the life of thomas telford-第47章

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and　disposition　of　the　materials　that　are　put　upon　it；　are　matters

quite　new　in　the　system　of　road…making　in　these　countries。〃

'Report　from　the　Select　Committee　on　the　Road　from　London　to

Holyhead　in　the　year　1819。'



*＇7＇　Evidence　of　William　Waterhouse　before　the　Select　Committee；

10th　March；　1819。





CHAPTER　XII。



THE　MENAI　AND　CONWAY　BRIDGES。



＇Image＇　Map　of　Menai　Strait　＇Ordnance　Survey＇



So　long　as　the　dangerous　Straits　of　Menai　had　to　be　crossed　in　an

open　ferry…boat；　the　communication　between　London　and　Holyhead　was

necessarily　considered　incomplete。　　While　the　roads　through　North

Wales　were　so　dangerous　as　to　deter　travellers　between　England　and

Ireland　from　using　that　route；　the　completion　of　the　remaining　link

of　communication　across　the　Straits　was　of　comparatively　little

importance。　　But　when　those　roads　had；　by　the　application　of　much

capital；　skill；　and　labour；　been　rendered　so　safe　and　convenient

that　the　mail　and　stage　coaches　could　run　over　them　at　the　rate　of

from　eight　to　ten　miles　an　hour；　the　bridging　of　the　Straits　became

a　measure　of　urgent　public　necessity。　　The　increased　traffic　by　this

route　so　much　increased　the　quantity　of　passengers　and　luggage；

that　the　open　boats　were　often　dangerously　overloaded；　and　serious

accidents；　attended　with　loss　of　life　and　property；　came　to　be　of

frequent　occurrence。



The　erection　of　a　bridge　over　the　Straits　had　long　been　matter　of

speculation　amongst　engineers。　　As　early　as　1776；　Mr。　Golborne

proposed　his　plan　of　an　embankment　with　a　bridge　in　the　middle　of　it；

and　a　few　years　later；　in　1785；　Mr。　Nichols　proposed　a　wooden

viaduct；　furnished　with　drawbridges　at　Cadnant　Island。　　Later　still；

Mr。　Rennie　proposed　his　design　of　a　cast　iron　bridge。　　But　none　of

these　plans　were　carried　out；　and　the　whole　subject　remained　in

abeyance　until　the　year　1810；　when　a　commission　was　appointed　to

inquire　and　report　as　to　the　state　of　the　roads　between　Shrewsbury；

Chester；　and　Holyhead。　　The　result　was；　that　Mr。　Telford　was　called

upon　to　report　as　to　the　most　effectual　method　of　bridging　the

Menai　Strait；　and　thus　completing　the　communication　with　the　port

of　embarkation　for　Ireland。



＇Image＇　Telford's　proposed　Cast　Iron　Bridge



Mr。　Telford　submitted　alternative　plans　for　a　bridge　over　the

Strait：　one　at　the　Swilly　Rock；　consisting　of　three　cast　iron

arches　of　260　feet　span；　with　a　stone　arch　of　100　feet　span　between

each　two　iron　ones；　to　resist　their　lateral　thrust；　and　another　at

Ynys…y…moch；　to　which　he　himself　attached　the　preference；

consisting　of　a　single　cast　iron　arch　of　500　feet　span；　the　crown

of　the　arch　to　be　100　feet　above　high　water　of　spring　tides；　and

the　breadth　of　the　roadway　to　be　40　feet。



The　principal　objection　taken　to　this　plan　by　engineers　generally；

was　the　supposed　difficulty　of　erecting　a　proper　centering　to

support　the　arch　during　construction；　and　the　mode　by　which

Mr。　Telford　proposed　to　overcome　this　may　be　cited　in　illustration

of　his　ready　ingenuity　in　overcoming　difficulties。　　He　proposed　to

suspend　the　centering　from　above　instead　of　supporting　it　from

below　in　the　usual　mannera　contrivance　afterwards　revived　by

another　very　skilful　engineer；　the　late　Mr。　Brunel。　　Frames；　50　feet

high；　were　to　be　erected　on　the　top　of　the　abutments；　and　on　these；

strong　blocks；　or　rollers　and　chains；　were　to　be　fixed；　by　means　of

which；　and　by　the　aid　of　windlasses　and　other　mechanical　powers；

each　separate　piece　of　centering　was　to　be　raised　into；　and

suspended　in；　its　proper　place。　　Mr。　Telford　regarded　this　method　of

constructing　centres　as　applicable　to　stone　as　well　as　to　iron

arches；　and　indeed　it　is　applicable；　as　Mr。　Brunel　held；　to　the

building　of　the　arch　itself。*＇1＇



＇Image＇　Proposed　Plan　of　Suspended　Centering



Mr。　Telford　anticipated　that；　if　the　method　recommended　by　him　were

successfully　adopted　on　the　large　scale　proposed　at　Menai；　all

difficulties　with　regard　to　carrying　bridges　over　deep　ravines

would　be　done　away　with；　and　a　new　era　in　bridge…building　begun。

For　this　and　other　reasonsbut　chiefly　because　of　the　much　greater

durability　of　a　cast　iron　bridge　compared　with　the　suspension

bridge　afterwards　adoptedit　is　matter　of　regret　that　he　was　not

permitted　to　carry　out　this　novel　and　grand　design。　　It　was；

however；　again　objected　by　mariners　that　the　bridge　would　seriously

affect；　if　not　destroy；　the　navigation　of　the　Strait；　and　this

plan；　like　Mr。　Rennie's；　was　eventually　rejected。



Several　years　passed；　and　during　the　interval　Mr。　Telford　was

consulted　as　to　the　construction　of　a　bridge　over　Runcorn　Gap　on

the　Mersey；　above　Liverpool。　　As　the　river　was　there　about　1200　feet

wide；　and　much　used　for　purposes　of　navigation；　a　bridge　of　the

ordinary　construction　was　found　inapplicable。　　But　as　he　was

required　to　furnish　a　plan　of　the　most　suitable　structure；　he

proceeded　to　consider　how　the　difficulties　of　the　case　were　to　be　met。

The　only　practicable　plan；　he　thought；　was　a　bridge　constructed　on

the　principle　of　suspension。　　Expedients　of　this　kind　had　long　been

employed　in　India　and　America；　where　wide　rivers　were　crossed　by

means　of　bridges　formed　of　ropes　and　chains；　and　even　in　this

country　a　suspension　bridge；　though　of　a　very　rude　kind；　had　long

been　in　use　near　Middleton　on　the　Tees；　where；　by　means　of　two

common　chains　stretched　across　the　river；　upon　which　a　footway　of

boards　was　laid；　the　colliers　were　enabled　to　pass　from　their

cottages　to　the　colliery　on　the　opposite　bank。



Captain　（afterwards　Sir　Samuel）　Brown　took　out　a　patent　for　forming

suspension　bridges　in　1817；　but　it　appears　that　Telford's　attention

had　been　directed　to　the　subject　before　this　time；　as　he　was　first

consulted　respecting　the　Runcorn　Bridge　in　the　year　1814；　when　he

proceeded　to　make　an　elaborate　series　of　experiments　on　the

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　wi