men of invention and industry-第21章

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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　from　the　vibrations　of　balances；　instead

of　those　of　a　pendulum　as　in　a　standing　clock。　　Mr。　Folkes；

President　of　the　Royal　Society；　when　presenting　the　gold　medal　to

Harrison　in　1749；　thus　describes　the　arrangement　of　his　new

machine。　　The　details　were　obtained　from　Harrison　himself；　who

was　present。　　He　had　made　use　of　two　balances　situated　in　the

same　plane；　but　vibrating　in　contrary　directions；　so　that　the　one

of　these　being　either　way　assisted　by　the　tossing　of　the　ship；

the　other　might　constantly　be　just　so　much　impeded　by　it　at　the

same　time。　　As　the　equality　of　the　times　of　the　vibrations　of　the

balance　of　a　pocket…watch　is　in　a　great　measure　owing　to　the

spiral　spring　that　lies　under　it；　so　the　same　was　here　performed

by　the　like　elasticity　of　four　cylindrical　springs　or　worms；

applied　near　the　upper　and　lower　extremities　of　the　two　balances

above　described。



Then　came　in　the　question　of　compensation。　　Harrison's　experience

with　the　compensation　pendulum　of　his　clock　now　proved　of　service

to　him。　　He　had　proceeded　to　introduce　a　similar　expedient　in　his

proposed　chronometer。　　As　is　well　known　to　those　who　are

acquainted　with　the　nature　of　springs　moved　by　balances；　the

stronger　those　springs　are；　the　quicker　the　vibrations　of　the

balances　are　performed；　and　vice　versa；　hence　it　follows　that

those　springs；　when　braced　by　cold；　or　when　relaxed　by　heat；　must

of　necessity　cause　the　timekeeper　to　go　either　faster　or　slower；

unless　some　method　could　be　found　to　remedy　the　inconvenience。



The　method　adopted　by　Harrison　was　his　compensation　balance；

doubtless　the　backbone　of　his　invention。　　His　〃thermometer　kirb；〃

he　himself　says；　〃is　composed　of　two　thin　plates　of　brass　and

steel；　riveted　together　in　several　places；　which；　by　the　greater

expansion　of　brass　than　steel　by　heat　and　contraction　by　cold；

becomes　convex　on　the　brass　side　in　hot　weather　and　convex　on　the

steel　side　in　cold　weather；　whence；　one　end　being　fixed；　the

other　end　obtains　a　motion　corresponding　with　the　changes　of　heat

and　cold；　and　the　two　pins　at　the　end；　between　which　the　balance

spring　passes；　and　which　it　alternately　touches　as　the　spring

bends　and　unbends　itself；　will　shorten　or　lengthen　the　spring；　as

the　change　of　heat　or　cold　would　otherwise　require　to　be　done　by

hand　in　the　manner　used　for　regulating　a　common　watch。〃　　Although

the　method　has　since　been　improved　upon　by　Leroy；　Arnold；　and

Earnshaw；　it　was　the　beginning　of　all　that　has　since　been　done　in

the　perfection　of　marine　chronometers。　　Indeed；　it　is　amazing　to

think　of　the　number　of　clever；　skilful；　and　industrious　men　who

have　been　engaged　for　many　hundred　years　in　the　production　of

that　exquisite　fabricso　useful　to　everybody；　whether　scientific

or　otherwise；　on　land　or　sea　the　modern　watch。



It　is　unnecessary　here　to　mention　in　detail　the　particulars　of

Harrison's　invention。　　These　were　published　by　himself　in　his

'Principles　of　Mr。　Harrison's　Timekeeper。'