.]

The three chief piers consist of groups of four columns of masonry,

each gradually tapering from 55 feet in diameter to 49 feet at the top,

and about 36 feet high. From these rise the huge cantilevers connected

together by girders 350 feet in length.

The centre of these three main piers rests on the island of Inchgarvie;

the two others are known as the Fife and the Queensferry piers

respectively, and are placed on the side of the deep water channels. In

addition to these three main piers are several others, some in shallow

water and some on land. The part of the bridge which they carry is

an ordinary girder of steel leading to the immense cantilevers. For

founding the shallow water piers, cofferdams were used; the caissons

with compressed air chambers being for the deep water structures.

They were put together on shore, launched, floated, steered to the

desired position, and sunk. One proved cranky and turned over, and was

only brought right after much expense and difficulty.

The cantilevers are bolted down to each pier by numbers of huge steel

ties, 24 feet in length and 2½ inches in diameter, embedded in the

masonry, there being 48 of these bolts or ties to each column. And now

as to these cantilevers.

Four huge tubular shafts, two on each side, rise from the group of

columns forming each pier, to the height of 350 feet. From these

shafts, which slope slightly inward, project the cantilevers, the upper

and lower parts being strongly braced together by diagonal ties. In

shape the gigantic brackets taper towards a point, the width decreasing

as much as from 120 feet at the commencement of the piers to 32 feet at

the ends. The wind, it is believed, will be more effectually resisted

by this means.

The cantilevers are hung back to back, one to some extent

counter-weighing the other. The component parts consist of cylinders

of steel or struts for resisting compression—these are the lower

parts; and ties of lattice-work made of steel plates for resisting

tension,—placed above.

Thus, then, from each of the three chief piers two pairs of gigantic

brackets project, each pair placed side by side and braced together,

and forming one composite cantilever jutting to the north and one to

the south. The rails run on sleepers placed lengthwise and fixed in

troughs of steel, so that should a train run off the line the wheels

will be caught by these supports.

It is calculated that there are about 45,000 tons of steel in the

bridge, and 120,000 cubic yards of masonry in the piers. The contract

price was £1,600,000, which works out at about £215 per foot; and the

contractors, who were able to obtain an admirable organisation of some

2000 men to carry out the magnificent design, were Messrs. Tancred,

Arrol, & Co. Some special tools for use in the work were planned by Sir

William Arrol. The bridge was opened by the Prince of Wales on the 4th

of March, 1890.

The success of this magnificent structure has assured the wider

adoption of the cantilever principle. Long-span bridges, in several

cases, have since been built on this design. Its engineers may claim

indeed to have widened the scope and possibilities of bridge-building.

Still, when another bridge was wanted over the Thames, at a busy spot,

crowded with shipping and near the historic Tower of London, another

kind of structure was adopted. What was it?

CHAPTER V.

THE TOWER BRIDGE.

“Why should they not have a drawbridge?”

“What! To draw up from each bank of the river?”

“No, I did not mean that exactly. Could they not get piers farther in

towards the centre of the stream, and let the drawbridge rise and fall

from them?”

“The river is too crowded for many piers.”

“It is. But I cannot help thinking a drawbridge—a bascule bridge as the

engineers call it—is the best solution of the difficulty.”

“Well, a bridge is wanted sufficiently low to spring from the flat

banks of the Thames for foot passengers and carriage traffic, and yet

sufficiently high to permit tall ships to pass underneath.”

“And apparently these two requirements are incompatible.”

“Not altogether,” remarks a third speaker.

“You are partly right in your idea of a drawbridge. That is Sir Horace

Jones’s idea. And, further, there is literally to be a high and also

a low-level bridge; for there are to be two levels—that is, two

roadways—one at a high, and one at a low, level across the middle span.”

“And is the low level to be a drawbridge—a roadway that can be drawn up

to permit vessels to pass? Is that so?”

“Exactly. And this drawbridge will be in two parts, one on either side;

they will be worked from two massive piers giving a clear span of 200

feet in the middle of the stream, through which span big vessels can

pass. The usual traffic of the river will be able to pass even when the

drawbridges are down.”

“And above the bascules or drawbridges will run the high-level bridge?”

“Yes, a girder bridge for footpaths, and people will reach it by lifts

and staircases in the piers—which, by-the-by, will be more like huge

towers. These towers will also contain the machinery for raising and

lowering the drawbridges.”

“And what sort of bridge will be used for the other spans—that is, to

cross the river between the piers and the shore?”

“Suspension bridges; so that the Tower Bridge as it will be called,

for it will cross the Thames by the Tower of London, will embody

the suspension, the bascule (or drawbridge), and the girder bridge

principles, while in the centre will be two levels.”

“It promises to be a splendid piece of work.”

“It does. And it is very much needed, for the congestion of traffic on

London Bridge is terrible.”

“And people have often to come round a long way to reach it.”

The promise of the Tower Bridge, as set forth by these speakers, has

been amply fulfilled. It is indeed a fine piece of work; and although

it does not embody any new idea, yet in its combination and development

of old principles and in its size it is very remarkable. It was opened

in June, 1894, and is, or was at the time of building, the biggest

bascule bridge in the world.

Within its handsome Gothic towers are steel columns of immense

strength, constituting the chief supports of the suspension bridges

and of the high-level footways. The architect was the late Sir Horace

Jones, and the engineer Mr. J. Wolfe Barry, while the cost was,

including land, about £1,170,000.

The problem was to combine a low-level bridge providing for ordinary

town traffic with a high level, under which ships could pass, and it

was accomplished by a union of principles. In its oldest shape the

drawbridge was probably a huge piece of timber, which was hauled up

and let down by chains over the moats of castles. In the Tower Bridge

there are two of such huge “flaps” or leaves, each about 100 feet long,

one rising and falling from each pier and meeting in the centre. Large

bascule bridges are usually constructed in this manner, and there is an

excellent specimen over the Ouse, for the passage of the North-Eastern

railway; one man at each half of the bridge can raise it in less than

two minutes. Another fine bascule may be seen at Copenhagen.

The bascules are raised and lowered by chains, which, in the case of

the Tower Bridge, are worked by superb hydraulic power from the massive

pier towers. When drawn up, which is done in less than five minutes,

the bascules are even with the sides of the towers, and full space is

given for the vessels to pass.

The two side spans of the bridge, crossed by the suspension bridges,

are wider than the centre, being 270 feet each, and the total length

of the whole bridge is 800 feet between the abutments. There are also

piers on the shoreward side for carrying the chains of the suspension

bridges at each extremity.

The massive tower piers, sunk 27 feet below the river bed, are built of

gray granite, and are also fitted with strong break-waters to resist

the action of the tide. The high-level bridges across the central

span are for foot passengers, and are 135 feet over high-water mark.

The bascule bridges, when closed for vehicular traffic, are 29½ feet

above high water, while the side suspension spans are 27 feet. The

roadway is 50 feet wide, which is also the width of the approaches. The

foot passenger traffic is never stopped, as persons can pass by the

hydraulic lifts or the stairways in the tower piers to the high-level

bridges above.

Sir Horace Jones died before the great work was completed, and was

succeeded by Mr. G. D. Stevenson, who had been his assistant. Sir

William Arrol & Co. supplied the iron and steel, and Sir William

Armstrong the hydraulic machinery. Various contractors carried out

different portions of the mighty work, which occupied about eight

years in building. Near by stands the ancient Tower of London, looking

not unkindly on the great constructive effort to which it has given its

name.

Sometimes a bridge is made movable by swinging it round on a pivot

instead of drawing it up on a hinge or axis; and sometimes, as in

the case of a bridge over the Arun for the Brighton and South Coast

Railway, it is made to slide on wheels backwards and forwards from

the abutment. Floating or pontoon bridges are made by placing planks

on pontoons, or boats anchored by cables. The longest in the world is

probably at Calcutta, across the Hooghly. It is 1530 feet in length,

there being twenty-eight pontoons in pairs. These are of iron, 160 feet

long, and with ends shaped like wedges; they support a road-way of

3-inch timbers, forty-eight feet wide, and raised on tressel work. An

opening can be made for ships by removing four pontoons and floating

them clear of the passage way.

Great bridges present some of the most remarkable triumphs of the

engineer. They rank beside the express locomotive and the ocean liner

as among the great constructive achievements of mankind. Daring in

design, and bold in execution and in sweep of span, they have been

developed along several principles; and so solidly have they been

built, so sound are the laws of their being, that it seems as though

they will live as long as the everlasting hills.

[Illustration]

[Illustration]

REMARKABLE TUNNELS AND THEIR CONSTRUCTION.

CHAPTER I.

HOW BRUNEL MADE A BORING-SHIELD.

“I watched the worm at work and took my idea from that tiny creature!”

“A worm! Was it an ordinary worm?”

“Oh no, it was the naval wood-worm—_Teredo Navalis_; it can bore its

way through the hardest timber. I was in a dockyard and I saw the

movements of this animal as it cut its way through the wood, and the

idea struck me that I could produce some machine of the kind for

successful tunnelling.”

“Well, it has been brilliantly successful.”

“I looked at the animal closely, and found that it was covered with

a couple of valvular shells in front; these shells seem to act as a

shield, and after many attempts I elaborated the boring-shield which

was used in hollowing out the Thames Tunnel.”

This statement, which we can imagine to have been made by Sir Marc

Isambard Brunel to a friend, is no doubt in substance quite true. A

writer in the “Edinburgh Encyclopædia” says, that Sir M. I. Brunel

informed him, “that the idea upon which his new plan of tunnelling is

founded, was suggested to him by the operations of the _Teredo_, a

testaceous worm, covered with a cylindrical shell, which eats its way

through the hardest wood.”

Two or three attempts had already been made to drive a tunnel under the

Thames, but they had ended in failure. In 1823, Brunel came forward

with another proposal, and he ultimately succeeded.

This illustrious engineer must not be confounded with his son—who was

also a celebrated engineer—Isambard Kingdom Brunel. There were two

Brunels, father and son, even as there were two Stephensons, George and

Robert.

Sir Marc Isambard Brunel, the father, whose most notable enterprise

was the Thames Tunnel, was a French farmer’s son, and after various

experiences in France and America settled in England in 1799, and

married the daughter of William Kingdom of Plymouth. He had already

succeeded as an engineer so well as to be appointed chief engineer of

New York, and a scheme for manufacturing block-pulleys by machinery for

vessels was accepted by the British Government, who paid him £17,000

for the invention. He was also engaged in the construction of Woolwich

Arsenal and Chatham Dockyard, etc., and in 1823 he came forward with

another proposal for the Thames Tunnel.

In that same year, his son, Isambard Kingdom Brunel, entered his

father’s office, and assisted in the construction of the tunnel. The

son subsequently became engineer to the Great Western Railway, and

designed the _Great Western_ steamship.

But though Brunel’s proposal for the tunnel was made public in 1823,

the work was not actually commenced until March, 1825. It was to cross

under the river from Wapping to Rotherhithe, and present two archways.

And if you had been down by the Rotherhithe bank of the Thames about

the latter date, you would have been surprised to see that instead of

hollowing out a shaft, proceedings began by raising a round tower.

A space was traced out, some 50 feet across, and bricklayers began to

build a circular hollow tower about 3 feet thick and 42 feet high.

This tower was strengthened by iron bars, etc., and then the excavation

commenced within. The soil was dug out and raised by an engine at the

top, which also pumped out water. And as the hollow proceeded, the

great shaft or tube of masonry sank gradually into it. Bricklayers

added to its summit until it reached a total height of 65 feet, which

in due course was sunk into the ground.

Thus, then, the engineer had, to commence with, a strong and reliable

brickwork shaft, 3 feet thick, by which men and materials could ascend

and descend in safety. A smaller shaft was also sunk deeper for

drainage.

And now the actual boring of the tunnel commenced. It was to be 38 feet

wide and 22½ feet in height. On New Year’s Day, 1826, the boring-shield

was placed below in the shaft. The shield was composed of 36 cells, 3

cells in height and 12 in breadth, with a workman to each.

The huge “shield” was placed before the earth to be excavated, and

a front board being removed, the soil behind it was dug out to a

specified extent, and the board was propped against the fresh surface

thus made. When the boards had all been placed thus, the cells were

pushed forward into the hollow then made. This was accomplished by

means of screws at the top and bottom of the shield, and which were set

against the completed brickwork behind.

For, while the labourers were working in front, the bricklayers behind

built up the sides and roof, and formed the floor of the tunnel, the

soil at the roof being supported by the shield until the masons had

completed their task.

For nine feet, the tunnel proceeded through clay, but then came an

unwelcome change. Wet, loose sand prevailed, and the work progressed

with peril for thirty-two days, when firmer ground was reached. Six

months passed and substantial headway was made, the tunnel being

completed to the extent of 260 feet.

Then, on the 14th of September, the startling intelligence came that

the engineer feared the river would burst in at the next tide. He had

found a cavity over the shield. Sure enough, at high tide, when the

river was brimming full, the workmen heard the ominous rattle of earth

falling on their shield, while gushes of water followed.

So excellent were the precautions, however, that no disastrous effects

followed, and Father Thames himself rolled earth or clay into the hole

and stopped it up. It was a warning, and emphasised the fear that

haunted the men’s minds all through the hazardous undertaking—the fear

that the river would break through and drown the tunnel.

In October, another small irruption took place, and was successfully

combated. Then, in the following January (1827), some clay fell, but

still no overwhelming catastrophe occurred. The ground grew so moist,

however, that it was examined on the other side. That is, the river

bed was inspected by the agency of a diving-bell, and some ominous

depressions were found. These were promptly filled by bags of clay.

It may be asked, Why had Brunel not gone deeper? Why had he not placed

a greater thickness of earth or clay between his work and the waters of

the Thames?

The answer is this—He had been informed by geologists that quicksand

prevailed lower down, and the shaft that he sank for drainage below

the level of the proposed tunnel, indicated that this view might be

correct. In fact, when he got down 80 feet, the soil gave way, and

water and sand rushed upwards. He was therefore apparently between the

Thames and the quicksand. The Tower Subway, constructed in 1869, and

driven through the solid London clay, is, however, 60 feet deep where

it commences at Tower Hill.

Work went steadily forward at Brunel’s tunnel until the 18th of May.

Mr. Beamish, the assistant engineer, was in the cutting on that day,

and as the tide rose he observed the water increase about the shield;

clay showed itself and gravel appeared. He had the clay closed up, and

went to encourage the pumpers. Suddenly, before he could get into the

cells, a great rush of sludge and water drove the men out of the cells,

extinguished the lights, floated the cement casks and boxes, and poured

forward and ever forward, filling the tunnel with the roaring of the

flood.

The Thames had broken in with a vengeance this time, and drowned the

tunnel.

CHAPTER II.

UNDER THE RIVER.

Happily no one lost his life.

The men retreated before the advancing wave, and as they went they met

Brunel. But the great engineer could do nothing just then, except,

like them, to retreat. The lights yet remaining flashed on the roaring

water, and then suddenly went out in darkness.

The foot of the staircase was reached, and it was found thronged with

the retreating workers. Higher and higher grew the surging flood;

Brunel ordered great speed; and scarcely were the men’s feet off the

lower stair when it was torn away.

On gaining the top, cries were heard; some calling for a rope, others

for a boat. Some one was below in the water! Brunel himself slipped

down an iron rod, another followed, and each fastening a rope to the

body of a man they found in the flood, he was soon drawn out of

danger. On calling the roll, every worker answered to his name. No life

was lost.

So far, good; but what was to be done now? The tunnel was full of

water. To pump it dry was impossible, for the tide poured in from the

Thames.

Again the diving-bell was used, and the hole was found in the bed of

the river. To stop it bags of clay, with hazel sticks, were employed;

and so difficult was the task that three thousand bags were utilised

in the process, and more than a month elapsed before the water was

subdued. Two months more passed before the earth washed in was removed,

and Brunel could examine the work.

He found it for the most part quite sound, though near the shield it

had been shorn of half its thickness of bricks. The chain of the shield

was snapped in twain, and irons belonging to the same apparatus had

been forced into the earth.

The men now proceeded with their task, and exhibited a cool courage

deserving of all praise. Earth and water frequently fell; foul gases

pervaded the stifling air, and sometimes exploded, or catching

fire, they would now and again dance over the water; and again and

again labourers would be carried away insensible from the poisonous

atmosphere. Complaints, such as skin eruptions, sickness, and

headaches, were common. Yet, in spite of every difficulty, the men

worked on in that damp and dripping and fœtid mine, haunted ever with

the dread of another flood.

And it came. On the 12th of August, 1828, some fifteen months after the

previous disaster, the ground bulged out, a large quantity fell, and a

violent rush of water followed; one man being washed out of his cell to

the wooden staging behind.

[Illustration: THE THAMES TUNNEL.]

The flow was so great that Brunel ordered all to retire. The water rose

so fast that when they had retreated a few feet it was up to their

waists, and finally Brunel had to swim to the stairs, and the rush of

water carried him up the shaft. Unhappily, about half-a-dozen lives

were lost at this catastrophe, and those who were rescued—about a dozen

in number—were extricated in an exhausted or fainting state. The roar

of the water in the shaft made a deafening noise; the news soon spread,

and the scene became very distressing as the relatives of the men

arrived.

Once more the hole in the bed of the Thames had to be stopped. Down

went the diving-bell, but it had to descend twice before the gap was

discovered. It was a hole some seven feet long, and four thousand tons

of earth, chiefly bags of clay, were used in filling it. Again the

tunnel was entered, and again the intrepid engineer found the work

sound.

But, alas, another difficulty had presented itself—one more difficult

to conquer even than stopping up huge holes in the bed of the Thames.

The tunnel was being cut by a Company, and its money had gone; nay,

more, its confidence had well nigh gone also. Work could not proceed

without money, and for seven years silence and desolation reigned in

those unfinished halls beneath the river.

Then the Government agreed to advance money, and work was again

commenced. But it proceeded very slowly, some weeks less than a foot

being cut, during others again three feet nine inches. The ground was

in fact a fluid mud, and the bed of the river had to be artificially

formed before the excavation could proceed in comparative safety.

Further, the tunnel was far deeper than any other work in the

neighbourhood, and all the water drained there—a difficulty which was

obviated by the construction of a shaft on the other side of the river.

The shield had also to be replaced. It had been so battered about by

the flood that another was necessary. As it kept up the earth above,

and also in front, the change was both arduous and perilous. But it was

accomplished without loss of life.

Three more irruptions of water occurred: the third in August, 1837,

the fourth in November, 1837, and the fifth in March, 1838. But the

engineer was more prepared for Father Thames’ unpleasant visits, and a

platform had been constructed by which the men could escape. Unhappily,

one life was lost, however, on the fourth occasion. A great rush of

soil also occurred in April, 1840, accompanied by a sinking of the

shore at Wapping over some seven hundred feet of surface. Happily this

occurred at low tide, and the chasm was filled with gravel and bags of

clay before the river rose high.

At length, on the 13th of August, 1841, Brunel descended the shaft at

Wapping, and entering a small cutting, passed through the shield in

the tunnel, amidst the cheers of the workmen. After all these years

of arduous toil, of anxious solicitude, and of hair-breadth escapes,

the end was near, and a passage under the Thames was cut. It was not

completed and open to the public, however, until the 25th of March,

1843, and then for foot passengers only.

The approaches for carriages remained to be constructed, and would have

been expensive works. They were to be immense circular roads, but they

were never made. Perhaps that deficiency contributed to the commercial

failure of the great engineering enterprise. In any case, the tunnel

never paid; the Company dissolved; and the tunnel passed over to the

East London Railway, who run trains through it. Its length is 1300

feet, while between it and the river there is a thickness of soil of

some fifteen feet.

Though a failure as a business, yet the tunnel was a great engineering

triumph. It was a marvel of perseverance, and of determined, arduous,

skilful toil against overwhelming difficulties. Eighteen years

passed before it was completed; and if the seven be deducted during

which the work was stopped, still eleven remain as the period of its

construction. Work occupying such a length of time must be costly.

Could it be shortened? Would tunnel-making machinery be developed and

improved so as to expedite the labour of years?

CHAPTER III.

THROUGH THE ALPS.

“Cut through the Alps? It is an impossibility; and it would never pay!”

“Yet they are about to do it. Sommeiller, an engineer, has invented, or

obtained, a rock-boring machine which promises to lighten the labour

considerably; and then, of course, they will shatter great quantities

of earth by explosives.”

“And what part of the Alps?”

“Through Mont Cenis. The tunnel will be about 7½ miles long, and the

mountain over it will rise 5400 feet at one point.”

“And when do they expect to finish it?”

“I cannot say. They will begin on the southern—that is, the

Italian—side first, and later on the French side. Through the tunnel

will pass one of the principal routes from the West to the East.”

This conversation, we may suppose, took place in 1857, the year when

the tunnel was commenced. For four years hand work was used, though

blasting was in operation from the first; but in 1861 drilling by

machinery was brought into play, and the rate of progress became much

greater.

The machine was the first practical boring apparatus for rock, and

was used first in making the Mont Cenis Tunnel. With explosives, as

gun-cotton, dynamite, etc., the time occupied in cutting tunnels has

been much reduced. Thus the Mont Cenis Tunnel occupied about thirteen

years, and cost three millions of pounds. The St. Gotthard—another

Alpine subway—occupied eight years, though it is 9¼ miles in length;

and the Arlberg—yet another Alpine tunnel—a little over 6 miles long,

occupied something more than three years.

Further, the railway of which the St. Gotthard Tunnel forms part, has

been commercially very successful. This tunnel was commenced in 1872

and completed in 1880, the same year that saw the beginning of the

Arlberg.

Tunnels through hard rock do not always need a lining of brickwork; but

if the soil be clay, or loose earth of any kind, the lining of brick

or stone must be brought up close to the scene of actual excavation.

The Mont Cenis is lined with stone or brick almost entirely, about 900

feet, however, being without such lining.

And now, how was the actual work of tunnelling carried on? It will be

seen at once that the problem was quite different from that of boring

fifteen feet under the Thames, and sometimes through watery mud. In

boring through mountains the quickest way of cutting and carting away

rock is one of the chief points to be considered. At the Mont Cenis

Tunnel the blasting took place by driving a series of shot holes into

the soil, all over the surface to be cut, filling them with explosives,

and firing them simultaneously in rings. Such explosives may be fired

by a time-fuse or by electricity, giving the workmen ample time to

escape out of reach. The shaken and shattered soil can then be cleared

away.

The blast holes in this small-shot system are about 1 to 1½ inch in

diameter, and from 1½ to 7 or 9 feet in the rock. The explosive is

forced to the end of each, and the hole is then tamped—that is, closed

with clay or sand—and fired in due time.

[Illustration: BORING MACHINE USED FOR THE MONT CENIS TUNNEL.]

The cutters for boring in rock are often diamond drills, the cutting

edges being furnished with a kind of diamond found in Brazil, of a

black colour and of great hardness. These are placed round the edge

of a cylinder of steel, to which iron pipes can be screwed as the

edge cuts its way deeper in the rock. The stuff cut out as the drill

revolves finds its way through the cylinder and the piping. There are,

however, a great number of boring machines of different kinds, hard

steel sometimes taking the place of the opaque diamonds for cutting

purposes. The compressed air with which many of the machines are

worked assisted in the St. Gotthard in the ventilation of the tunnel,

frequently a great consideration, as the space is so small and the gas

from explosions often so great.

The Mont Cenis Tunnel marks a transition period in tunnelling. During

the four years that hand labour was used, the average rate of progress

was but nine inches a-day on either side; but when the rock-drills

worked by compressed air were introduced, the speed was five times as

great. Still further, at the Arlberg Tunnel through the Tyrolese Alps

the average rate of progress was 9·07 yards per day, and the cost £108

per lineal yard; while the cost of the Mont Cenis was £226 per lineal

yard. These figures show immense progress in economy and in speed.

The St. Gotthard Tunnel was begun in 1872, and the machine drills were

used throughout. A heading was first cut about eight feet square, and

the hollow thus gained was afterwards enlarged and finally sunk to the

desired level. Several Ferroux drills were used, placed on a carriage,

and an average charge of 1¾ lbs. of dynamite placed in the holes made.

After firing, the compressed air was discharged and the shattered soil

was cleared away.

In the Arlberg Tunnel a chief heading was driven, and then shafts

opened up enabling smaller headings to be driven on both hands. Drills

worked by hydraulic power were used, as well as drills worked by air,

and, after the explosions, water spray was thrown out to assist in

clearing and purifying the air. Ventilators also were used, which

injected air at the rate of more than 8000 cubic feet per minute.

Speedy transit of the earth excavated and the materials for masonry

were also effected, it being estimated that some 900 tons of earth had

to be taken out of each end, and about 350 tons of masonry had to be

brought in, every day.

Tunnels through huge thicknesses of rock or under rivers can only

be cut from the two opposite ends. Where possible, however, other

shafts have been sunk along the line the subway was to take, and thus

excavation might continue at several places along the line of route,

the shafts being used for ventilation and for the conveyance of the

excavated soil.

But the use of machine drills and of blasting explosives, with improved

appliances for ventilation, have, with possibly some rare exceptions,

rendered these methods obsolete. According to Pliny the tunnel for

draining Lake Fucino was the greatest work of his day. It was over

3½ miles long, and cut under Monte Salviano. Forty shafts were sunk

in cutting it, also sloping galleries, and huge copper buckets were

used to carry away the earth. It is stated that this tunnel—some ten

feet high, by six wide—occupied 30,000 men eleven years. Compare this

with the Arlberg, or even the Gotthard, double and treble the length,

occupying much less time. Sir Benjamin Baker has calculated that the

Fucino tunnel could now be cut in eleven months.

Gunpowder gave some advance on old Roman methods of tunnelling. The

improved explosives and rock-drills have gone further.

Even as the Mont Cenis shows a transition period, so the Arlberg may

be said to emphasise a triumph of the methods then indicated. So great

have been the improvements of the rock-boring machinery, of the power

of the blasts, and the speedy ventilation following the explosions, and

of the quick transit of materials, that we shall most likely hear no

more of sinking numerous shafts along the route.

But what of subaqueous tunnels? Violent explosives are hardly suitable

for excavation a few feet under a turbid river. What is to be done,

when cutting under a full and treacherous stream?

CHAPTER IV.

UNDER WATER AGAIN.

“How to cross the Thames at Blackwall, far east of the Tower Bridge?”

That was a problem which the citizens of London had to face in the

latter part of the nineteenth century.

An immense population dwelt on either side, and some means of easy

communication became a pressing necessity. Should it be effected by

means of a bridge, fixed or floating, or by means of a tunnel?

Finally a tunnel was decided upon, with sloping approaches on either

side. Its entire length was to be 6200 feet including the approaches;

but herein lay the danger and the difficulty—it was to be driven only

seven feet below the bed of the river, and through loose soil and

gravel.

How then was this perilous task to be accomplished? If the great river

burst through Brunel’s fifteen feet, would it not be much more likely

to rush through this seven feet of loose soil?

But the engineers in charge had an appliance in hand, which was unknown

to Brunel—viz., a compressed air chamber, a piece of apparatus which

has facilitated several great engineering achievements, besides the

Blackwall Tunnel.

When the excavation of the tunnel was commenced, a stout apartment

was formed at the end of the cutting, into which air was pumped until

it exerted a pressure of some thirty-five pounds to a square inch, in

addition to its usual weight.

This is generally reckoned at an average of 14·7 pounds to a square

inch. We are so used to this pressure that we do not feel it; but let

us enter a room where the air has been much more compressed, as in

this air-chamber, and serious consequences would be likely to ensue,

especially at first.

The human body, however, has a wonderful power of adaptability, and

after a time some men get used to the change and can work in the

compressed air without injury. But at first it may cause bleeding from

the nose and ears, sometimes indeed affecting the hearing more or less

seriously, and also causing great pain.

The reason for using this compressed air chamber was to keep out Father

Thames. The great pressure of the air resisted the great pressure of

the water, and held up the seven feet of soil between.

Powerful engines were maintained at work to provide for the pressure

of the air, and the chamber in which the compressed air was kept was

entered and left by the workmen through an “air-lock”—that is, a small

ante-chamber having two doors, one leading to the compressed air and

the other to the ordinary atmosphere, and neither being opened at the

same time.

The men, then, worked in this compressed air chamber, which prevented

irruptions of the river. But the method of excavation was also another

safeguard, both against irruptions of water and of earth.

In essence, it was much the same as that pursued in boring the tunnel

for the South London Electric Railway; that, however, was through thick

clay and about 10½ feet in diameter, and this was 27 feet across, and

through loose and stony stuff. The shield, instead of containing as in

Brunel’s time a number of cells, consisted of an immense iron cylinder,

weighing some 250 tons; closed in front, but having a door in the

closed part; the rim of the cylinder round this part having a sharp

edge for cutting into the soil.

[Illustration: THE ENTRANCE TO THE AIR-LOCK.

(_Men waiting to enter the Compressed Air-Chamber through the Door._)]

The door being opened, the men found themselves face to face with the

earth to be excavated. They cut away as well as they could, perhaps

about 2½ feet deep, throwing the earth into trucks in the compressed

air chamber; these trucks would be afterwards hauled away through the

air-lock by electricity, and the huge iron cylinder would be pushed

forward by means of hydraulic power. Twenty-eight hydraulic “jacks”

were employed, and they forced forward the 250 ton cylinder with its

cutting edge, when the men would resume working through the door as

before.

Behind them, the hole of the tunnel thus cut out was being lined.

First, it was built round with iron plates a couple of inches thick.

This plating was fixed in segments, and formed a huge pipe a little

smaller than the actual hollow in the earth. Through holes in the

immense piping, liquid cement was forced, thus plugging up the space

entirely between the earth and the iron, and forming an outer ring of

cement.

Within, the tunnel was completed by a facing of glazed tiles, placed on

a thickness of 14 inches of concrete. A road-way was laid 16 feet wide,

flanked by footpaths of 3 feet, 2 inches, on either side. The subway is

lighted by electricity, and staircases on the banks lead down to it for

foot passengers. The stairways give entrance to the tunnel not far from

the river, and much nearer than the commencement of the carriage-way

approaches.

At the northern side, the slope down commences near the East India

Dock entrance, and turns out of the East India Dock Road. The slope

is fairly gradual—about one in thirty-four—and it passes under the

Blackwall line of the Great Eastern Railway, and near to Poplar

Station. The part of the tunnel near to this point—that is the part

between the river and the open slope—was executed by what is called

“cut and cover” work—that is, a huge trench was dug, then arched in and

covered over.

“Cut and cover” work also took place on the south side; and there, at

the foot of an immense excavation ninety feet down, and with its sides

held up by huge timbers, might have been seen a river of water which

had drained in and was being pumped up quickly by powerful machinery.

Not far distant, the shaft was being sunk for the staircase. In

principle, the sinking of the shaft was conducted much as Brunel’s

shaft at the Thames Tunnel, only it was built up of iron instead of

brick. Imagine a big gasometer with a scaffold near the top, where men

are busy building the walls higher and higher by adding on plate after

plate of iron. On reaching the scaffold you find that there are two

great cylinders of iron, one standing inside the other, and concrete

is being filled in between them. Men also are down below digging out

the earth which is being swung up in iron buckets; and as the soil is

gradually removed, the immense double iron and concrete cylinder slowly

sinks by its own weight.

In this manner, the great shaft was sunk nearly ninety feet, and within

it the staircase has been built, giving entrance for foot passengers,

not far from the river. Thus, on either side are sloping entrances to

the tunnel, and also, nearer the water, stairways of descent down great

shafts.

Engineers have also found their way beneath other great English

rivers—the Severn and the Mersey. Much water had to be dealt with

in the cutting of the Severn Tunnel. This important work, four and

one-third miles long, was driven in some places forty-five feet under

sandstone, and at the Salmon Pool—a hollow in the river bed—the tunnel

was thirty feet under soil called trias marl. Much greater space,

therefore, exists here between the tunnel and river than at Blackwall.

But the river burst through. The work was begun in 1873, and completed

in 1886.

Six years after its commencement the tunnel was drowned, so to speak,

for a long time by a large spring of water which burst out from

limestone, and arrangements had to be made to provide for this flood.

It is now conducted by a subsidiary tunnel or channel to a huge shaft,

where it is raised by pumps of sufficient strength. Then there was the

perilous Salmon Pool to be dealt with. The river burst through here,

and the rent had to be stopped with clay. The tunnel is twenty-six

feet wide by twenty feet high, and is cut through Pennant stone,

shale, and marl. It is lined with Staffordshire vitrified bricks

throughout—seventy-five million bricks it is estimated being used. The

works are ventilated by a huge fan, and pumping continually proceeds,

something like twenty-six million gallons of water, it is said, being

raised in the twenty-four hours. The tunnel, of which the engineers

were Messrs. Hawkshaw, Son, Hayter & Richardson, and Mr. T. A. Walker,

Contractor, is for the use of the Great Western Railway, and saves

that Company’s Welsh and Irish trains to Milford a long way round by

Gloucester.

[Illustration: THE BORING MACHINE USED IN THE PRELIMINARY CONSTRUCTION

OF THE ENGLISH CHANNEL TUNNEL.]

In cutting the Mersey Tunnel, which was completed in 1886, machinery

was used for some of the work. The machine bored partly to a diameter

of seven feet four inches, but hand labour had to be largely depended

upon. The plan pursued was to sink a shaft on either side of the

river and drive a heading, sloping upward through the sandstone to

the centre; this heading acting as a drain for any water which might

appear. The thickness between the arch of the tunnel and the river bed

is thirty feet at its least, and the tunnel, which occupied about six

years in construction, and of which the engineers were Messrs. Brunlees

& Fox, is provided with pumps raising some thirteen million gallons

of water daily. As in the case of the Severn Tunnel, ventilation is

provided for by huge fans.

A boring machine was also used in the preliminary efforts for the

construction of a tunnel under the English Channel. Holes, seven

feet across and to the length of 2000 yards, have been bored by a

compressed air machine, working with two arms furnished with teeth of

steel. The construction of the tunnel is held to be quite feasible from

an engineering point of view, and it is believed that it would pass

through strata impervious to water, such as chalk marl and grey chalk.

Still, the huge tunnel at Blackwall, which was carried out by Mr.

Binnie, Chief Engineer of the London County Council, with Mr. Greathead

and Sir Benjamin Baker as Consulting Engineers, is probably one of the

most daring and stupendous enterprises of the kind ever undertaken. To

hollow out a subway hundreds of feet long under the Thames, only seven

feet from the bed of the great river, and through loose gravelly soil,

was a great triumph. It was achieved not by uncalculating bravery, but

by a wise combination of cool courage, superb skill, and admirable

foresight.

To design effectively, to provide for contingencies, to be daunted

by no difficulties—these qualities help to produce the Triumphs of

Engineers, as well as do great inventive skill, the power of adapting

principles to varying circumstances, and high-spirited enterprise in

planning and conducting noble and useful works. These works may well

rank among the great achievements of man’s effort and the wonders of

the world.

THE END.

LORIMER AND GILLIES, PRINTERS, EDINBURGH.