heroes of the telegraph-第15章

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Mr。　Field　then　directed　his　efforts　to　the　completion　of　the　trans…　oceanic　section。　　He　induced　the　American　Government　to　despatch　Lieutenant　Berryman；　in　the　Arctic；　and　the　British　Admiralty　to　send　Lieutenant：　　Dayman；　in　the　Cyclops；　to　make　a　special　survey　along　the　proposed　route　of　the　cable。　　These　soundings　　revealed　the　existence　of　a　submarine　hill　dividing　the　'telegraph　plateau'　from　the　shoal　water　on　the　coast　of　Ireland；　but　its　slope　was　gradual　and　easy。

Till　now　the　enterprise　had　been　purely　American；　and　the　funds　provided　by　American　capitalists；　with　the　exception　of　a　few　shares　held　by　Mr。　J。　W。　Brett。　　But　seeing　that　the　cable　was　to　land　on　British　soil；　it　was　fitting　that　the　work　should　be　international；　and　that　the　British　people　should　be　asked　to　contribute　towards　the　manufacture　and　submersion　of　the　cable。　　Mr。　Field　therefore　proceeded　to　London；　and　with　the　assistance　of　Mr。　Brett　the　Atlantic　Telegraph　Company　was　floated。　　Mr。　Field　himself　supplied　a　quarter　of　the　needed　capital；　and　we　may　add　that　Lady　Byron；　and　Mr。　Thackeray；　the　novelist；　were　among　the　shareholders。

The　design　of　the　cable　was　a　subject　of　experiment　by　Professor　Morse　and　others。　　It　was　known　that　the　conductor　should　be　of　copper；　possessing　a　high　conductivity　for　the　electric　current；　and　that　its　insulating　jacket　of　gutta…percha　should　offer　a　great　resistance　to　the　leakage　of　the　current。　　Moreover；　experience　had　shown　that　the　protecting　sheath　or　armour　of　the　core　should　be　light　and　flexible　as　well　as　strong；　in　order　to　resist　external　violence　and　allow　it　to　be　lifted　for　repair。　　There　was　another　consideration；　however；　which　at　this　time　was　rather　a　puzzle。　　As　early　as　1823　Mr。　（afterwards　Sir）　Francis　Ronalds　had　observed　that　electric　signals　were　retarded　in　passing　through　an　insulated　wire　or　core　laid　under　ground；　and　the　same　effect　was　noticeable　on　cores　immersed　in　water；　and　particularly　on　the　lengthy　cable　between　England　and　the　Hague。　　Faraday　showed　that　it　was　caused　by　induction　between　the　electricity　in　the　wire　and　the　earth　or　water　surrounding　it。　　A　core；　in　fact；　is　an　attenuated　Leyden　jar；　the　wire　of　the　core；　its　insulating　jacket；　and　the　soil　or　water　around　it　stand　respectively　for　the　inner　tinfoil；　the　glass；　and　the　outer　tinfoil　of　the　jar。　　When　the　wire　is　charged　from　a　battery；　the　electricity　induces　an　opposite　charge　in　the　water　as　it　travels　along；　and　as　the　two　charges　attract　each　other；　the　exciting　charge　is　restrained。　　The　speed　of　a　signal　through　the　conductor　of　a　submarine　cable　is　thus　diminished　by　a　drag　of　its　own　making。　　The　nature　of　the　phenomenon　was　clear；　but　the　laws　which　governed　it　were　still　a　mystery。　　It　became　a　serious　question　whether；　on　a　long　cable　such　as　that　required　for　the　Atlantic；　the　signals　might　not　be　so　sluggish　that　the　work　would　hardly　pay。　　Faraday　had　said　to　Mr。　Field　that　a　signal　would　take　'about　a　second；'　and　the　American　was　satisfied；　but　Professor　Thomson　enunciated　the　law　of　retardation；　and　cleared　up　the　whole　matter。　　He　showed　that　the　velocity　of　a　signal　through　a　given　core　was　inversely　proportional　to　the　square　of　the　length　of　the　core。　That　is　to　say；　in　any　particular　cable　the　speed　of　a　signal　is　diminished　to　one…fourth　if　the　length　is　doubled；　to　one…ninth　if　it　is　trebled；　to　one…sixteenth　if　it　is　quadrupled；　and　so　on。　　It　was　now　possible　to　calculate　the　time　taken　by　a　signal　in　traversing　the　proposed　Atlantic　line　to　a　minute　fraction　of　a　second；　and　to　design　the　proper　core　for　a　cable　of　any　given　length。

The　accuracy　of　Thomson's　law　was　disputed　in　1856　by　Dr。　Edward　O。　Wildman　Whitehouse；　the　electrician　of　the　Atlantic　Telegraph　Company；　who　had　misinterpreted　the　results　of　his　own　experiments。　　Thomson　disposed　of　his　contention　in　a　letter　to　the　ATHENAEUM；　and　the　directors　of　the　company　saw　that　he　was　a　man　to　enlist　in　their　adventure。　　It　is　not　enough　to　say　the　young　Glasgow　professor　threw　himself　heart　and　soul　into　their　work。　　He　descended　in　their　midst　like　the　very　genius　of　electricity；　and　helped　them　out　of　all　their　difficulties。　　In　1857　he　published　in　the　ENGINEER　the　whole　theory　of　the　mechanical　forces　involved　in　the　laying　of　a　submarine　cable；　and　showed　that　when　the　line　is　running　out　of　the　ship　at　a　constant　speed　in　a　uniform　depth　of　water；　it　sinks　in　a　slant　or　straight　incline　from　the　point　where　it　enters　the　water　to　that　where　it　touches　the　bottom。

To　these　gifts　of　theory；　electrical　and　mechanical；　Thomson　added　a　practical　boon　in　the　shape　of　the　reflecting　galvanometer；　or　mirror　instrument。　　This　measurer　of　the　current　was　infinitely　more　sensitive　than　any　which　preceded　it；　and　enables　the　electrician　to　detect　the　slightest　flaw　in　the　core　of　a　cable　during　its　manufacture　and　submersion。　　Moreover；　it　proved　the　best　apparatus　for　receiving　the　messages　through　a　long　cable。　　The　Morse　and　other　instruments；　however　suitable　for　land　lines　and　short　cables；　were　all　but　useless　on　the　Atlantic　line；　owing　to　the　retardation　of　the　signals；　but　the　mirror　instrument　sprang　out　of　Thomson's　study　of　this　phenomenon；　and　was　designed　to　match　it。　　Hence　this　instrument；　through　being　the　fittest　for　the　purpose；　drove　the　others　from　the　field；　and　allowed　the　first　Atlantic　cables　to　be　worked　on　a　profitable　basis。

The　cable　consisted　of　a　strand　of　seven　copper　wires；　one　weighing　107　pounds　a　nautical　mile　or　knot；　covered　with　three　coats　of　gutta…　percha；　weighing　261　pounds　a　knot；　and　wound　with　tarred　hemp；　over　which　a　sheath　of　eighteen　strands；　each　of　seven　iron　wires；　was　laid　in　a　close　spiral。　　It　weighed　nearly　a　ton　to　the　mile；　was　flexible　as　a　rope；　and　able　to　withstand　a　pull　of　several　tons。　　It　was　made　conjointly　by　Messrs。　Glass；　Elliot　&　Co。；　of　Greenwich；　and　Messrs。　R。　S。　Newall　&　Co。；　of　Liverpool。

The　British　Government　promised　Mr。　Field　a　subsidy　of　L1；400　a　year；　and　the　loan　of　ships　to　lay　the　cable。　　He　solicited　an　equal　help　from　Congress；　but　a　large　number　of　the　senators；　actuated　by　a　national　jealousy　of　England；　and　looking　to　the　fact　that　both　ends　of　the　line　were　to　lie　in　British　territory；　opposed　the　grant。　It　appeared　to　these　far…sighted　politicians　that　England；　the　hereditary　foe；　was　'literally　crawling　under　the　sea　to　get　some　advantage　over　the　United　States。'　The　Bill　was　only　passed　by　a　majority　of　a　single　vote。　　In　the　House　of　Representatives　it　encountered　a　similar　hostility；　but　was　ultimately　signed　by　President　Pierce。

The　Agamemnon；　a　British　man…of…war　fitted　out　for　the　purpose；　took　in　the　section　made　at　Greenwich；　and　the　Niagara；　an　American　warship；　that　made　at　Liverpool。　　The　vessels　and　their　consorts　met　in　the　bay　of　Valentia　Island；　on　the　south…west　coast　of　Ireland；　where　on　August　5；　1857；　the　shore　end　of　the　cable　was　landed　from　the　Niagara。　　It　was　a　memorable　scene。　　The　ships　in　the　bay　were　dressed　in　bunting；　and　the　Lord　Lieutenant　of　Ireland　stood　on　the　beach；　attended　by　his　following；　to　receive　the　end　from　the　American　sailors。　　Visitors　in　holiday　attire　collected　in　groups　to　watch　the　operations；　and　eagerly　joined　with　his　excellency　in　helping　to　pull　the　wire　ashore。　　When　it　was　landed；　the　Reverend　Mr。　Day；　of　Kenmore；　offered　up　a　prayer；　asking　the　Almighty　to　prosper　the　undertaking；　Next　day　the　expedition　sailed；　but　ere　the　Niagara　had　proceeded　five　miles　on　her　way　the　shore…end　parted；　and　the　repairing　of　it　delayed　the　start　for　another　day。

At　first　the　Niagara　went　slowly　ahead　to　avoid　a　mishap；　but　as　the　cable　ran　out　easily　she　increased　her　speed。　　The　night　fell；　but　hardly　a　soul　slept。　　The　utmost　vigilance　was　maintained　throughout　the　vessel。　Apart　from　the　noise　of　the　paying…out　machinery；　there　was　an　awful　stillness　on　board。　　Men　walked　about　with　a　muffled　step；　or　spoke　in　whispers；　as　if　they　were　afraid　the　sound　of　their　voices　would　break　the　slender　line。　　It　seemed　as　though　a　great　and　valued　friend　lay　at　the　point　of　death。

The　submarine　hill；　with　its　dangerous　slope；　was　passed　in　safety；　and　the　'telegraph　plateau；'　n