meteorology-第4节

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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　among　the　same　constellations　（for　it　is　clearly　a



greatest　circle）；　whereas；　since　the　sun　does　not　remain　in　the　same



place；　what　is　hidden　from　it　differs　at　different　times。　Consequently



with　the　change　of　the　sun's　position　the　milky　way　ought　to　change



its　position　too：　but　we　find　that　this　does　not　happen。　Besides；　if



astronomical　demonstrations　are　correct　and　the　size　of　the　sun　is



greater　than　that　of　the　earth　and　the　distance　of　the　stars　from



the　earth　many　times　greater　than　that　of　the　sun　（just　as　the　sun



is　further　from　the　earth　than　the　moon）；　then　the　cone　made　by　the



rays　of　the　sun　would　terminate　at　no　great　distance　from　the　earth；



and　the　shadow　of　the　earth　（what　we　call　night）　would　not　reach　the



stars。　On　the　contrary；　the　sun　shines　on　all　the　stars　and　the



earth　screens　none　of　them。



　　（3）　There　is　a　third　theory　about　the　milky　way。　Some　say　that　it　is



a　reflection　of　our　sight　to　the　sun；　just　as　they　say　that　the



comet　is。



　　But　this　too　is　impossible。　For　if　the　eye　and　the　mirror　and　the



whole　of　the　object　were　severally　at　rest；　then　the　same　part　of



the　image　would　appear　at　the　same　point　in　the　mirror。　But　if　the



mirror　and　the　object　move；　keeping　the　same　distance　from　the　eye



which　is　at　rest；　but　at　different　rates　of　speed　and　so　not　always　at



the　same　interval　from　one　another；　then　it　is　impossible　for　the　same



image　always　to　appear　in　the　same　part　of　the　mirror。　Now　the



constellations　included　in　the　circle　of　the　milky　way　move；　and　so



does　the　sun；　the　object　to　which　our　sight　is　reflected；　but　we　stand



still。　And　the　distance　of　those　two　from　us　is　constant　and



uniform；　but　their　distance　from　one　another　varies。　For　the　Dolphin



sometimes　rises　at　midnight；　sometimes　in　the　morning。　But　in　each



case　the　same　parts　of　the　milky　way　are　found　near　it。　But　if　it　were



a　reflection　and　not　a　genuine　affection　of　these　this　ought　not　to　be



the　case。



　　Again；　we　can　see　the　milky　way　reflected　at　night　in　water　and



similar　mirrors。　But　under　these　circumstances　it　is　impossible　for



our　sight　to　be　reflected　to　the　sun。



　　These　considerations　show　that　the　milky　way　is　not　the　path　of



one　of　the　planets；　nor　the　light　of　imperceptible　stars；　nor　a



reflection。　And　those　are　the　chief　theories　handed　down　by　others



hitherto。



　　Let　us　recall　our　fundamental　principle　and　then　explain　our



views。　We　have　already　laid　down　that　the　outermost　part　of　what　is



called　the　air　is　potentially　fire　and　that　therefore　when　the　air



is　dissolved　by　motion；　there　is　separated　off　a　kind　of　matter…and　of



this　matter　we　assert　that　comets　consist。　We　must　suppose　that　what



happens　is　the　same　as　in　the　case　of　the　comets　when　the　matter



does　not　form　independently　but　is　formed　by　one　of　the　fixed　stars　or



the　planets。　Then　these　stars　appear　to　be　fringed；　because　matter