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the life of thomas telford-第65章

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tenacity of wrought iron bars; with the object of employing this

material in his proposed structure。  After he had made upwards of

two hundred tests of malleable iron of various qualities; he

proceeded to prepare his design of a bridge; which consisted of a

central opening of 1000 feet span; and two side openings of 500

feet each; supported by pyramids of masonry placed near the

low…water lines。  The roadway was to be 30 feet wide; divided into

one central footway and two distinct carriageways of 12 feet each。

At the same time he prepared and submitted a model of the central

opening; which satisfactorily stood the various strains which were

applied to it。  This Runcorn design of 1814 was of a very

magnificent character; perhaps superior even to that of the Menai

Suspension Bridge; afterwards erected; but unhappily the means were

not forthcoming to carry it into effect。  The publication of his

plan and report had; however; the effect of directing public

attention to the construction of bridges on the suspension

principle; and many were shortly after designed and erected by

Telford and other engineers in different parts of the kingdom。



Mr。 Telford continued to be consulted by the Commissioners of the

Holyhead Roads as to the completion of the last and most important

link in the line of communication between London and Holyhead;

by bridging the Straits of Menai; and at one of their meetings in

1815; shortly after the publication of his Runcorn design; the

inquiry was made whether a bridge upon the same principle was not

applicable in this particular case。  The engineer was instructed

again to examine the Straits and submit a suitable plan and

estimate; which he proceeded to do in the early part of 1818。

The site selected by him as the most favourable was that which had

been previously fixed upon for the projected cast iron bridge;

namely at Ynys…y…mochthe shores there being bold and rocky;

affording easy access and excellent foundations; while by spanning

the entire channel between the low…water lines; and the roadway

being kept uniformly 100 feet above the highest water at spring tide;

the whole of the navigable waterway would be left entirely

uninterrupted。  The distance between the centres of the supporting

pyramids was proposed to be of the then unprecedented width of 550

feet; and the height of the pyramids 53 feet above the level of the

roadway。  The main chains were to be sixteen in number; with a

deflection of 37 feet; each composed of thirty…six bars of

half…inch…square iron; so placed as to give a square of six on each

side; making the whole chain about four inches in diameter; welded

together for their whole length; secured by bucklings; and braced

round with iron wire; while the ends of these great chains were to

be secured by a mass of masonry built over stone arches between

each end of the supporting piers and the adjoining shore。  Four of

the arches were to be on the Anglesea; and three on the

Caernarvonshire side; each of them of 52 feet 6 inches span。

The roadway was to be divided; as in the Runcorn design with a

carriage way 12 feet wide on each side; and a footpath of 4 feet in

the middle。  Mr。 Telford's plan was supported by Mr。 Rennie and other

engineers of eminence; and the Select Committee of the House of Commons;

being satisfied as to its practicability; recommended Parliament to

pass a Bill and to make a grant of money to enable the work to be

carried into effect。



'Image' Outline of Menai Bridge



The necessary Act passed in the session of 1819; and Mr。 Telford

immediately proceeded to Bangor to make preparations for beginning

the works。  The first proceeding was to blast off the inequalities

of the surface of the rock called Ynys…y…moch; situated on the

western or Holyhead side of the Strait; at that time accessible

only at low water。  The object was to form an even surface upon it

for the foundation of the west main pier。  It used to be at this

point; where the Strait was narrowest; that horned cattle were

driven down; preparatory to swimming them across the channel to the

Caernarvon side; when the tide was weak and at its lowest ebb。  The

cattle were; nevertheless; often carried away; the current being

too strong for the animals to contend against it。



At the same time; a landing…quay was erected on Ynys…y…moch; which

was connected with the shore by an embankment carrying lines of

railway。  Along these; horses drew the sledges laden with stone

required for the work; the material being brought in barges from

the quarries opened at Penmon Point; on the north…eastern extremity

of the Isle of Anglesea; a little to the westward of the northern

opening of the Strait。  When the surface of the rock had been

levelled and the causeway completed; the first stone of the main

pier was laid by Mr。 W。A。 Provis; the resident engineer; on the

10th of August; 1819; but not the slightest ceremony was observed

on the occasion。



Later in the autumn; preparations were made for proceeding with the

foundations of the eastern main pier on the Bangor side of the

Strait。  After excavating the beach to a depth of 7 feet; a solid

mass of rock was reached; which served the purpose of an immoveable

foundation for the pier。  At the same; time workshops were erected;

builders; artisans; and labourers were brought together from

distant quarters; vessels and barges were purchased or built for

the special purpose of the work; a quay was constructed at Penmon

Point for loading the stones for the piers; and all the requisite

preliminary arrangements were made for proceeding with the building

operations in the ensuing spring。



A careful specification of the masonry work was drawn up; and the

contract was let to Messrs。  Stapleton and Hall; but as they did not

proceed satisfactorily; and desired to be released from the contract;

it was relet on the same terms to Mr。 John Wilson; one of Mr。 Telford's

principal contractors for mason work on the Caledonian Canal。

The building operations were begun with great vigour early in 1820。

The three arches on the Caernarvonshire side and the four on the

Anglesea side were first proceeded with。  They are of immense

magnitude; and occupied four years in construction; having been

finished late in the autumn of 1824。  These piers are 65 feet in

height from high…water line to the springing of the arches; the

span of each being 52 feet 6 inches。  The work of the main piers

also made satisfactory progress; and the masonry proceeded so

rapidly that stones could scarcely be got from the quarries in

sufficient quantity to keep the builders at work。  By the end of

June about three hundred men were employed。



The two principal piers; each 153 feet in height; upon which the

main chains of the bridge were to be suspended; were built with

great care and under rigorous inspection。  In these; as indeed in

most of the masonry of the bridge; Mr。 Telford adopted the same

practice which he had employed in his previous bridge structures;

that of leaving large void spaces; commencing above high water mark

and continuing them up perpendicularly nearly to the level of the

roadway。  〃I have elsewhere expressed my conviction;〃 he says; when

referring to the mode of constructing these piers; 〃that one of the

most important improvements which I have been able to introduce

into masonry consists in the preference of cross…walls to rubble;

in the structure of a pier; or any other edifice requiring strength。

Every stone and joint in such walls is open to inspection in the

progress of the work; and even afterwards; if necessary; but a

solid filling of rubble conceals itself; and may be little better

than a heap of rubbish confined by side walls。〃  The walls of these

main piers were built from within as well as from without all the

way up; and the inside was as carefully and closely cemented with

mortar as the external face。  Thus the whole pier was bound firml

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