a history of science-1-第46节

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his　is　more　distinctly　proved　from　observations　of　the　sea　and　sky；　for　here　the　evidence　of　the　senses　and　common　observation　is　alone　requisite。　The　convexity　of　the　sea　is　a　further　proof　of　this　to　those　who　have　sailed；　for　they　cannot　perceive　lights　at　a　distance　when　placed　at　the　same　level　as　their　eyes；　and　if　raised　on　high　they　at　once　become　perceptible　to　vision　though　at　the　same　time　farther　removed。　So　when　the　eye　is　raised　it　sees　what　before　was　utterly　imperceptible。　Homer　speaks　of　this　when　he　says：

〃　'Lifted　up　on　the　vast　wave　he　quickly　beheld　afar。'　Sailors　as　they　approach　their　destination　behold　the　shore　continually　raising　itself　to　their　view；　and　objects　which　had　at　first　seemed　low　begin　to　lift　themselves。　Our　gnomons；　also；　are；　among　other　things；　evidence　of　the　revolution　of　the　heavenly　bodies；　and　common…sense　at　once　shows　us　that　if　the　depth　of　the　earth　were　infinite　such　a　revolution　could　not　take　place。〃＇1＇　Elsewhere　Strabo　criticises　Eratosthenes　for　having　entered　into　a　long　discussion　as　to　the　form　of　the　earth。　This　matter；　Strabo　thinks；　〃should　have　been　disposed　of　in　the　compass　of　a　few　words。〃　Obviously　this　doctrine　of　the　globe's　sphericity　had；　in　the　course　of　600　years；　become　so　firmly　established　among　the　Greek　thinkers　as　to　seem　almost　axiomatic。　We　shall　see　later　on　how　the　Western　world　made　a　curious　recession　from　this　seemingly　secure　position　under　stimulus　of　an　Oriental　misconception。　As　to　the　size　of　the　globe；　Strabo　is　disposed　to　accept　without　particular　comment　the　measurements　of　Eratosthenes。　He　speaks；　however；　of　〃more　recent　measurements；〃　referring　in　particular　to　that　adopted　by　Posidonius；　according　to　which　the　circumference　is　only　about　one　hundred　and　eighty　thousand　stadia。　Posidonius；　we　may　note　in　passing；　was　a　contemporary　and　friend　of　Cicero；　and　hence　lived　shortly　before　the　time　of　Strabo。　His　measurement　of　the　earth　was　based　on　observations　of　a　star　which　barely　rose　above　the　southern　horizon　at　Rhodes　as　compared　with　the　height　of　the　same　star　when　observed　at　Alexandria。　This　measurement　of　Posidonius；　together　with　the　even　more　famous　measurement　of　Eratosthenes；　appears　to　have　been　practically　the　sole　guide　as　to　the　size　of　the　earth　throughout　the　later　periods　of　antiquity；　and；　indeed；　until　the　later　Middle　Ages。　As　becomes　a　writer　who　is　primarily　geographer　and　historian　rather　than　astronomer；　Strabo　shows　a　much　keener　interest　in　the　habitable　portions　of　the　globe　than　in　the　globe　as　a　whole。　He　assures　us　that　this　habitable　portion　of　the　earth　is　a　great　island；　〃since　wherever　men　have　approached　the　termination　of　the　land；　the　sea；　which　we　designate　ocean；　has　been　met　with；　and　reason　assures　us　of　the　similarity　of　this　place　which　our　senses　have　not　been　tempted　to　survey。〃　He　points　out　that　whereas　sailors　have　not　circumnavigated　the　globe；　that　they　had　not　been　prevented　from　doing　so　by　any　continent；　and　it　seems　to　him　altogether　unlikely　that　the　Atlantic　Ocean　is　divided　into　two　seas　by　narrow　isthmuses　so　placed　as　to　prevent　circumnavigation。　〃How　much　more　probable　that　it　is　confluent　and　uninterrupted。　This　theory；〃　he　adds；　〃goes　better　with　the　ebb　and　flow　of　the　ocean。　Moreover　（and　here　his　reasoning　becomes　more　fanciful）；　the　greater　the　amount　of　moisture　surrounding　the　earth；　the　easier　would　the　heavenly　bodies　be　supplied　with　vapor　from　thence。〃　Yet　he　is　disposed　to　believe；　following　Plato；　that　the　tradition　〃concerning　the　island　of　Atlantos　might　be　received　as　something　more　than　idle　fiction；　it　having　been　related　by　Solon；　on　the　authority　of　the　Egyptian　priests；　that　this　island；　almost　as　large　as　a　continent；　was　formerly　in　existence　although　now　it　had　disappeared。〃＇2＇　In　a　word；　then；　Strabo　entertains　no　doubt　whatever　that　it　would　be　possible　to　sail　around　the　globe　from　Spain　to　India。　Indeed；　so　matter…of…fact　an　inference　was　this　that　the　feat　of　Columbus　would　have　seemed　less　surprising　in　the　first　century　of　our　era　than　it　did　when　actually　performed　in　the　fifteenth　century。　The　terrors　of　the　great　ocean　held　the　mariner　back；　rather　than　any　doubt　as　to　where　he　would　arrive　at　the　end　of　the　voyage。　Coupled　with　the　idea　that　the　habitable　portion　of　the　earth　is　an　island；　there　was　linked　a　tolerably　definite　notion　as　to　the　shape　of　this　island。　This　shape　Strabo　likens　to　a　military　cloak。　The　comparison　does　not　seem　peculiarly　apt　when　we　are　told　presently　that　the　length　of　the　habitable　earth　is　more　than　twice　its　breadth。　This　idea；　Strabo　assures　us；　accords　with　the　most　accurate　observations　〃both　ancient　and　modern。〃　These　observations　seemed　to　show　that　it　is　not　possible　to　live　in　the　region　close　to　the　equator；　and　that；　on　the　other　hand；　the　cold　temperature　sharply　limits　the　habitability　of　the　globe　towards　the　north。　All　the　civilization　of　antiquity　clustered　about　the　Mediterranean；　or　extended　off　towards　the　east　at　about　the　same　latitude。　Hence　geographers　came　to　think　of　the　habitable　globe　as　having　the　somewhat　lenticular　shape　which　a　crude　map　of　these　regions　suggests。　We　have　already　had　occasion　to　see　that　at　an　earlier　day　Anaxagoras　was　perhaps　influenced　in　his　conception　of　the　shape　of　the　earth　by　this　idea；　and　the　constant　references　of　Strabo　impress　upon　us　the　thought　that　this　long；　relatively　narrow　area　of　the　earth's　surface　is　the　only　one　which　can　be　conceived　of　as　habitable。　Strabo　had　much　to　tell　us　concerning　zones；　which；　following　Posidonius；　he　believes　to　have　been　first　described　by　Parmenides。　We　may　note；　however；　that　other　traditions　assert　that　both　Thales　and　Pythagoras　had　divided　the　earth　into　zones。　The　number　of　zones　accepted　by　Strabo　is　five；　and　he　criticises　Polybius　for　making　the　number　six。　The　five　zones　accepted　by　Strabo　are　as　follows：　the　uninhabitable　torrid　zone　lying　in　the　region　of　the　equator；　a　zone　on　either　side　of　this　extending　to　the　tropic；　and　then　the　temperate　zones　extending　in　either　direction　from　the　tropic　to　the　arctic　regions。　There　seems　to　have　been　a　good　deal　of　dispute　among　the　scholars　of　the　time　as　to　the　exact　arrangement　of　these　zones；　but　the　general　idea　that　the　north…temperate　zone　is　the　part　of　the　earth　with　which　the　geographer　deals　seemed　clearly　established。　That　the　south…temperate　zone　would　also　present　a　habitable　area　is　an　idea　that　is　sometimes　suggested；　though　seldom　or　never　distinctly　expressed。　It　is　probable　that　different　opinions　were　held　as　to　this；　and　no　direct　evidence　being　available；　a　cautiously　scientific　geographer　like　Strabo　would　naturally　avoid　the　expression　of　an　opinion　regarding　it。　Indeed；　his　own　words　leave　us　somewhat　in　doubt　as　to　the　precise　character　of　his　notion　regarding　the　zones。　Perhaps　we　shall　do　best　to　quote　them：　〃Let　the　earth　be　supposed　to　consist　of　five　zones。　（1）　The　equatorial　circle　described　around　it。　（2）　Another　parallel　to　this；　and　defining　the　frigid　zone　of　the　northern　hemisphere。　（3）　A　circle　passing　through　the　poles　and　cutting　the　two　preceding　circles　at　right…　angles。　The　northern　hemisphere　contains　two　quarters　of　the　earth；　which　are　bounded　by　the　equator　and　circle　passing　through　the　poles。　Each　of　these　quarters　should　be　supposed　to　contain　a　four…sided　district；　its　northern　side　being　of　one…half　of　the　parallel　next　the　pole；　its　southern　by　the　half　of　the　equator；　and　its　remaining　sides　by　two　segments　of　the　circle　drawn　through　the　poles；　opposite　to　each　other；　and　equal　in　length。　In　one　of　these　（which　of　them　is　of　no　consequence）　the　earth　which　we　inhabit　is　situated；　surrounded　by　a　sea　and　similar　to　an　island。　This；　as　we　said　before；　is　evident　both　to　our　senses　and　to　our　reason。　But　let　any　one　doubt　this；　it　makes　no　difference　so　far　as　geography　is　concerned　whether　you　believe　the　portion　of　the　earth　which　we　inhabit　to　be　an　island　or　only　admit　what　we　know　from　experience　namely；　that　whether　you　start　from　the　east　or　the　west　you　may　sail　all　around　it。　Certain　intermediate　spaces　may　have　been　left　（unexplored）；　but　these　are　as　likely　to　be　occupied　by　sea　as　uninhabited　land。　The　object　of　the　geographer　is　to　describe　known　countries。　Those　which　are　unknown　he　passes　over　equally　with　those　beyond　the　limits　of　the　inhabited　earth。　It　will；　therefore；　be　sufficient　for　describing　the　contour　of　the　island　we　have　been　speaking　of；　if　we　join　by　a　right　line　the　outmost　points　which；　up　to　this　time；　have　been　explored　by　voyagers　along　the　coast　on　either　side。〃＇3＇　We　may　pass　over　the　specific　criticisms　of　Strabo　upon　various　explorations　that　seem　to　have　been　of　great　interest　to　his　contemporaries；　including　an　alleged　trip　of　one　Eudoxus　out　into　the　Atlantic；　and　the　journeyings　of　Pytheas　in　the　far　north。　It　is　Pytheas；　we　may　add；　who　was　cited　by　Hipparchus　as　having　made　the　mistaken　observation　that　the　length　of　the　shadow　of　the　gnomon　is　the　same　at　Marseilles　and　Byzantium；　hence　that　these　two　places　are　on　the　same　parallel。　Modern　commentators　have　defended　Pytheas　as　regards　this　observation；　claiming　that　it　was　Hipparchus　and　not　Pytheas　who　made　th