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。