the life of thomas telford-第57节

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is　not　above　twelve　feet　in　the　steepest　part　of　the　hill；　and　two

carriages　cannot　pass　without　the　greatest　danger。　　Between　this

hill　and　Rhyddlanfair　there　are　a　number　of　dangerous　precipices；

steep　hills；　and　difficult　narrow　turnings。　　From　Corwen　to

Llangollen　the　road　is　very　narrow；　long；　and　steep；　has　no　side

fence；　except　about　a　foot　and　a　half　of　mould　or　dirt；　which　is

thrown　up　to　prevent　carriages　falling　down　three　or　four　hundred

feet　into　the　river　Dee。　　Stage…coaches　have　been　frequently

overturned　and　broken　down　from　the　badness　of　the　road；　and　the

mails　have　been　overturned；　but　I　wonder　that　more　and　worse

accidents　have　not　happened；　the　roads　are　so　bad。〃Evidence　of

Mr。　William　Akers；　of　the　Post…office；　before　Committee　of　the

House　of　Commons；　1st　June；　1815。



*＇6＇　The　Select　Committee　of　the　House　of　Commons；　in　reporting　as

to　the　manner　in　which　these　works　were　carried　out；　stated　as

follows：　〃The　professional　execution　of　the　new　works　upon　this

road　greatly　surpasses　anything　of　the　same　kind　in　these

countries。　　The　science　which　has　been　displayed　in　giving　the

general　line　of　the　road　a　proper　inclination　through　a　country

whose　whole　surface　consists　of　a　succession　of　rocks；　bogs；

ravines；　rivers；　and　precipices；　reflects　the　greatest　credit　upon

the　engineer　who　has　planned　them；　but　perhaps　a　still　greater

degree　of　professional　skill　has　been　shown　in　the　construction；　or

rather　the　building；　of　the　road　itself。　　The　great　attention　which

Mr。　Telford　has　devoted；　to　give　to　the　surface　of　the　road　one

uniform　and　moderately　convex　shape；　free　from　the　smallest

inequality　throughout　its　whole　breadth；　the　numerous　land　drains；

and；　when　necessary；　shores　and　tunnels　of　substantial　masonry；

with　which　all　the　water　arising　from　springs　or　falling　in　rain　is

instantly　carried　off；　the　great　care　with　which　a　sufficient

foundation　is　established　for　the　road；　and　the　quality；　solidity；

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　erecte