the life of thomas telford-第44章

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Paine；　however；　was　by　this　time　carried　away　by　the　fervour　of　the

French　Revolution；　having　become　a　member　of　the　National

Convention；　as　representative　for　Calais。　　The　〃Friends　of　Man；〃

whose　cause　he　had　espoused；　treated　him　scurvily；　imprisoning　him

in　the　Luxembourg；　where　he　lay　for　eleven　months。　　Escaped　to

America；　we　find　him　in　1803　presenting　to　the　American　Congress　a

memoir　on　the　construction　of　Iron　Bridges；　accompanied　by　several

models。　　It　does　not　appear；　however；　that　Paine　ever　succeeded　in

erecting　an　iron　bridge。　　He　was　a　restless；　speculative；　unhappy

being；　and　it　would　have　been　well　for　his　memory　if；　instead　of

penning　shallow　infidelity；　he　had　devoted　himself　to　his　original

idea　of　improving　the　communications　of　his　adopted　country。

In　the　meantime；　however；　the　bridge　exhibited　at　Paddington　had

produced　important　results。　The　manufacturers　agreed　to　take　it

back　as　part　of　their　debt；　and　the　materials　were　afterwards　used

in　the　construction　of　the　noble　bridge　over　the　Wear　at　Sunderland；

which　was　erected　in　1796。



The　project　of　constructing　a　bridge　at　this　place；　where　the　rocky

banks　of　the　Wear　rise　to　a　great　height　oh　both　sides　of　the

river；　is　due　to　Rowland　Burdon；　Esq。；　of　Castle　Eden；　under　whom

Mr。　T。　Wilson　served　as　engineer　in　carrying　out　his　design。

The　details　differed　in　several　important　respects　from　the　proposed

bridge　of　Paine；　Mr。　Burdon　introducing　several　new　and　original

features；　more　particularly　as　regarded　the　framed　iron　panels

radiating　towards　the　centre　in　the　form　of　voussoirs；　for　the

purpose　of　resisting　compression。　　Mr。　Phipps；　C。E。；　in　a　report

prepared　by　him　at　the　instance　of　the　late　Robert　Stephenson；

under　whose　superintendence　the　bridge　was　recently　repaired；

observes；　with　respect　to　the　original　design；〃We　should　probably

make　a　fair　division　of　the　honour　connected　with　this　unique

bridge；　by　conceding　to　Burdon　all　that　belongs　to　a　careful

elaboration　and　improvement　upon　the　designs　of　another；　to　the

boldness　of　taking　upon　himself　the　great　responsibility　of

applying。　　this　idea　at　once　on　so　magnificent　a　scale；　and　to　his

liberality　and　public　spirit　in　furnishing　the　requisite　funds

＇to　the　amount　of　22；000L。＇；　but　we　must　not　deny　to　Paine　the　credit

of　conceiving　the　construction　of　iron　bridges　of　far　larger　span

than　had　been　made　before　his　time；　or　of　the　important　examples

both　as　models　and　large　constructions　which　he　caused　to　be　made

and　publicly　exhibited。　　In　whatever　shares　the　merit　of　this　great

work　may　be　apportioned；　it　must　be　admitted　to　be　one　of　the

earliest　and　greatest　triumphs　of　the　art　of　bridge　construction。〃

Its　span　exceeded　that　of　any　arch　then　known；　being　236　feet；　with

a　rise　of　34　feet；　the　springing　commencing　at　95　feet　above　the

bed　of　the　river；　and　its　height　was　such　as　to　allow　vessels　of

300　tons　burden　to　sail　underneath　without　striking　their　masts。

Mr。　Stephenson　characterised　the　bridge　as　〃a　structure　which；　as

regards　its　proportions　and　the　small　quantity　of　material　employed

in　its　construction；　will　probably　remain　unrivalled。〃



＇Image＇　Wear　Bridge；　at　Sunderland。



The　same　year　in　which　Burdon's　Bridge　was　erected　at　Sunderland；

Telford　was　building　his　first　iron　bridge　over　the　Severn　at

Buildwas；　at　a　point　about　midway　between　Shrewsbury　and　Bridgenorth。

An　unusually　high　flood　having　swept　away　the　old　bridge　in　the

Year　1795；　he　was　called　upon；　as　surveyor　for　the　county；　to

supply　the　plan　of　a　new　one。　　Having　carefully　examined　the　bridge

at　Coalbrookdale；　and　appreciated　its　remarkable　merits；　he

determined　to　build　the　proposed　bridge　at　Buildwas　of　iron；　and　as

the　waters　came　down　with　great　suddenness　from　the　Welsh　mountains；

he　further　resolved　to　construct　it　of　only　one　arch；　so　as　to

afford　the　largest　possible　water…way。



He　had　some　difficulty　in　inducing　the　Coalbrookdale　iron…masters；

who　undertook　the　casting　of　the　girders；　to　depart　from　the　plan

of　the　earlier　structure；　but　he　persisted　in　his　design；　which　was

eventually　carried　out。　　It　consisted　of　a　single　arch　of　130　feet

span；　the　segment　of　a　very　large　circle；　calculated　to　resist　the

tendency　of　the　abutments　to　slide　inwards；　which　had　been　a　defect

of　the　Coalbrookdale　bridge；　the　flat　arch　being　itself　sustained

and　strengthened　by　an　outer　ribbed　one　on　each　side；　springing

lower　than　the　former　and　also　rising　higher；　somewhat　after　the

manner　of　timber…trussing。　　Although　the　span　of　the　new　bridge　was

30　feet　wider　than　the　Coalbrookdale　bridge；　it　contained　less　than

half　the　quantity　of　iron；　Buildwas　bridge　containing　173；　whereas

the　other　contained　378　tons。　　The　new　structure　was；　besides；

extremely　elegant　in　form；　and　when　the　centres　were　struck；　the

arch　and　abutments　stood　perfectly　firm；　and　have　remained　so　to

this　day。　　But　the　ingenious　design　of　this　bridge　will　be　better

explained　by　the　following　representation　than　by　any　description

in　words。*＇4＇　　The　bridge　at　Buildwas；　however；　was　not　Telford's

first　employment　of　iron　in　bridge…building；　for；　the　year　before

its　erection；　we　find　him　writing　to　his　friend　at　Langholm　that　he

had　recommended　an　iron　aqueduct　for　the　Shrewsbury　Canal；

〃on　a　principle　entirely　new；〃　and　which　he　was　〃endeavouring　to

establish　with　regard　to　the　application　of　iron。〃*＇5＇　　This　iron

aqueduct　had　been　cast　and　fixed；　and　it　was　found　to　effect　so

great　a　saving　in　masonry　and　earthwork；　that　he　was　afterwards

induced　to　apply　the　same　principle；　as　we　have　already　seen；

in　different　forms；　in　the　magnificent　aqueducts　of　Chirk　and

Pont…Cysylltau。



The　uses　of　cast　iron　in　canal　construction　became　more　obvious

with　every　year's　successive　experience；　and　Telford　was　accustomed

to　introduce　it　in　many　cases　where　formerly　only　timber　or　stone

had　been　used。　　On　the　Ellesmere；　and　afterwards　on　the　Caledonial

Canal；　he　adopted　cast　iron　lock…gates；　which　were　found　to　answer

well；　being　more　durable　than　timber；　and　not　liable　like　it　to

shrink　and　expand　with　alternate　dryness　and　wet。　　The　turnbridges

which　he　applied　to　his　canals；　in　place　of　the　old　drawbridges；

were　also　of　cast　iron；　and　in　some　cases　even　the　locks　were　of

the　same　material。　　Thus；　on　a　part　of　the　Ellesmere　Canal　opposite

Beeston　Castle；　in　Cheshire；　where　a　couple　of　locks；　together

rising　17　feet；　having　been　built　on　a　stratum　of　quicksand；　were

repeatedly　undermined；　the　idea　of　constructing　the　entire　locks　of

cast　iron　was　suggested；　and　this　unusual　application　of　the　new

material　was　accomplished　with　entirely　satisfactory　results。



But　Telford's　principal　employment　of　cast　iron　was　in　the

construction　of　road　bridges；　in　which　he　proved　himself　a　master。

His　experience　in　these　structures　had　become　very　extensive。

During　the　time　that　he　held　the　office　of　surveyor　to　the　county

of　Salop；　he　erected　no　fewer　than　forty…two；　five　of　which　were　of

iron。　　Indeed；　his　success　in　iron　bridge…building　so　much

emboldened　him；　that　in　1801；　when　Old　London　Bridge　had　become　so

rickety　and　inconvenient　that　it　was　found　necessary　to　take　steps

to　rebuild　or　remove　it；　he　proposed　the　daring　plan　of　a　cast　iron

bridge　of　a　single　arch　of　not　less　than　600　feet　span；　the　segment

of　a　circle　l450　feet　in　diameter。　　In　preparing　this　design　we

find　that　he　was　associated　with　a　Mr。　Douglas；　to　whom　many

allusions　are　made　in　his　private　letters。*＇6＇　　The　design　of　this

bridge　seems　to　have　arisen　out　of　a　larger　　project　for　the

improvement　of　the　port　of　London。　　In　a　private　letter　of　Telford's；

dated　the　13th　May；　1800；　he　says：



〃I　have　twice　attended　the　Select　Committee　on　the　Fort　of　London；

Lord　Hawkesbury；　Chairman。　　The　subject　has　now　been　agitated　for

four　years；　and　might　have　been　so　for　many　more；　if　Mr。　Pitt　had

not　taken　the　business　out　of　the　hands　of　the　Gen