the life of thomas telford-第66章

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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

united　by　a　connecting　horizontal　avenue　or　cavern；　sufficiently

capacious　to　enable　the　workmen　to　fix　the　strong　iron　frames；

composed　principally　of　thick　flat　cast　iron　plates；　which　were

engrafted　deeply　into　the　rock；　and　strongly　bound　together　by　the

iron　work　passing　along　the　horizontal　avenue；　so　that；　if　the　iron

held；　the　chains　could　only　yield　by　tearing　up　the　whole　mass　of

solid　rock　under　which　they　were　thus　firmly　bound。



A　similar　method　of　anchoring　the　main　chains　was　adopted　on　the

Caernarvonshire　side。　　A　thick　bank　of　earth　had　there　to　be　cut

through；　and　a　solid　mass　of　masonry　built　in　its　place；　the　rock

being　situated　at　a　greater　distance　from　the　main　pier；　involving

a　greater　length　of　suspending　chain；　and　a　disproportion　in　the

catenary　or　chord　line　on　that　side　of　the　bridge。　　The　excavation

and　masonry　thereby　rendered　necessary　proved　a　work　of　vast

labour；　and　its　execution　occupied　a　considerable　time；　but　by　the

beginning　of　the　year　1825　the　suspension　pyramids；　the　land　piers

and　arches；　and　the　rock　tunnels；　had　all　been　completed；　and　the

main　chains　were　firmly　secured　in　them；　the　work　being

sufficiently　advanced　to　enable　the　suspending　of　the　chains　to　be

proceeded　with。　　This　was　by　far　the　most　difficult　and　anxious　part

of　the　undertaking。



With　the　same　careful　forethought　and　provision　for　every

contingency　which　had　distinguished　the　engineer's　procedure　in　the

course　of　the　work；　he　had　made　frequent　experiments　to　ascertain

the　actual　power　which　would　be　required　to　raise　the　main　chains

to　their　proper　curvature。　　A　valley　lay　convenient　for　the　purpose；

a　little　to　the　west　of　the　bridge　on　the　Anglesea　side。

Fifty…seven　of　the　intended　vertical　suspending　rods；　each　nearly

ten　feet　long　and　an　inch　square；　having　been　fastened　together；　a

piece　of　chain　was　attached　to　one　end　to　make　the　chord　line　570

feet　in　length；　and　experiments　having　been　made　and　comparisons

drawn；　Mr。　Telford　ascertained　that　the　absolute　weight　of　one　of

the　main　chains　of　the　bridge　between　the　points　of　suspension　was

23　1/2　tons；　requiring　a　strain　of　39　1/2　tons　to　raise　it　to　its

proper　curvature。　　On　this　calculation　the　necessary　apparatus

required　for　the　hoisting　was　prepared。　　The　mode　of　action　finally

determined　on　for　lifting　the　main　chains；　and　fixing　them　into

their　places；　was　to　build　the　central　portion　of　each　upon　a　raft

450　feet　long　and　6　feet　wide；　then　to　float　it　to　the　site　of　the

bridge；　and　lift　it　into　its　place　by　capstans　and　proper　tackle。



At　length　all　was　ready　for　hoisting　the　first　great　chain；　and

about　the　middle　of　April；　1825；　Mr。　Telford　left　London　for　Bangor

to　superintend　the　operations。　　An　immense　assemblage　collected　to

witness　the　sight；　greater　in　number　than　any　that　had　been

collected　in　the　same　place　since　the　men　of　Anglesea；　in　their

war…paint；　rushing　down　to　the　beach；　had　shrieked　defiance　across

the　Straits　at　their　Roman　invaders　on　the　Caernarvon　shore。

Numerous　boats　arrayed　in　gay　colours　glided　along　the　waters；　the

daythe　26th　of　Aprilbeing　bright；　calm；　and　in　every　way

propitious。



At　half…past　two；　about　an　hour　before　high　water；　the　raft　bearing

the　main　chain　was　cast　off　from　near　Treborth　Mill；　on　the

Caernarvon　side。　　Towed　by　four　boats；　it　began　gradually　to　move

from　the　shore；　and　with　the　assistance　of　the　tide；　which　caught

it　at　its　further　end；　it　swung　slowly　and　majestically　round　to

its　position　between　the　main　piers；　where　it　was　moored。　　One　end

of　the　chain　was　then　bolted　to　that　which　hung　down　the　face　of

the　Caernarvon　pier；　whilst　the　other　was　attached　to　ropes

connected　with　strong　capstans　fixed　on　the　Anglesea　side；　the

ropes　passing　by　means　of　blocks　over　the　top　of　the　pyramid　of　the

Anglesea　pier。　　The　capstans　for　hauling　in　the　ropes　bearing　the

main　chain；　were　two　in　number；　manned　by　about　150　labourers。　　When

all　was　ready；　the　signal　was　given　to　〃Go　along！〃　　A　Band　of　fifers

struck　up　a　lively　tune；　the　capstans　were　instantly　in　motion；　and

the　men　stepped　round　in　a　steady　trot。　　All　went　well。　　The　ropes

gradually　coiled　in。　　As　the　strain　increased；　the　pace　slackened　a

little；　but　〃Heave　away；　now　she　comes！〃　was　sung　out。　　Round　went

the　men；　and　steadily　and　safely　rose　the　ponderous　chain。



＇Image＇　Cut　of　Bridge；　showing　state　of　Suspension　Chain



The　tide　had　by　this　time　turned；　and　bearing　upon　the　side　of　the

raft；　now　getting　freer　of　its　load；　the　current　floated　it　away

from　under　the　middle　of　the　chain　still　resting　on　it；　and　it

swung　easily　off　into　the　water。　　Until　this　moment　a　breath　less

silence　pervaded　the　watching　multitude；　and　nothing　was　heard

among　the　working　party　on　the　Anglesea　side　but　the　steady　tramp

of　the　men　at　the　capstans；　the　shrill　music　of　the　fife；　and　the

occasional　order　to　〃Hold　on！〃　or　〃Go　along！〃　　But　no　sooner　was　the

raft　seen　floating　away；　and　the　great　chain　safely　swinging　in　the

air；　than　a　tremendous　cheer　burst　forth　along　both　sides　of　the

Straits。



The　rest　of　the　work　was　only　a　matter　of　time。　　The　most　anxious

moment　had　passed。　　In　an　hour　and　thirty…five　minutes　after　the

commencement　of　the　hoisting；　the　chain　was　raised　to　its　proper

curvature；　and　fastened　to　the　land　portion　of　it　which　had　been

previously　placed　over　the　top　of　the　Anglesea　pyramid。　　Mr。　Telford

ascended　to　the　point　of　fastening；　and　satisfied　himself　that　a

continuous　and　safe　connection　had　been　formed　from　the　Caernarvon

fastening　on　the　rock　to　that　on　Anglesea。　　The　announcement　of　the

fact　was　followed　by　loud　and　prolonged　cheering　from　the　workmen；

echoed　by　the　spectators；　and　extending　along　the　Straits　on　both

sides；　until　it　seemed　to　die　away　along　the　shores　in　the　distance。

Three　foolhardy　workmen；　excited　by　the　day's　p