heroes of the telegraph-第4部分

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orks　of　a　watch；　so　that　it　revolved　at　a　high　velocity；　and　observed　the　reflections　of　his　three　sparks　in　it。　　The　points　of　the　wire　were　so　arranged　that　if　the　sparks　were　instantaneous；　their　reflections　would　appear　in　one　straight　line；　but　the　middle　one　was　seen　to　lag　behind　the　others；　because　it　was　an　instant　later。　　The　electricity　had　taken　a　certain　time　to　travel　from　the　ends　of　the　wire　to　the　middle。　　This　time　was　found　by　measuring　the　amount　of　lag；　and　comparing　it　with　the　known　velocity　of　the　mirror。　　Having　got　the　time；　he　had　only　to　compare　that　with　the　length　of　half　the　wire；　and　he　found　that　the　velocity　of　electricity　was　288；000　miles　a　second。

Till　then；　many　people　had　considered　the　electric　discharge　to　be　instantaneous；　but　it　was　afterwards　found　that　its　velocity　depended　on　the　nature　of　the　conductor；　its　resistance；　and　its　electro…static　capacity。　　Faraday　showed；　for　example；　that　its　velocity　in　a　submarine　wire；　coated　with　insulator　and　surrounded　with　water；　is　only　144；000　miles　a　second；　or　still　less。　　Wheatstone's　device　of　the　revolving　mirror　was　afterwards　employed　by　Foucault　and　Fizeau　to　measure　the　velocity　of　light。

In　1835；　at　the　Dublin　meeting　of　the　British　Association；　Wheatstone　showed　that　when　metals　were　volatilised　in　the　electric　spark；　their　light；　examined　through　a　prism；　revealed　certain　rays　which　were　characteristic　of　them。　　Thus　the　kind　of　metals　which　formed　the　sparking　points　could　be　determined　by　analysing　the　light　of　the　spark。　This　suggestion　has　been　of　great　service　in　spectrum　analysis；　and　as　applied　by　Bunsen；　Kirchoff；　and　others；　has　led　to　the　discovery　of　several　new　elements；　such　as　rubidium　and　thallium；　as　well　as　increasing　our　knowledge　of　the　heavenly　bodies。　　Two　years　later；　he　called　attention　to　the　value　of　thermo…electricity　as　a　mode　of　generating　a　current　by　means　of　heat；　and　since　then　a　variety　of　thermo…piles　have　been　invented；　some　of　which　have　proved　of　considerable　advantage。

Wheatstone　abandoned　his　idea　of　transmitting　intelligence　by　the　mechanical　vibration　of　rods；　and　took　up　the　electric　telegraph。　　In　1835　he　lectured　on　the　system　of　Baron　Schilling；　and　declared　that　the　means　were　already　known　by　which　an　electric　telegraph　could　be　made　of　great　service　to　the　world。　　He　made　experiments　with　a　plan　of　his　own；　and　not　only　proposed　to　lay　an　experimental　line　across　the　Thames；　but　to　establish　it　on　the　London　and　Birmingham　Railway。　　Before　these　plans　were　carried　out；　however；　he　received　a　visit　from　Mr。　Fothergill　Cooke　at　his　house　in　Conduit　Street　on　February　27；　1837；　which　had　an　important　influence　on　his　future。

Mr。　Cooke　was　an　officer　in　the　Madras　army；　who；　being　home　on　furlough；　was　attending　some　lectures　on　anatomy　at　the　University　of　Heidelberg；　where；　on　March　6；　1836；　he　witnessed　a　demonstration　with　the　telegraph　of　Professor　Moncke；　and　was　so　impressed　with　its　importance；　that　he　forsook　his　medical　studies　and　devoted　all　his　efforts　to　the　work　of　introducing　the　telegraph。　　He　returned　to　London　soon　after；　and　was　able　to　exhibit　a　telegraph　with　three　needles　in　January；　1837。　　Feeling　his　want　of　scientific　knowledge；　he　consulted　Faraday　and　Dr。　Roget；　the　latter　of　whom　sent　him　to　Wheatstone。

At　a　second　interview；　Mr。　Cooke　told　Wheatstone　of　his　intention　to　bring　out　a　working　telegraph；　and　explained　his　method。　　Wheatstone；　according　to　his　own　statement；　remarked　to　Cooke　that　the　method　would　not　act；　and　produced　his　own　experimental　telegraph。　　Finally；　Cooke　proposed　that　they　should　enter　into　a　partnership；　but　Wheatstone　was　at　first　reluctant　to　comply。　　He　was　a　well…known　man　of　science；　and　had　meant　to　publish　his　results　without　seeking　to　make　capital　of　them。　　Cooke；　on　the　other　hand；　declared　that　his　sole　object　was　to　make　a　fortune　from　the　scheme。　　In　May　they　agreed　to　join　their　forces；　Wheatstone　contributing　the　scientific；　and　Cooke　the　administrative　talent。　　The　deed　of　partnership　was　dated　November　19；　1837。　　A　joint　patent　was　taken　out　for　their　inventions；　including　the　five…needle　telegraph　of　Wheatstone；　and　an　alarm　worked　by　a　relay；　in　which　the　current；　by　dipping　a　needle　into　mercury；　completed　a　local　circuit；　and　released　the　detent　of　a　clockwork。

The　five…needle　telegraph；　which　was　mainly；　if　not　entirely；　due　to　Wheatstone；　was　similar　to　that　of　Schilling；　and　based　on　the　principle　enunciated　by　Amperethat　is　to　say；　the　current　was　sent　into　the　line　by　completing　the　circuit　of　the　battery　with　a　make　and　break　key；　and　at　the　other　end　it　passed　through　a　coil　of　wire　surrounding　a　magnetic　needle　free　to　turn　round　its　centre。　　According　as　one　pole　of　the　battery　or　the　other　was　applied　to　the　line　by　means　of　the　key；　the　current　deflected　the　needle　to　one　side　or　the　other。　　There　were　five　separate　circuits　actuating　five　different　needles。　　The　latter　were　pivoted　in　rows　across　the　middle　of　a　dial　shaped　like　a　diamond；　and　having　the　letters　of　the　alphabet　arranged　upon　it　in　such　a　way　that　a　letter　was　literally　pointed　out　by　the　current　deflecting　two　of　the　needles　towards　it。

An　experimental　line；　with　a　sixth　return　wire；　was　run　between　the　Euston　terminus　and　Camden　Town　station　of　the　London　and　North　Western　Railway　on　July　25；　1837。　　The　actual　distance　was　only　one　and　a　half　mile；　but　spare　wire　had　been　inserted　in　the　circuit　to　increase　its　length。　　It　was　late　in　the　evening　before　the　trial　took　place。　　Mr。　Cooke　was　in　charge　at　Camden　Town；　while　Mr。　Robert　Stephenson　and　other　gentlemen　looked　on；　and　Wheatstone　sat　at　his　instrument　in　a　dingy　little　room；　lit　by　a　tallow　candle；　near　the　booking…office　at　Euston。　　Wheatstone　sent　the　first　message；　to　which　Cooke　replied；　and　'never；'　said　Wheatstone；　'did　I　feel　such　a　tumultuous　sensation　before；　as　when；　all　alone　in　the　still　room；　I　heard　the　needles　click；　and　as　I　spelled　the　words；　I　felt　all　the　magnitude　of　the　invention　pronounced　to　be　practicable　beyond　cavil　or　dispute。'

In　spite　of　this　trial；　however；　the　directors　of　the　railway　treated　the　'new…fangled'　invention　with　indifference；　and　requested　its　removal。　　In　July；　1839；　however；　it　was　favoured　by　the　Great　Western　Railway；　and　a　line　erected　from　the　Paddington　terminus　to　West　Drayton　station；　a　distance　of　thirteen　miles。　　Part　of　the　wire　was　laid　underground　at　first；　but　subsequently　all　of　it　was　raised　on　posts　along　the　line。　　Their　circuit　was　eventually　extended　to　Slough　in　1841；　and　was　publicly　exhibited　at　Paddington　as　a　marvel　of　science；　which　could　transmit　fifty　signals　a　distance　of　280；000　miles　in　a　minute。　The　price　of　admission　was　a　shilling。

Notwithstanding　its　success；　the　public　did　not　readily　patronise　the　new　invention　until　its　utility　was　noised　abroad　by　the　clever　capture　of　the　murderer　Tawell。　　Between　six　and　seven　o'clock　one　morning　a　woman　named　Sarah　Hart　was　found　dead　in　her　home　at　Salt　Hill；　and　a　man　had　been　observed　to　leave　her　house　some　time　before。　　The　police　knew　that　she　was　visited　from　time　to　time　by　a　Mr。　John　Tawell；　from　Berkhampstead；　where　he　was　much　respected；　and　on　inquiring　and　arriving　at　Slough；　they　found　that　a　person　answering　his　description　had　booked　by　a　slow　train　for　London；　and　entered　a　first…class　carriage。　　The　police　telegraphed　at　once　to　Paddington；　giving　the　particulars；　and　desiring　his　capture。　　'He　is　in　the　garb　of　a　Quaker；'　ran　the　message；　'with　a　brown　coat　on；　which　reaches　nearly　to　his　feet。'　There　was　no　'Q'　in　the　alphabet　of　the　five…needle　instrument；　and　the　clerk　at　Slough　began　to　spell　the　word　'Quaker'　with　a　'kwa'；　but　when　he　had　got　so　far　he　was　interrupted　by　the　clerk　at　Paddington；　who　asked　him　to　'repent。'　The　repetition　fared　no　better；　until　a　boy　at　Paddington　suggested　that　Slough　should　be　allowed　to　finish　the　word。　　'Kwaker'　was　understood；　and　as　soon　as　Tawell　stepped　out　on　the　platform　at　Paddington　he　was　'shadowed'　by　a　detective；　who　followed　him　into　a　New　Road　omnibus；　and　arrested　him　in　a　coffee　tavern。

Tawell　was　tried　for　the　murder　of　the　woman；　and　astounding　revelations　were　made　as　to　his　character。　　Transported　in　1820　for　the　crime　of　forgery；　he　obtained　a　ticket…of…leave；　and　started　as　a　chemist　in　Sydney；　where　he　flourished；　and　after　fifteen　years　left　it　a　rich　man。　Returning　to　England；　he　married　a　Quaker　lady　as　his　second　wife。　　He　confessed　to　the　murder　of　Sarah　Hart；　by　prussic　acid；　his　motive　being　a　dread　of　their　relations　becoming　known。

Tawell　was　executed；　and　the　notoriety　of　the　case　brought　the　telegraph　into　repute。　　Its　advantages　as　a　rapid　means　of　conveying　intelligence　and　detecting　criminals　had　been　signally　demonstrated；　and　it　was　soon　adopted　on　a　more　extensive　scale。

In　1845　Wheatstone　introduced　two　improved　forms　of　the　apparatus；　namely；　the　'single'　and　the　'double'　needle　instruments；　in　which　the　signals　were　made　by　the　successive　deflections　of　the　needles。　　Of　these；　the　single…needle　instrument；　requiring　only　one　wire；　is　still　in　use。

In　1841　a　difference　arose　between　Cooke　and　Wheatstone　as　to　the　share　of　each　in