men of invention and industry-第15章

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experiments。　　He　was　no　doubt　to　a　certain　extent　influenced　by

the　reward　of　20；000L。　which　the　English　Government　had　offered

for　an　instrument　that　should　enable　the　longitude　to　be　more

accurately　determined　by　navigators　at　sea　than　was　then

possible；　and　it　was　with　the　object　of　obtaining　pecuniary

assistance　to　assist　him　in　completing　his　chronometer　that

Harrison　had；　in　1728；　made　his　first　visit　to　London　to　exhibit

his　drawings。



The　Act　of　Parliament　offering　this　superb　reward　was　passed　in

1714；　fourteen　years　before；　but　no　attempt　had　been　made　to

claim　it。　　It　was　right　that　England；　then　rapidly　advancing　to

the　first　position　as　a　commercial　nation；　should　make　every

effort　to　render　navigation　less　hazardous。　　Before　correct

chronometers　were　invented；　or　good　lunar　tables　were

prepared；＇7＇　the　ship；　when　fairly　at　sea；　out　of　sight　of　land；

and　battling　with　the　winds　and　tides；　was　in　a　measure　lost。　　No

method　existed　for　accurately　ascertaining　the　longitude。　　The

ship　might　be　out　of　its　course　for　one　or　two　hundred　miles；　for

anything　that　the　navigator　knew；　and　only　the　wreck　of　his　ship

on　some　unknown　coast　told　of　the　mistake　that　he　had　made　in　his

reckoning。



It　may　here　be　mentioned　that　it　was　comparatively　easy　to

determine　the　latitude　of　a　ship　at　sea　every　day　when　the　sun

was　visible。　　The　latitudethat　is；　the　distance　of　any　spot

from　the　equator　and　the　polemight　be　found　by　a　simple

observation　with　the　sextant。　　The　altitude　of　the　sun　at　noon　is

found；　and　by　a　short　calculation　the　position　of　the　ship　can　be

ascertained。



The　sextant；　which　is　the　instrument　universally　used　at　sea；　was

gradually　evolved　from　similar　instruments　used　from　the　earliest

times。　　The　object　of　this　instrument　has　always　been　to　find　the

angular　distance　between　two　bodiesthat　is　to　say；　the　angle

contained　by　two　straight　lines；　drawn　from　those　bodies　to　meet

in　the　observer's　eye。　　The　simplest　instrument　of　this　kind　may

be　well　represented　by　a　pair　of　compasses。　　If　the　hinge　is　held

to　the　eye；　one　leg　pointed　to　the　distant　horizon；　and　the　other

leg　pointed　to　the　sun；　the　position　of　the　two　legs　will　show

the　angular　distance　of　the　sun　from　the　horizon　at　the　moment　of

observation。



Until　the　end　of　the　seventeenth　century；　the　instrument　used　was

of　this　simple　kind。　　It　was　generally　a　large　quadrant；　with　one

or　two　bars　moving　on　a　hinge；to　all　intents　and　purposes　a

huge　pair　of　compasses。　　The　direction　of　the　sight　was　fixed　by

the　use　of　a　slit　and　a　pointer；　much　as　in　the　ordinary　rifle。　

This　instrument　was　vastly　improved　by　the　use　of　a　telescope；

which　not　only　allowed　fainter　objects　to　be　seen；　but　especially

enabled　the　sight　to　be　accurately　directed　to　the　object

observed。



The　instruments　of　the　pre…telescopic　age　reached　their　glory　in

the　hands　of　Tycho　Brahe。　　He　used　magnificent　instruments　of　the

simple　〃pair　of　compasses〃　kindcircles；　quadrants；　and

sextants。　　These　were　for　the　most　part　ponderous　fixed

instruments　of　little　or　no　use　for　the　purposes　of　navigation。　

But　Tycho　Brahe's　sextant　proved　the　forerunner　of　the　modern

instrument。　　The　general　structure　is　the　same；　but　the　vast

improvement　of　the　modern　sextant　is　due；　firstly；　to　the　use　of

the　reflecting　mirror；　and；　secondly；　to　the　use　of　the　telescope

for　accurate　sighting。　　These　improvements　were　due　to　many

scientific　mento　William　Gascoigne；　who　first　used　the

telescope；　about　1640；　to　Robert　Hooke；　who；　in　1660；　proposed　to

apply　it　to　the　quadrant；　to　Sir　Isaac　Newton；　who　designed　a

reflecting　quadrant；＇8＇　and　to　John　Hadley；　who　introduced　it。　

The　modern　sextant　is　merely　a　modification　of　Newton's　or

Badley's　quadrant；　and　its　present　construction　seems　to　be

perfect。



It　therefore　became　possible　accurately　to　determine　the　position

of　a　ship　at　sea　as　regarded　its　latitude。　　But　it　was　quite

different　as　regarded　the　longitude　that　is；　the　distance　of　any

place　from　a　given　meridian；　eastward　or　westward。　　In　the　case

of　longitude　there　is　no　fixed　spot　to　which　reference　can　be

made。　　The　rotation　of　the　earth　makes　the　existence　of　such　a

spot　impossible。　　The　question　of　longitude　is　purely　a　question

of　TIME。　　The　circuit　of　the　globe；　east　and　west；　is　simply

represented　by　twenty…four　hours。　　Each　place　has　its　own　time。　

It　is　very　easy　to　determine　the　local　time　at　any　spot　by

observations　made　at　that　spot。　　But；　as　time　is　always　changing；

the　knowledge　of　the　local　time　gives　no　idea　of　the　actual

position；　and　still　less　of　a　moving　objectsay；　of　a　ship　at

sea。　　But　if；　in　any　locality；　we　know　the　local　time；　and　also

the　local　time　of　some　other　locality　at　that　momentsay；　of　the

Observatory　at　Greenwich　we　can；　by　comparing　the　two　local

times；　determine　the　difference　of　local　times；　or；　what　is　the

same　thing；　the　difference　of　longitude　between　the　two　places。　

It　was　necessary　therefore　for　the　navigator　to　be　in　possession

of　a　first…rate　watch　or　chronometer；　to　enable　him　to　determine

accurately　the　position　of　his　ship　at　sea；　as　respected　the

longitude。



Before　the　middle　of　the　eighteenth　century　good　watches　were

comparatively　unknown。　　The　navigator　mainly　relied；　for　his

approximate　longitude；　upon　his　Dead　Reckoning；　without　any

observation　of　the　heavenly　bodies。　　He　depended　upon　the

accuracy　of　the　course　which　he　had　steered　by　the　compass；　and

the　mensuration　of　the　ship's　velocity　by　an　instrument　called

the　Log；　as　well　as　by　combining　and　rectifying　all　the

allowances　for　drift；　lee…way；　and　so　on；　according　to　the　trim

of　the　ship；　but　all　of　these　were　liable　to　much　uncertainty；

especially　when　the　sea　was　in　a　boisterous　condition。　　There　was

another　and　independent　course　which　might　have　been

adoptedthat　is；　by　observation　of　the　moon；　which　is　constantly

moving　amongst　the　stars　from　west　to　east。　　But　until　the　middle

of　the　eighteenth　century　good　lunar　tables　were　as　much　unknown

as　good　watches。



Hence　a　method　of　ascertaining　the　longitude；　with　the　same

degree　of　accuracy　which　is　attainable　in　respect　of　latitude；

had　for　ages　been　the　grand　desideratum　for　men　〃who　go　down　to

the　sea　in　ships。〃　　Mr。　Macpherson；　in　his　important　work

entitled　'The　Annals　of　Commerce；'　observes；　〃Since　the　year

1714；　when　Parliament　offered　a　reward　of　20；000L。　for　the　best

method　of　ascertaining　the　longitude　at　sea；　many　schemes　have

been　devised；　but　all　to　little　or　no　purpose；　as　going　generally

upon　wrong　principles；　till　that　heaven…taught　artist　Mr。　John

Harrison　arose；〃　and　by　him；　as　Mr。　Macpherson　goes　on　to　say；

the　difficulty　was　conquered；　having　devoted　to　it　〃the　assiduous

studies　of　a　long　life。〃



The　preamble　of　the　Act　of　Parliament　in　question　runs　as

follows：　〃Whereas　it　is　well　known　by　all　that　are　acquainted

with　the　art　of　navigation　that　nothing　is　so　much　wanted　and

desired　at　sea　as　the　discovery　of　the　longitude；　for　the　safety

and　quickness　of　voyages；　the　preservation　of　ships　and　the　lives

of　men；〃　and　so　on。　　The　Act　proceeds　to　constitute　certain

persons　commissioners　for　the　discovery　of　the　longitude；　with

power　to　receive　and　experiment　upon　proposals　for　that　purpose；

and　to　grant　sums　of　money　not　exceeding　2000L。　to　aid　in　such

experiments。　　It　will　be　remembered　from　what　has　been　above

stated；　that　a　reward　of　10；000L。　was　to　be　given　to　the　person

who　should　contrive　a　method　of　determining　the　longitude　within

one　degree　of　a　great　circle；　or　60　geographical　miles；　15；000L。　

within　40　geographical　miles；　and　20；000L。　within　30　geographical

miles。



It　will；　in　these　days；　be　scarcely　believed　that　little　more

than　a　hundred　and　fifty　years　ago　a　prize　of　not　less　than　ten

thousand　pounds　should　have　been　offered　for　a　method　of

determining　the　longitude　within　sixty　miles；　and　that　double　the

amount　should　have　been　offered　for　a　method　of　determining　it

within　thirty　miles！　　The　amount　of　these　rewards　is　sufficient

proof　of　the　fearful　necessity　for　improvement　which　then　existed

in　the　methods　of　navigation。　　And　yet；　from　the　date　of　the

passing　of　the　Act　in　1714　until　the　year　1736；　when　Harrison

finished　his　first　timepiece；　nothing　had　been　done　towards

ascertaining　the　longitude　more　accurately；　even　within　the　wide

limits　specified　by　the　Act　of　Parliament。　　Although　several

schemes　had　been　projected；　none　of　them　had　proved　successful；

and　the　offered　rewards　therefore　still　remained　unclaimed。



To　return　to　Harrison。　　After　reaching　his　home　at　Barrow；　after

his　visit　to　London　in　1728；　he　began　his　experiments　for　the

construction　of　a　marine　chronometer。　　The　task　was　one　of　no

small　difficulty。　　It　was　necessary　to　provide　against

irregularities　arising　from　the　motion　of　a　ship　at　sea；　and　to

obviate　the　effect　of　alternations　of　temperature　in　the　machine

itself；　as　well　as　the　oil　with　which　it　was　lubricated。　　A

thousand　obstacles　presented　themselves；　but　they　were　not　enough

to　deter　Harrison　from　grappling　with　the　work　he　had　set　himself

to　perform。



Every　one　knows　the　beautiful　machinery　of　a　timepiece；　and　the

perfect　tools　required　to　produce　such　a　machine。　　Some　of　these

tools　Harrison　procured　in　London；　but　the　greater　number　he

provided　for　himself；　and　many　entirely　new　adaptations　were

required　for　his　chronometer。　　As　wood　could　no　longer　be

exclusively　employed；　as　in　his　first　clock；　he　had　to　teach

himself　to　work　accurately　and　minutely　in　brass　and　other

metals。　　Having　been　unable　to　obtain　any　assistance　from　the

Board　of　Longitude；　he　was　under　the　necessity；　while　carrying

forward　his　experiments；　of　maintaining　himself　by　still　working

at　his　trade　of　a　carpenter　and　joiner。　　This　will　account　for

the　very　long　period　that　elapsed　before　he　could　bring　his

chronometer　to　such　a　state　as　that　it　might　be　tried　with　any

approach　to　certainty　in　its　operations。



Harrison；　besides　his　intentness　and　earnestness；　was　a　cheerful

and　hopeful　man。　　He　had　a　fine　taste　for　music；　and　organised

and　led　the　choir　of　the　village　church；　which　attained　a　high

degree　of　perfection。　　He　invented　a　curious　monochord；　which　was

not　less　accurate　than　his　clocks　in　the　mensuration　of　time。　

His　ear　was　distressed　by　the　ringing　of　bells　out　of　tune；　and

he　set　himself　to　remedy　them。　　At　the　parish　church　of　Hull；　for

instance；　the　bells　were　harsh　and　disagreeable；　and　by　the

authority　of　the　vicar　and　churchwardens　he　was　allowed　to　put

them　into　a　state　of　exact　tune；　so　that　they　proved　entirely

melodious。



But　the　great　work　of　his　life　was　his　marine　chronometer。　　He

found　it　necessary；　in　the　first　place；　to　alter　the　first　mover

of　his　clock　to　a　spring　wound　up；　so　that　the　regularity　of　the

motion　might　be　derived　f