Itwasnecessarythereforeforthenavigatortobeinpossessionofafirst-ratewatchorchronometer，toenablehimtodetermineaccuratelythepositionofhisshipatsea，asrespectedthelongitude。
　　Beforethemiddleoftheeighteenthcenturygoodwatcheswerecomparativelyunknown。Thenavigatormainlyrelied，forhisapproximatelongitude，uponhisDeadReckoning，withoutanyobservationoftheheavenlybodies。Hedependedupontheaccuracyofthecoursewhichhehadsteeredbythecompass，andthemensurationoftheship’svelocitybyaninstrumentcalledtheLog，aswellasbycombiningandrectifyingalltheallowancesfordrift，lee-way，andsoon，accordingtothetrimoftheship；butallofthesewereliabletomuchuncertainty，especiallywhentheseawasinaboisterouscondition。Therewasanotherandindependentcoursewhichmighthavebeenadopted——thatis，byobservationofthemoon，whichisconstantlymovingamongstthestarsfromwesttoeast。Butuntilthemiddleoftheeighteenthcenturygoodlunartableswereasmuchunknownasgoodwatches。
　　Henceamethodofascertainingthelongitude，withthesamedegreeofaccuracywhichisattainableinrespectoflatitude，hadforagesbeenthegranddesideratumformen“whogodowntotheseainships。“Mr。Macpherson，inhisimportantworkentitled’TheAnnalsofCommerce，’observes，“Sincetheyear1714，whenParliamentofferedarewardof20，000L。forthebestmethodofascertainingthelongitudeatsea，manyschemeshavebeendevised，butalltolittleornopurpose，asgoinggenerallyuponwrongprinciples，tillthatheaven-taughtartistMr。JohnHarrisonarose；“andbyhim，asMr。Macphersongoesontosay，thedifficultywasconquered，havingdevotedtoit“theassiduousstudiesofalonglife。“
　　ThepreambleoftheActofParliamentinquestionrunsasfollows:“Whereasitiswellknownbyallthatareacquaintedwiththeartofnavigationthatnothingissomuchwantedanddesiredatseaasthediscoveryofthelongitude，forthesafetyandquicknessofvoyages，thepreservationofshipsandthelivesofmen，“andsoon。TheActproceedstoconstitutecertainpersonscommissionersforthediscoveryofthelongitude，withpowertoreceiveandexperimentuponproposalsforthatpurpose，andtograntsumsofmoneynotexceeding2000L。toaidinsuchexperiments。Itwillberememberedfromwhathasbeenabovestated，thatarewardof10，000L。wastobegiventothepersonwhoshouldcontriveamethodofdeterminingthelongitudewithinonedegreeofagreatcircle，or60geographicalmiles；15，000L。
　　within40geographicalmiles；and20，000L。within30geographicalmiles。
　　Itwill，inthesedays，bescarcelybelievedthatlittlemorethanahundredandfiftyyearsagoaprizeofnotlessthantenthousandpoundsshouldhavebeenofferedforamethodofdeterminingthelongitudewithinsixtymiles，andthatdoubletheamountshouldhavebeenofferedforamethodofdeterminingitwithinthirtymiles！Theamountoftheserewardsissufficientproofofthefearfulnecessityforimprovementwhichthenexistedinthemethodsofnavigation。Andyet，fromthedateofthepassingoftheActin1714untiltheyear1736，whenHarrisonfinishedhisfirsttimepiece，nothinghadbeendonetowardsascertainingthelongitudemoreaccurately，evenwithinthewidelimitsspecifiedbytheActofParliament。Althoughseveralschemeshadbeenprojected，noneofthemhadprovedsuccessful，andtheofferedrewardsthereforestillremainedunclaimed。
　　ToreturntoHarrison。AfterreachinghishomeatBarrow，afterhisvisittoLondonin1728，hebeganhisexperimentsfortheconstructionofamarinechronometer。Thetaskwasoneofnosmalldifficulty。Itwasnecessarytoprovideagainstirregularitiesarisingfromthemotionofashipatsea，andtoobviatetheeffectofalternationsoftemperatureinthemachineitself，aswellastheoilwithwhichitwaslubricated。A
　　thousandobstaclespresentedthemselves，buttheywerenotenoughtodeterHarrisonfromgrapplingwiththeworkhehadsethimselftoperform。
　　Everyoneknowsthebeautifulmachineryofatimepiece，andtheperfecttoolsrequiredtoproducesuchamachine。SomeofthesetoolsHarrisonprocuredinLondon，butthegreaternumberheprovidedforhimself；andmanyentirelynewadaptationswererequiredforhischronometer。Aswoodcouldnolongerbeexclusivelyemployed，asinhisfirstclock，hehadtoteachhimselftoworkaccuratelyandminutelyinbrassandothermetals。HavingbeenunabletoobtainanyassistancefromtheBoardofLongitude，hewasunderthenecessity，whilecarryingforwardhisexperiments，ofmaintaininghimselfbystillworkingathistradeofacarpenterandjoiner。Thiswillaccountfortheverylongperiodthatelapsedbeforehecouldbringhischronometertosuchastateasthatitmightbetriedwithanyapproachtocertaintyinitsoperations。
　　Harrison，besideshisintentnessandearnestness，wasacheerfulandhopefulman。Hehadafinetasteformusic，andorganisedandledthechoirofthevillagechurch，whichattainedahighdegreeofperfection。Heinventedacuriousmonochord，whichwasnotlessaccuratethanhisclocksinthemensurationoftime。
　　Hisearwasdistressedbytheringingofbellsoutoftune，andhesethimselftoremedythem。AttheparishchurchofHull，forinstance，thebellswereharshanddisagreeable，andbytheauthorityofthevicarandchurchwardenshewasallowedtoputthemintoastateofexacttune，sothattheyprovedentirelymelodious。
　　Butthegreatworkofhislifewashismarinechronometer。Hefounditnecessary，inthefirstplace，toalterthefirstmoverofhisclocktoaspringwoundup，sothattheregularityofthemotionmightbederivedfromthevibrationsofbalances，insteadofthoseofapendulumasinastandingclock。Mr。Folkes，PresidentoftheRoyalSociety，whenpresentingthegoldmedaltoHarrisonin1749，thusdescribesthearrangementofhisnewmachine。ThedetailswereobtainedfromHarrisonhimself，whowaspresent。Hehadmadeuseoftwobalancessituatedinthesameplane，butvibratingincontrarydirections，sothattheoneofthesebeingeitherwayassistedbythetossingoftheship，theothermightconstantlybejustsomuchimpededbyitatthesametime。Astheequalityofthetimesofthevibrationsofthebalanceofapocket-watchisinagreatmeasureowingtothespiralspringthatliesunderit，sothesamewashereperformedbythelikeelasticityoffourcylindricalspringsorworms，appliedneartheupperandlowerextremitiesofthetwobalancesabovedescribed。
　　Thencameinthequestionofcompensation。Harrison’sexperiencewiththecompensationpendulumofhisclocknowprovedofservicetohim。Hehadproceededtointroduceasimilarexpedientinhisproposedchronometer。Asiswellknowntothosewhoareacquaintedwiththenatureofspringsmovedbybalances，thestrongerthosespringsare，thequickerthevibrationsofthebalancesareperformed，andviceversa；henceitfollowsthatthosesprings，whenbracedbycold，orwhenrelaxedbyheat，mustofnecessitycausethetimekeepertogoeitherfasterorslower，unlesssomemethodcouldbefoundtoremedytheinconvenience。
　　ThemethodadoptedbyHarrisonwashiscompensationbalance，doubtlessthebackboneofhisinvention。His“thermometerkirb，“
　　hehimselfsays，“iscomposedoftwothinplatesofbrassandsteel，rivetedtogetherinseveralplaces，which，bythegreaterexpansionofbrassthansteelbyheatandcontractionbycold，becomesconvexonthebrasssideinhotweatherandconvexonthesteelsideincoldweather；whence，oneendbeingfixed，theotherendobtainsamotioncorrespondingwiththechangesofheatandcold，andthetwopinsattheend，betweenwhichthebalancespringpasses，andwhichitalternatelytouchesasthespringbendsandunbendsitself，willshortenorlengthenthespring，asthechangeofheatorcoldwouldotherwiserequiretobedonebyhandinthemannerusedforregulatingacommonwatch。“AlthoughthemethodhassincebeenimproveduponbyLeroy，Arnold，andEarnshaw，itwasthebeginningofallthathassincebeendoneintheperfectionofmarinechronometers。Indeed，itisamazingtothinkofthenumberofclever，skilful，andindustriousmenwhohavebeenengagedformanyhundredyearsintheproductionofthatexquisitefabric——sousefultoeverybody，whetherscientificorotherwise，onlandorseathemodernwatch。
　　ItisunnecessaryheretomentionindetailtheparticularsofHarrison’sinvention。Thesewerepublishedbyhimselfinhis’PrinciplesofMr。Harrison’sTimekeeper。’Itmay，however，bementionedthatheinventedamethodbywhichthechronometermightbekeptgoingwithoutlosinganyportionoftime。Thiswasduringtheprocessofwindingup，whichwasdoneonceinaday。
　　Whilethemainspringwasbeingwoundup，asecondaryonepreservedthemotionofthewheelsandkeptthemachinegoing。
　　Aftersevenyears’labour，duringwhichHarrisonencounteredandovercamenumerousdifficulties，heatlastcompletedhisfirstmarinechronometer。Heplaceditinasortofmoveableframe，somewhatresemblingwhatthesailorscalla’compassjumble，’butmuchmoreartificiallyandcuriouslymadeandarranged。InthisstatethechronometerwastriedfromtimetotimeinalargebargeontheriverHumber，inroughaswellasinsmoothweather，anditwasfoundtogoperfectly，withoutlosingamomentoftime。
　　SuchwastheconditionofHarrison’schronometerwhenhearrivedwithitinLondonin1735，inordertoapplytothecommissionersappointedforprovidingapublicrewardforthediscoveryofthelongitudeatsea。HefirstshowedittoseveralmembersoftheRoyalSociety，whocordiallyapprovedofit。Fiveofthemostprominentmembers——Dr。Bailey，Dr。Smith，Dr。Bradley，Mr。JohnMachin，andMr。GeorgeGraham——furnishedHarrisonwithacertificate，statingthattheprinciplesofhismachineformeasuringtimepromisedaverygreatandsufficientdegreeofexactness。Inconsequenceofthiscertificate，themachine，attherequestoftheinventor，andattherecommendationoftheLordsoftheAdmiralty，wasplacedonboardaman-of-war。
　　SirCharlesWager，thenfirstLordoftheAdmiralty，wrotetothecaptainoftheCenturion，statingthattheinstrumenthadbeenapprovedbymathematiciansasthebestthathadbeenmadeformeasuringtime；andrequestinghiskindtreatmentofMr。
　　Harrison，whowastoaccompanyittoLisbon。CaptainProctoransweredtheFirstLordfromSpithead，datedMayl7th，1736，promisinghisattentiontoHarrison’scomfort，butintimatinghisfearthathehadattemptedimpossibilities。Itisalwayssowithanewthing。Thefirststeam-engine，thefirstgaslight，thefirstlocomotive，thefirststeamboattoAmerica，thefirstelectrictelegraph，wereallimpossibilities！
　　ThisfirstchronometerbehavedverywellontheoutwardvoyageintheCenturion。Itwasnotaffectedbytheroughestweather，orbytheworkingoftheshipthroughtherollingwavesoftheBayofBiscay。Itwasbroughtback，withHarrison，intheOrfordman-of-war，whenitsgreatutilitywasprovedinaremarkablemanner，although，fromthevoyagebeingnearlyonameridian，theriskoflosingthelongitudewascomparativelysmall。Yetthefollowingwasthecertificateofthecaptainoftheship，datedthe24thJune，1737:“Whenwemadetheland，thesaidland，accordingtomyreckoningandothers，oughttohavebeentheStart；but，beforeweknewwhatlanditwas，JohnHarrisondeclaredtomeandtherestoftheship’scompanythat，accordingtohisobservationswithhismachine，itoughttobetheLizard——thewhich，indeed，itwasfoundtobe，hisobservationshowingtheshiptobemorewestthanmyreckoning，aboveonedegreeandtwenty-sixmiles，“——thatis，nearlyninetymilesoutofitscourse！
　　Sixdayslater——thatis，onthe30thJune——theBoardofLongitudemet，whenHarrisonwaspresent，andproducedthechronometerwithwhichhehadmadethevoyagetoLisbonandback。Theminutestates:“Mr。JohnHarrisonproducedanewinventedmachine，inthenatureofclockwork，wherebyheproposestokeeptimeatseawithmoreexactnessthanbyanyotherinstrumentormethodhithertocontrived，inordertothediscoveryofthelongitudeatsea；andproposestomakeanothermachineofsmallerdimensionswithinthespaceoftwoyears，wherebyhewillendeavourtocorrectsomedefectswhichhehathfoundinthatalreadyprepared，soastorenderthesamemoreperfect；whichmachine，whencompleted，heisdesirousofhavingtriedinoneofHisMajesty’sshipsthatshallbeboundtotheWestIndies；butatthesametimerepresentedthatheshouldnotbeable，byreasonofhisnecessitouscircumstances，togoonandfinishhissaidmachinewithoutassistance，andrequestedthathemaybefurnishedwiththesumof500L。，toputhiminacapacitytoperformthesame，andtomakeaperfectexperimentthereof。“
　　Theresultofthemeetingwasthat500L。wasorderedtobepaidtoHarrison，onemoietyassoonasconvenient，andtheotherwhenhehasproducedacertificatefromthecaptainofoneofHisMajesty’sshipsthathehasputthemachineonboardintothecaptain’spossession。Mr。GeorgeGraham，whowasconsulted，urgedthattheCommissionersshouldgrantHarrisonatleast1000L。，buttheyonlyawardedhimhalfthesum，andatfirstonlyamoietyoftheamountvoted。AttherecommendationofLordMonson，whowaspresent，Harrisonacceptedthe250L。asahelptowardstheheavyexpenseswhichhehadalreadyincurred，andwasagainabouttoincur，inperfectingtheinvention。Hewasinstructedtomakehisnewchronometeroflessdimensions，astheoneexhibitedwascumbersomeandheavy，andoccupiedtoomuchspaceonboard。
　　Heaccordinglyproceededtomakehissecondchronometer。Itoccupiedaspaceofonlyabouthalfthesizeofthefirst。Heintroducedseveralimprovements。Helessenedthenumberofthewheels，andtherebydiminishedfriction。Butthegeneralarrangementremainedthesame。Thissecondmachinewasfinishedin1739。Itwasmoresimpleinitsarrangement，andlesscumbrousinitsdimensions。Itansweredevenbetterthanthefirst，andthoughitwasnottriedatseaitsmotionsweresufficientlyexactforfindingthelongitudewithinthenearestlimitsproposedbyActofParliament。
　　Notsatisfiedwithhistwomachines，Harrisonproceededtomakeathird。Thiswasofanimprovedconstruction，andoccupiedstilllessspace，thewholeofthemachineanditsapparatusstandinguponanareaofonlyfoursquarefeet。ItwasinsuchforwardnessinJanuary，1741，thatitwasexhibitedbeforetheRoyalSociety，andtwelveofthemostprominentmemberssignedacertificateof“itsgreatandexcellentuse，aswellfordeterminingthelongitudeatseaasforcorrectingthechartsofthecoasts。“Thetestimonialconcluded:“WedorecommendMr。