meteorology-第3章

按键盘上方向键 ← 或 → 可快速上下翻页，按键盘上的 Enter 键可回到本书目录页，按键盘上方向键 ↑ 可回到本页顶部！
————未阅读完？加入书签已便下次继续阅读！




dried　up　by　the　sun　on　its　course。　When　it　moves　towards　the　south



it　has　no　lack　of　the　necessary　moisture；　but　because　the　segment　of



its　circle　which　is　above　the　horizon　is　small；　and　that　below　it　many



times　as　large；　it　is　impossible　for　the　sun　to　be　reflected　to　our



sight；　either　when　it　approaches　the　southern　tropic；　or　at　the　summer



solstice。　Hence　in　these　regions　it　does　not　develop　a　tail　at　all。



But　when　it　is　visible　in　the　north　it　assumes　a　tail　because　the



arc　above　the　horizon　is　large　and　that　below　it　small。　For　under



these　circumstances　there　is　nothing　to　prevent　our　vision　from



being　reflected　to　the　sun。



　　These　views　involve　impossibilities；　some　of　which　are　common　to　all



of　them；　while　others　are　peculiar　to　some　only。



　　This　is　the　case；　first；　with　those　who　say　that　the　comet　is　one　of



the　planets。　For　all　the　planets　appear　in　the　circle　of　the　zodiac；



whereas　many　comets　have　been　seen　outside　that　circle。　Again　more



comets　than　one　have　often　appeared　simultaneously。　Besides；　if



their　tail　is　due　to　reflection；　as　Aeschylus　and　Hippocrates　say；



this　planet　ought　sometimes　to　be　visible　without　a　tail　since；　as



they　it　does　not　possess　a　tail　in　every　place　in　which　it　appears。



But；　as　a　matter　of　fact；　no　planet　has　been　observed　besides　the



five。　And　all　of　them　are　often　visible　above　the　horizon　together



at　the　same　time。　Further；　comets　are　often　found　to　appear；　as　well



when　all　the　planets　are　visible　as　when　some　are　not；　but　are



obscured　by　the　neighbourhood　of　the　sun。　Moreover　the　statement



that　a　comet　only　appears　in　the　north；　with　the　sun　at　the　summer



solstice；　is　not　true　either。　The　great　comet　which　appeared　at　the



time　of　the　earthquake　in　Achaea　and　the　tidal　wave　rose　due　west；　and



many　have　been　known　to　appear　in　the　south。　Again　in　the　archonship



of　Euclees；　son　of　Molon；　at　Athens　there　appeared　a　comet　in　the



north　in　the　month　Gamelion；　the　sun　being　about　the　winter



solstice。　Yet　they　themselves　admit　that　reflection　over　so　great　a



space　is　an　impossibility。



　　An　objection　that　tells　equally　against　those　who　hold　this　theory



and　those　who　say　that　comets　are　a　coalescence　of　the　planets　is；



first；　the　fact　that　some　of　the　fixed　stars　too　get　a　tail。　For



this　we　must　not　only　accept　the　authority　of　the　Egyptians　who　assert



it；　but　we　have　ourselves　observed　the　fact。　For　a　star　in　the　thigh



of　the　Dog　had　a　tail；　though　a　faint　one。　If　you　fixed　your　sight



on　it　its　light　was　dim；　but　if　you　just　glanced　at　it；　it　appeared



brighter。　Besides；　all　the　comets　that　have　been　seen　in　our　day



have　vanished　without　setting；　gradually　fading　away　above　the



horizon；　and　they　have　not　left　behind　them　either　one　or　more



stars。　For　instance　the　great　comet　we　mentioned　before　appeared　to



the　west　in　winter　in　frosty　weather　when　the　sky　was　clear；　in　the



archonship　of　Asteius。　On　the　first　day　it　set　before　the　sun　and



was　then　not　seen。　On　the　next　day　it　was　seen；　being　ever　so　little



behind　the　sun　and　immediately　setting。　But　its　light　extended　over



a　third　part　of　the　sky　like　a　leap；　so　that　people　called　it　a



'path'。　This　comet　receded　as　far　as　Orion's　belt　and　there　dissolved。



Democritus　however；　insists　upon　the　truth　of　his　view　and　affirms



that　certain　stars　have　been　seen　when　comets　dissolve。　But　on　his



theory　this　ought　not　to　occur　occasionally　but　always。　Besides；　the



Egyptians　affirm　that　conjunctions　of　the　planets　with　one　another；



and　with　the　fixed　stars；　take　place；　and　we　have　ourselves　observed



Jupiter　coinciding　with　one　of　the　stars　in　the　Twins　and　hiding　it；



and　yet　no　comet　was　formed。　Further；　we　can　also　give　a　rational



proof　of　our　point。　It　is　true　that　some　stars　seem　to　be　bigger



than　others；　yet　each　one　by　itself　looks　indivisible。　Consequently；



just　as；　if　they　really　had　been　indivisible；　their　conjunction



could　not　have　created　any　greater　magnitude；　so　now　that　they　are　not



in　fact　indivisible　but　look　as　if　they　were；　their　conjunction　will



not　make　them　look　any　bigger。



　　Enough　has　been　said；　without　further　argument；　to　show　that　the



causes　brought　forward　to　explain　comets　are　false。







　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　7







　　We　consider　a　satisfactory　explanation　of　phenomena　inaccessible



to　observation　to　have　been　given　when　our　account　of　them　is　free



from　impossibilities。　The　observations　before　us　suggest　the　following



account　of　the　phenomena　we　are　now　considering。　We　know　that　the



dry　and　warm　exhalation　is　the　outermost　part　of　the　terrestrial　world



which　falls　below　the　circular　motion。　It；　and　a　great　part　of　the　air



that　is　continuous　with　it　below；　is　carried　round　the　earth　by　the



motion　of　the　circular　revolution。　In　the　course　of　this　motion　it



often　ignites　wherever　it　may　happen　to　be　of　the　right　consistency；



and　this　we　maintain　to　be　the　cause　of　the　'shooting'　of　scattered



'stars'。　We　may　say；　then；　that　a　comet　is　formed　when　the　upper



motion　introduces　into　a　gathering　of　this　kind　a　fiery　principle



not　of　such　excessive　strength　as　to　burn　up　much　of　the　material



quickly；　nor　so　weak　as　soon　to　be　extinguished；　but　stronger　and



capable　of　burning　up　much　material；　and　when　exhalation　of　the



right　consistency　rises　from　below　and　meets　it。　The　kind　of　comet



varies　according　to　the　shape　which　the　exhalation　happens　to　take。　If



it　is　diffused　equally　on　every　side　the　star　is　said　to　be　fringed；



if　it　stretches　out　in　one　direction　it　is　called　bearded。　We　have



seen　that　when　a　fiery　principle　of　this　kind　moves　we　seem　to　have



a　shooting…star：　similarly　when　it　stands　still　we　seem　to　have　a　star



standing　still。　We　may　compare　these　phenomena　to　a　heap　or　mass　of



chaff　into　which　a　torch　is　thrust；　or　a　spark　thrown。　That　is　what



a　shooting…star　is　like。　The　fuel　is　so　inflammable　that　the　fire　runs



through　it　quickly　in　a　line。　Now　if　this　fire　were　to　persist　instead



of　running　through　the　fuel　and　perishing　away；　its　course　through　the



fuel　would　stop　at　the　point　where　the　latter　was　densest；　and　then



the　whole　might　begin　to　move。　Such　is　a　comet…like　a　shooting…star



that　contains　its　beginning　and　end　in　itself。



　　When　the　matter　begins　to　gather　in　the　lower　region　independently



the　comet　appears　by　itself。　But　when　the　exhalation　is　constituted　by



one　of　the　fixed　stars　or　the　planets；　owing　to　their　motion；　one　of



them　becomes　a　comet。　The　fringe　is　not　close　to　the　stars　themselves。



Just　as　haloes　appear　to　follow　the　sun　and　the　moon　as　they　move；　and



encircle　them；　when　the　air　is　dense　enough　for　them　to　form　along



under　the　sun's　course；　so　too　the　fringe。　It　stands　in　the　relation



of　a　halo　to　the　stars；　except　that　the　colour　of　the　halo　is　due　to



reflection；　whereas　in　the　case　of　comets　the　colour　is　something　that



appears　actually　on　them。



　　Now　when　this　matter　gathers　in　relation　to　a　star　the　comet



necessarily　appears　to　follow　the　same　course　as　the　star。　But　when



the　comet　is　formed　independently　it　falls　behind　the　motion　of　the



universe；　like　the　rest　of　the　terrestrial　world。　It　is　this　fact；



that　a　comet　often　forms　independently；　indeed　oftener　than　round



one　of　the　regular　stars；　that　makes　it　impossible　to　maintain　that



a　comet　is　a　sort　of　reflection；　not　indeed；　as　Hippocrates　and　his



school　say；　to　the　sun；　but　to　the　very　star　it　is　alleged　to



accompany…in　fact；　a　kind　of　halo　in　the　pure　fuel　of　fire。



　　As　for　the　halo　we　shall　explain　its　cause　later。



　　The　fact　that　comets　when　frequent　foreshadow　wind　and　drought



must　be　taken　as　an　indication　of　their　fiery　constitution。　For



their　origin　is　plainly　due　to　the　plentiful　supply　of　that　secretion。



Hence　the　air　is　necessarily　drier　and　the　moist　evaporation　is　so



dissolved　and　dissipated　by　the　quantity　of　the　hot　exhalation　as



not　readily　to　condense　into　water。…But　this　phenomenon　too　shall　be



explained　more　clearly　later　when　the　time　comes　to　speak　of　the



winds。…So　when　there　are　many　comets　and　they　are　dense；　it　is　as　we



say；　and　the　years　are　clearly　dry　and　windy。　When　they　are　fewer



and　fainter　this　effect　does　not　appear　in　the　same　degree；　though



as　a　rule　the　is　found　to　be　excessive　either　in　duration　or　strength。



For　instance　when　the　stone　at　Aegospotami　fell　out　of　the　air…it



had　been　carried　up　by　a　wind　and　fell　down　in　the　daytime…then　too



a　comet　happened　to　have　appeared　in　the　west。　And　at　the　time　of



the　great　comet　the　winter　was　dry　and　north　winds　prevailed；　and



the　wave　was　due　to　an　opposition　of　winds。　For　in　the　gulf　a　north



wind　blew　and　outside　it　a　violent　south　wind。　Again　in　the　archonship



of　Nicomachus　a　comet　appeared　for　a　few　days　about　the　equinoctial



circle　（this　one　had　not　risen　in　the　west）；　and　simultaneously　with



it　there　happened　the　storm　at　Corinth。



　　That　there　are　few　comets　and　that　they　appear　rarely　and　outside



the　tropic　circles　more　than　within　them　is　due　to　the　motion　of　the



sun　and　the　stars。　For　this　motion　does　not　only　cause　the　hot



principle　to　be　secreted　but　also　dissolves　it　when　it　is　gathering。



But　the　chief　reason　is　that　most　of　this　stuff　collects　in　the　region



of　the　milky　way。







　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　8







　　Let　us　now　explain　the　origin；　cause；　and　nature　of　the　milky　way。



And　here　too　let　us　begin　by　discussing　the　statements　of　others　on



the　subject。



　　（1）　Of　the　so…called　Pythagoreans　some　say　that　this　is　the　path



of　one　of　the　stars　that　fell　from　heaven　at　the　time　of　Phaethon's



downfall。　Others　say　that　the　sun　used　once　to　move　in　this　circle　and



that　this　region　was　scorched　or　met　with　some　other　affection　of　this



kind；　because　of　the　sun　and　its　motion。



　　But　it　is　absurd　not　to　see　that　if　this　were　the　reason　the



circle　of　the　Zodiac　ought　to　be　affected　in　the　same　way；　and



indeed　more　so　than　that　of　the　milky　way；　since　not　the　sun　only



but　all　the　planets　move　in　it。　We　can　see　the　whole　of　this　circle



（half　of　it　being　visible　at　any　time　of　the　night）；　but　it　shows　no



signs　of　any　such　affection　except　where　a　part　of　it　touches　the



circle　of　the　milky　way。



　　（2）　Anaxagoras；　Democritus；　and　their　schools　say　that　the　milky　way



is　the　light　of　certain　stars。　For；　they　say；　when　the　sun　passes



below　the　earth　some　of　the　stars　are　hidden　from　it。　Now　the　light　of



those　on　which　the　sun　shines　is　invisible；　being　obscured　by　the　of



the　sun。　But　the　milky　way　is　the　peculiar　light　of　those　stars



which　are　shaded　by　the　earth　from　the　sun's　rays。



　　This；　too；　is　obviously　impossible。　The　milky　way　is　always



unchanged　and　amo