HewasbornatFoulby，intheparishofWragby，nearPontefract，Yorkshire，inMarch，1693。Hisfather，HenryHarrison，wascarpenterandjoinertoSirRowlandWinn，owneroftheNostellPrioryestate。Thepresenthousewasbuiltbythebaronetonthesiteoftheancientpriory。HenryHarrisonwasasortofretainerofthefamily，andlongcontinuedintheirService。
　　Littleisknownoftheboy’seducation。Itwascertainlyofaveryinferiordescription。LikeGeorgeStephenson，Harrisonalwayshadagreatdifficultyinmakinghimselfunderstood，eitherbyspeechorwriting。Indeed，everyboard-schoolboynowreceivesabettereducationthanJohnHarrisondidahundredandeightyyearsago。Buteducationdoesnotaltogethercomebyreadingandwriting。Theboywaspossessedofvigorousnaturalabilities。Hewasespeciallyattractedbyeverymachinethatmoveduponwheels。Theboywas’fathertotheman。’Whensixyearsold，andlyingsickofsmall-pox，agoingwatchwasplaceduponhispillow，whichaffordedhiminfinitedelight。
　　WhensevenyearsoldhewastakenbyhisfathertoBarrow，nearBarton-on-Humber，whereSirRowlandWinnhadanotherresidenceandestate。HenryHarrisonwasstillactingasthebaronet’scarpenterandjoiner。IncourseoftimeyoungHarrisonjoinedhisfatherintheworkshop，andprovedofgreatusetohim。Hisopportunitiesforacquiringknowledgewerestillveryfew，butheappliedhispowersofobservationandhisworkmanshipuponthethingswhichwerenearesthim。Heworkedinwood，andtowoodhefirstturnedhisattention。
　　Hewasstillfondofmachinesgoinguponwheels。Hehadenjoyedthesightofthebigwatchgoinguponbrasswheelswhenhewasaboy；but，nowthathewasaworkmaninwood，heproposedtomakeaneight-dayclock，withwheelsofthismaterial。Hemadetheclockin1713，whenhewastwentyyearsold，[4]sothathemusthavemadediligentuseofhisopportunities。Hehadofcoursedifficultiestoencounter，andnothingcanbeaccomplishedwithoutthem；foritisdifficultiesthattrainthehabitsofapplicationandperseverance。Buthesucceededinmakinganeffectiveclock，whichcountedthetimewithregularity。Thisclockisstillinexistence。ItistobeseenattheMuseumofPatents，SouthKensington；andwhenwevisiteditafewmonthsagoitwasgoing，andstillmarkingthemomentsastheypassed。
　　Itiscontainedinacaseaboutsixfeethigh，withaglassfront，showingapendulumandtwoweights。Overtheclockisthefollowinginscription:
　　“ThisclockwasmadeatBarrow，Lincolnshire，intheyear1715，byJohnHarrison，celebratedastheinventorofanauticaltimepiece，orchronometer，whichgainedtherewardof20，000L。，offeredbytheBoardofLongitude，A。D。1767。
　　“Thisclockstrikesthehour，indicatesthedayofthemonth，andwithoneexceptiontheescapementthewheelsareentirelymadeofwood。“
　　This，however，wasonlyabeginning。Harrisonproceededtomakebetterclocks；andthenhefounditnecessarytointroducemetal，whichwasmorelasting。Hemadepivotsofbrass，whichmovedmoreconvenientlyinsocketsofwoodwiththeuseofoil。Healsocausedtheteethofhiswheelstorunagainstcylindricalrollersofwood，fixedbybrasspins，ataproperdistancefromtheaxisofthepinions；andthustoaconsiderableextentremovedtheinconveniencesoffriction。
　　InthemeantimeHarrisoneagerlyimprovedeveryincidentfromwhichhemightderivefurtherinformation。TherewasaclergymanwhocameeverySundaytothevillagetoofficiateintheneighbourhood；andhavingheardofthesedulousapplicationoftheyoungcarpenter，helenthimamanuscriptcopyofProfessorSaunderson’sdiscourses。Thatblindprofessorhadpreparedseverallecturesonnaturalphilosophyfortheuseofhisstudents，thoughtheywerenotintendedforpublication。YoungHarrisonnowproceededtocopythemout，togetherwiththediagrams。Sometimes，indeed，hespentthegreaterpartofthenightinwritingordrawing。
　　Aspartofhisbusiness，heundertooktosurveyland，andtorepairclocksandwatches，besidescarryingonhistradeofacarpenter。Hesoonobtainedaconsiderableknowledgeofwhathadbeendoneinclocksandwatches，andwasabletodonotonlywhatthebestprofessionalworkershaddone，buttostrikeoutentirelynewlightsintheclockandwatch-makingbusiness。Hefoundoutamethodofdiminishingfrictionbyaddingajointtothepalletsofthependulum，wherebytheyweremadetoworkinthenatureofrollersofalargeradius，withoutanysliding，asusual，upontheteethofthewheel。Heconstructedaclockontherecoilingprinciple，whichwentperfectly，andneverlostaminutewithinfourteenyears。SirEdmundDenisonBeckettsaysthatheinventedthismethodinordertosavehimselfthetroubleofgoingsofrequentlytooiltheescapementofaturretclock，ofwhichhehadcharge；thoughtherewereotherinfluencesatworkbesidesthis。
　　Buthismostimportantinvention，atthisearlyperiodofhislife，washiscompensationpendulum。Everyoneknowsthatmetalsexpandwithheatandcontractbycold。Thependulumoftheclockthereforeexpandedinsummerandcontractedinwinter，therebyinterferingwiththeregulargoingoftheclock。Huygenshadbyhiscylindricalchecksremovedthegreatirregularityarisingfromtheunequallengthsoftheoscillations；butthependulumwasaffectedbythetossingofashipatsea，andwasalsosubjecttoavariationinweight，dependingontheparalleloflatitude。Graham，thewell-knownclock-maker，inventedthemercurialcompensationpendulum，consistingofaglassorironjarfilledwithquicksilverandfixedtotheendofthependulumrod。Whentherodwaslengthenedbyheat，thequicksilverandthejarwhichcontaineditweresimultaneouslyexpandedandelevated，andthecentreofoscillationwasthuscontinuedatthesamedistancefromthepointofsuspension。
　　Butthedifficulty，toacertainextent，remainedunconquereduntilHarrisontookthematterinhand。Heobservedthatallrodsofmetaldonotaltertheirlengthsequallybyheat，or，onthecontrary，becomeshorterbycold，butsomemoresensiblythanothers。AfterinnumerableexperimentsHarrisonatlengthcomposedaframesomewhatresemblingagridiron，inwhichthealternatebarswereofsteelandofbrass，andsoarrangedthatthosewhichexpandedthemostwerecounteractedbythosewhichexpandedtheleast。Bythismeansthependulumcontainedthepowerofequalisingitsownaction，andthecentreofoscillationcontinuedatthesameabsolutedistancefromthepointofsuspensionthroughallthevariationsofheatandcoldduringtheyear。[5]
　　Thusbytheyear1726，whenhewasonlythirty-threeyearsold，Harrisonhadfurnishedhimselfwithtwocompensationclocks，inwhichalltheirregularitiestowhichthesemachinesweresubject，wereeitherremovedorsohappilybalanced，onemetalagainsttheother，thatthetwoclockskepttimetogetherindifferentpartsofhishouse，withoutthevariationofmorethanasinglesecondinthemonth。Oneofthem，indeed，whichhekeptbyhimforhisownuse，andconstantlycomparedwithafixedstar，didnotvarysomuchasonewholeminuteduringthetenyearsthathecontinuedinthecountryafterfinishingthemachine。[6]
　　Living，ashedid，notfarfromthesea，Harrisonnextendeavouredtoarrangehistimekeeperforpurposesofnavigation。
　　HetriedhisclockinavesselbelongingtoBarton-on-Humber；buthiscompensatingpendulumcouldtherebeofcomparativelylittleuse；foritwasliabletobetossedhitherorthitherbythesuddenmotionsoftheship。Hefounditnecessary，therefore，tomountachronometer，orportabletimekeeper，whichmightbetakenfromplacetoplace，andsubjectedtotheviolentandirregularmotionofashipatsea，withoutaffectingitsrateofgoing。Itwasevidenttohimthatthefirstmovermustbechangedfromaweightandpendulumtoaspringwoundupandacompensatingbalance。
　　Henowappliedhisgeniusinthisdirection。Afterponderingoverthesubject，heproceededtoLondonin1728，andexhibitedhisdrawingstoDr。Halley，thenAstronomer-Royal。TheDoctorreferredhimtoMr。GeorgeGraham，thedistinguishedhorologer，inventorofthedead-beatescapementandthemercurialpendulum。
　　AfterexaminingthedrawingsandholdingsomeconversewithHarrison，Grahamperceivedhimtobeamanofuncommonmerit，andgavehimeveryencouragement。Herecommendedhim，however，tomakehismachinebeforeagainapplyingtotheBoardofLongitude。
　　HarrisonreturnedhometoBarrowtocompletehistask，andmanyyearselapsedbeforeheagainappearedinLondontopresenthisfirstchronometer。
　　TheremarkablesuccesswhichHarrisonhadachievedinhiscompensatingpendulumcouldnotbuturgehimontofurtherexperiments。Hewasnodoubttoacertainextentinfluencedbytherewardof20，000L。whichtheEnglishGovernmenthadofferedforaninstrumentthatshouldenablethelongitudetobemoreaccuratelydeterminedbynavigatorsatseathanwasthenpossible；anditwaswiththeobjectofobtainingpecuniaryassistancetoassisthimincompletinghischronometerthatHarrisonhad，in1728，madehisfirstvisittoLondontoexhibithisdrawings。
　　TheActofParliamentofferingthissuperbrewardwaspassedin1714，fourteenyearsbefore，butnoattempthadbeenmadetoclaimit。ItwasrightthatEngland，thenrapidlyadvancingtothefirstpositionasacommercialnation，shouldmakeeveryefforttorendernavigationlesshazardous。Beforecorrectchronometerswereinvented，orgoodlunartableswereprepared，[7]theship，whenfairlyatsea，outofsightofland，andbattlingwiththewindsandtides，wasinameasurelost。Nomethodexistedforaccuratelyascertainingthelongitude。Theshipmightbeoutofitscourseforoneortwohundredmiles，foranythingthatthenavigatorknew；andonlythewreckofhisshiponsomeunknowncoasttoldofthemistakethathehadmadeinhisreckoning。
　　Itmayherebementionedthatitwascomparativelyeasytodeterminethelatitudeofashipatseaeverydaywhenthesunwasvisible。Thelatitude——thatis，thedistanceofanyspotfromtheequatorandthepole——mightbefoundbyasimpleobservationwiththesextant。Thealtitudeofthesunatnoonisfound，andbyashortcalculationthepositionoftheshipcanbeascertained。
　　Thesextant，whichistheinstrumentuniversallyusedatsea，wasgraduallyevolvedfromsimilarinstrumentsusedfromtheearliesttimes。Theobjectofthisinstrumenthasalwaysbeentofindtheangulardistancebetweentwobodies——thatistosay，theanglecontainedbytwostraightlines，drawnfromthosebodiestomeetintheobserver’seye。Thesimplestinstrumentofthiskindmaybewellrepresentedbyapairofcompasses。Ifthehingeisheldtotheeye，onelegpointedtothedistanthorizon，andtheotherlegpointedtothesun，thepositionofthetwolegswillshowtheangulardistanceofthesunfromthehorizonatthemomentofobservation。
　　Untiltheendoftheseventeenthcentury，theinstrumentusedwasofthissimplekind。Itwasgenerallyalargequadrant，withoneortwobarsmovingonahinge，——toallintentsandpurposesahugepairofcompasses。Thedirectionofthesightwasfixedbytheuseofaslitandapointer，muchasintheordinaryrifle。
　　Thisinstrumentwasvastlyimprovedbytheuseofatelescope，whichnotonlyallowedfainterobjectstobeseen，butespeciallyenabledthesighttobeaccuratelydirectedtotheobjectobserved。
　　Theinstrumentsofthepre-telescopicagereachedtheirgloryinthehandsofTychoBrahe。Heusedmagnificentinstrumentsofthesimple“pairofcompasses“kind——circles，quadrants，andsextants。Thesewereforthemostpartponderousfixedinstrumentsoflittleornouseforthepurposesofnavigation。
　　ButTychoBrahe’ssextantprovedtheforerunnerofthemoderninstrument。Thegeneralstructureisthesame；butthevastimprovementofthemodernsextantisdue，firstly，totheuseofthereflectingmirror，and，secondly，totheuseofthetelescopeforaccuratesighting。Theseimprovementswereduetomanyscientificmen——toWilliamGascoigne，whofirstusedthetelescope，about1640；toRobertHooke，who，in1660，proposedtoapplyittothequadrant；toSirIsaacNewton，whodesignedareflectingquadrant；[8]andtoJohnHadley，whointroducedit。
　　ThemodernsextantismerelyamodificationofNewton’sorBadley’squadrant，anditspresentconstructionseemstobeperfect。
　　Itthereforebecamepossibleaccuratelytodeterminethepositionofashipatseaasregardeditslatitude。Butitwasquitedifferentasregardedthelongitudethatis，thedistanceofanyplacefromagivenmeridian，eastwardorwestward。Inthecaseoflongitudethereisnofixedspottowhichreferencecanbemade。Therotationoftheearthmakestheexistenceofsuchaspotimpossible。ThequestionoflongitudeispurelyaquestionofTIME。Thecircuitoftheglobe，eastandwest，issimplyrepresentedbytwenty-fourhours。Eachplacehasitsowntime。
　　Itisveryeasytodeterminethelocaltimeatanyspotbyobservationsmadeatthatspot。But，astimeisalwayschanging，theknowledgeofthelocaltimegivesnoideaoftheactualposition；andstilllessofamovingobject——say，ofashipatsea。Butif，inanylocality，weknowthelocaltime，andalsothelocaltimeofsomeotherlocalityatthatmoment——say，oftheObservatoryatGreenwichwecan，bycomparingthetwolocaltimes，determinethedifferenceoflocaltimes，or，whatisthesamething，thedifferenceoflongitudebetweenthetwoplaces。