the life of thomas telford-第58节

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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　firmly

together；　and　the　utmost　strength　given；　while　the　weight　of　the

superstructure　upon　the　lower　parts　of　the　work　was　reduced　to　its

minimum。



＇Image＇　Section　of　Main　Pier



Over　the　main　piers；　the　small　arches　intended　for　the　roadways

were　constructed；　each　being　15　feet　to　the　springing　of　the　arch；

and　9　feet　wide。　　Upon　these　arches　the　masonry　was　carried

upwards；　in　a　tapering　form；　to　a　height　of　53　feet　above　the

level　of　the　road。　　As　these　piers　were　to　carry　the　immense　weight

of　the　suspension　chains；　great　pains　were　taken　with　their

construction；　and　all　the　stones；　from　top　to　bottom；　were　firmly

bound　together　with　iron　dowels　to　prevent　the　possibility　of　their

being　separated　or　bulged　by　the　immense　pressure　they　had　to

withstand。



The　most　important　point　in　the　execution　of　the　details　of　the

bridge；　where　the　engineer　had　no　past　experience　to　guide　him；　was

in　the　designing　and　fixing　of　the　wrought　iron　work。　　Mr。　Telford

had　continued　his　experiments　as　to　the　tenacity　of　bar　iron；　until

he　had　obtained　several　hundred　distinct　tests；　and　at　length；

after　the　most　mature　delilberation；　the　patterns　and　dimensions

were　finally　arranged　by　him；　and　the　contract　for　the　manufacture

of　the　whole　was　let　to　Mr。　Hazeldean；　of　Shrewsbury；　in　the　year

1820。　　The　iron　was　to　be　of　the　best　Shropshire；　drawn　at　Upton

forge；　and　finished　and　proved　at　the　works；　under　the　inspection

of　a　person　appointed　by　the　engineer。



＇Image＇　Cut　showing　fixing　of　the　chains　in　the　rock



The　mode　by　which　the　land　ends　of　these　enormous　suspension　chains

were　rooted　to　the　solid　ground　on　either　side　of　the　Strait；　was

remarkably　ingenious　and　effective。　　Three　oblique　tunnels　were　made

by　blasting　the　rock　on　the　Anglesea　side；　they　were　each　about　six

feet　in　diameter；　the　excavations　being　carried　down　an　inclined

plane　to　the　depth　of　about　twenty　yards。　　A　considerable　width　of

rock　lay　between　each　tunnel；　but　at　the　bottom　they　were　all