the life of thomas telford- 第36部分

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　General　Committee；

and　got　it　referred　to　a　Select　Committee。　　Last　year　they

recommended　that　a　system　of　docks　should　be　formed　in　a　large　bend

of　the　river　opposite　Greenwich；　called　the　Isle　of　Dogs；　with　a

canal　across　the　neck　of　the　bend。　　This　part　of　the　contemplated

improvements　is　already　commenced；　and　is　proceeding　as　rapidly　as

the　nature　of　the　work　will　admit。　　It　will　contain　ship　docks　for

large　vessels；　such　as　East　and　West　Indiamen；　whose　draught　of

water　is　considerable。



〃There　are　now　two　other　propositions　under　consideration。　　One　is

to　form　another　system　of　docks　at　Wapping；　and　the　other　to　take

down　London　Bridge；　rebuild　it　of　such　dimensions　as　to　admit　of

ships　of　200　tons　passing　under　it；　and　form　a　new　pool　for　ships

of　such　burden　between　London　and　Blackfriars　Bridges；　with　a　set

of　regular　wharves　on　each　side　of　the　river。　　This　is　with　the　view

of　saving　lighterage　and　plunderage；　and　bringing　the　great　mass　of

commerce　so　much　nearer　to　the　heart　of　the　City。　　This　last　part　of

the　plan　has　been　taken　up　in　a　great　measure　from　some　statements

I　made　while　in　London　last　year；　and　I　have　been　called　before　the

Committee　to　explain。　　I　had　previously　prepared　a　set　of　plans　and

estimates　for　the　purpose　of　showing　how　the　idea　might　be　carried

out；　and　thus　a　considerable　degree　of　interest　has　been　excited　on

the　subject。　　It　is　as　yet；　however；　very　uncertain　how　far　the

plans　will　be　carried　out。　　It　is　certainly　a　matter　of　great

national　importance　to　render　the　Port　of　London　as　perfect　as

possible。〃*＇7＇



Later　in　the　same　year　he　writes　that　his　plans　and　propositions

have　been　approved　and　recommended　to　be　carried　out；　and　he

expects　to　have　the　execution　of　them。　　〃If　they　will　provide　the

ways　and　means；〃　says　he；　〃and　give　me　elbow…room；　I　see　my　way　as

plainly　as　mending　the　brig　at　the　auld　burn。〃　　In　November；　1801；

he　states　that　his　view　of　London　Bridge；　as　proposed　by　him；　has

been　published；　and　much　admired。　　On　the　l4th　of　April；　1802；　he

writes；　〃I　have　got　into　mighty　favour　with　the　Royal　folks。　　I　have

received　notes　written　by　order　of　the　King；　the　Prince　of　Wales；

Duke　of　York；　and　Duke　of　Kent；　about　the　bridge　print；　and　in

future　it　is　to　be　dedicated　to　the　King。〃



The　bridge　in　question　was　one　of　the　boldest　of　Telford's　designs。

He　proposed　by　his　one　arch　to　provide　a　clear　headway　of　65　feet

above　high　water。　　The　arch　was　to　consist　of　seven　cast　iron　ribs；

in　segments　as　large　as　possible；　and　they　were　to　be　connected　by

diagonal　cross…bracing；　disposed　in　such　a　manner　that　any　part　of

the　ribs　and　braces　could　be　taken　out　and　replaced　without　injury

to　the　stability　of　the　bridge　or　interruption　to　the　traffic　over　it。

The　roadway　was　to　be　90　feet　wide　at　the　abutments　and　45　feet

in　the　centre；　the　width　of　the　arch　being　gradually　contracted

towards　the　crown　in　order　to　lighten　the　weight　of　the　structure。

The　bridge　was　to　contain　6500　tons　of　iron；　and　the　cost　of　the

whole　was　to　be　262；289L。



＇Image＇　Telford's　proposed　One…arched　Bridge　over　the　Thames。



The　originality　of　the　design　was　greatly　admired；　though　there

were　many　who　received　with　incredulity　the　proposal　to　bridge　the

Thames　by　a　single　arch；　and　it　was　sarcastically　said　of　Telford

that　he　might　as　well　think　of　〃setting　the　Thames　on　fire。〃

Before　any　outlay　was　incurred　in　building　the　bridge；　the　design

was　submitted　to　the　consideration　of　the　most　eminent　scientific

and　practical　men　of　the　day；　after　which　evidence　was　taken　at

great　length　before　a　Select　Committee　which　sat　on　the　subject。

Among　those　examined　on　the　occasion　were　the　venerable　James　Watt

of　Birmingham；　Mr。　John　Rennie；　Professor　Button　of　Woolwich；

Professors　Playfair　and　Robison　of　Edinburgh；　Mr。　Jessop；

Mr。Southern；　and　Dr。　Maskelyne。　　Their　evidence　will　still　be　found

interesting　as　indicating　the　state　at　which　constructive　science

had　at　that　time　arrived　in　England。*＇8＇　　There　was　a　considerable

diversity　of　opinion　among　the　witnesses；　as　might　have　been

expected；　for　experience　was　as　yet　very　limited　as　to　the

resistance　of　cast　iron　to　extension　and　compression。　　Some　of　them

anticipated　immense　difficulty　in　casting　pieces　of　metal　of　the

necessary　size　and　exactness；　so　as　to　secure　that　the　radiated

joints　should　be　all　straight　and　bearing。　　Others　laid　down　certain

ingenious　theories　of　the　arch；　which　did　not　quite　square　with　the

plan　proposed　by　the　engineer。　　But；　as　was　candidly　observed　by

Professor　Playfair　in　concluding　his　report〃It　is　not　from

theoretical　men　that　the　most　valuable　information　in　such　a　case

as　the　present　is　to　be　expected。　　When　a　mechanical　arrangement

becomes　in　a　certain　degree　complicated；　it　baffles　the　efforts　of

the　geometer；　and　refuses　to　submit　to　even　the　most　approved

methods　of　investigation。　　This　holds　good　particularly　of　bridges；

where　the　principles　of　mechanics；　aided　by　all　the　resources　of

the　higher　geometry；　have　not　yet　gone　further　than　to　determine

the　equilibrium　of　a　set　of　smooth　wedges　acting　on　one　another　by

pressure　only；　and　in　such　circumstances　as；　except　in　a

philosophical　experiment；　can　hardly　ever　be　realised。　　It　is；

therefore；　from　men　educated　in　the　school　of　daily　practice　and

experience；　and　who　to　a　knowledge　of　general　principles　have

added；　from　the　habits　of　their　profession；　a　certain　feeling　of

the　justness　or　insufficiency　of　any　mechanical　contrivance；　that

the　soundest　opinions　on　a　matter　of　this　kind　can　be　obtained。〃



It　would　appear　that　the　Committee　came　to　the　general　conclusion

that　the　construction　of　the　proposed　bridge　was　practicable　and

safe；　for　the　river　was　contracted　to　the　requisite　width；　and　the

preliminary　works　were　actually　begun。　　Mr。　Stephenson　says　the

design　was　eventually　abandoned；　owing　more　immediately　to　the

difficulty　of　constructing　the　approaches　with　such　a　head　way；

which　would　have　involved　the　formation　of　extensive　inclined

planes　from　the　adjoining　streets；　and　thereby　led　to　serious

inconvenience；　and　the　depreciation　of　much　valuable　property　on

both　sides　of　the　river。*＇9＇　　Telford's　noble　design　of　his　great

iron　bridge　over　the　Thames；　together　with　his　proposed　embankment

of　the　river；　being　thus　definitely　abandoned；　he　fell　back　upon

his　ordinary　business　as　an　architect　and　engineer；　in　the　course

of　which　he　designed　and　erected　several　stone　bridges　of

considerable　magnitude　and　importance。



In　the　spring　of　1795；　after　a　long　continued　fall　of　snow；　a

sudden　thaw　raised　a　heavy　flood　in　the　Severn；　which　carried　away

many　bridgesamongst　others　one　at　Bewdley；　in　Worcestershire；

when　Telford　was　called　upon　to　supply　a　design　for　a　new　structure。

At　the　same　time；　he　was　required　to　furnish　a　plan　for　a　new

bridge　near　the　town　of　Bridgenorth；　〃in　short；〃　he　wrote　to　his

friend；　〃I　have　been　at　it　night　and　day。〃　　So　uniform　a　success　had

heretofore　attended　the　execution　of　his　designs；　that　his

reputation　as　a　bridge…builder　was　universally　acknowledged。

〃Last　week；〃　he　says；　〃Davidson　and　I　struck　the　centre　of　an　arch

of　76　feet　span；　and　this　is　the　third　which　has　been　thrown　this

summer；　none　of　which　have　shrunk　a　quarter　of　an　inch。〃



Bewdley　Bridge　is　a　handsome　and　substantial　piece　of　masonry。

The　streets　on　either　side　of　it　being　on　low　ground；　land　arches

were　　provided　at　both　ends　for　the　passage　of　the　flood　waters；

and　as　the　Severn　was　navigable　at　the　point　crossed；　it　was

considered　necessary　to　allow　considerably　greater　width　in　the

river　arches　than　had　been　the　case　in　the　former　structure。

The　arches　were　three　in　numberone　of　60　feet　span　and　two　of　52

feet；　the　land　arches　being　of　9　feet　span。　　The　works　were

proceeded　with　and　the　bridge　was