men of invention and industry-第20章

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against　the　other；　that　the　two　clocks　kept　time　together　in

different　parts　of　his　house；　without　the　variation　of　more　than

a　single　second　in　the　month。　　One　of　them；　indeed；　which　he　kept

by　him　for　his　own　use；　and　constantly　compared　with　a　fixed

star；　did　not　vary　so　much　as　one　whole　minute　during　the　ten

years　that　he　continued　in　the　country　after　finishing　the

machine。＇6＇



Living；　as　he　did；　not　far　from　the　sea；　Harrison　next

endeavoured　to　arrange　his　timekeeper　for　purposes　of　navigation。



He　tried　his　clock　in　a　vessel　belonging　to　Barton…on…Humber；　but

his　compensating　pendulum　could　there　be　of　comparatively　little

use；　for　it　was　liable　to　be　tossed　hither　or　thither　by　the

sudden　motions　of　the　ship。　　He　found　it　necessary；　therefore；　to

mount　a　chronometer；　or　portable　timekeeper；　which　might　be　taken

from　place　to　place；　and　subjected　to　the　violent　and　irregular

motion　of　a　ship　at　sea；　without　affecting　its　rate　of　going。　　It

was　evident　to　him　that　the　first　mover　must　be　changed　from　a

weight　and　pendulum　to　a　spring　wound　up　and　a　compensating

balance。



He　now　applied　his　genius　in　this　direction。　　After　pondering

over　the　subject；　he　proceeded　to　London　in　1728；　and　exhibited

his　drawings　to　Dr。　Halley；　then　Astronomer…Royal。　　The　Doctor

referred　him　to　Mr。　George　Graham；　the　distinguished　horologer；

inventor　of　the　dead…beat　escapement　and　the　mercurial　pendulum。　

After　examining　the　drawings　and　holding　some　converse　with

Harrison；　Graham　perceived　him　to　be　a　man　of　uncommon　merit；　and

gave　him　every　encouragement。　　He　recommended　him；　however；　to

make　his　machine　before　again　applying　to　the　Board　of　Longitude。



Harrison　returned　home　to　Barrow　to　complete　his　task；　and　many

years　elapsed　before　he　again　appeared　in　London　to　present　his

first　chronometer。



The　remarkable　success　which　Harrison　had　achieved　in　his

compensating　pendulum　could　not　but　urge　him　on　to　further

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