meteorology-第4章

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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



of　this　kind　follows　their　course。　In　the　same　way；　a　certain　kind



of　matter　follows　the　sun；　and　we　explain　the　halo　as　a　reflection



from　it　when　the　air　is　of　the　right　constitution。　Now　we　must



assume　that　what　happens　in　the　case　of　the　stars　severally　happens　in



the　case　of　the　whole　of　the　heavens　and　all　the　upper　motion。　For



it　is　natural　to　suppose　that；　if　the　motion　of　a　single　star



excites　a　flame；　that　of　all　the　stars　should　have　a　similar　result；



and　especially　in　that　region　in　which　the　stars　are　biggest　and



most　numerous　and　nearest　to　one　another。　Now　the　circle　of　the　zodiac



dissolves　this　kind　of　matter　because　of　the　motion　of　the　sun　and　the



planets；　and　for　this　reason　most　comets　are　found　outside　the



tropic　circles。　Again；　no　fringe　appears　round　the　sun　or　moon：　for



they　dissolve　such　matter　too　quickly　to　admit　of　its　formation。　But



this　circle　in　which　the　milky　way　appears　to　our　sight　is　the



greatest　circle；　and　its　position　is　such　that　it　extends　far



outside　the　tropic　circles。　Besides　the　region　is　full　of　the



biggest　and　brightest　constellations　and　also　of　what　called



'scattered'　stars　（you　have　only　to　look　to　see　this　clearly）。　So



for　these　reasons　all　this　matter　is　continually　and　ceaselessly



collecting　there。　A　proof　of　the　theory　is　this：　In　the　circle



itself　the　light　is　stronger　in　that　half　where　the　milky　way　is



divided；　and　in　it　the　constellations　are　more　numerous　and　closer



to　one　another　than　in　the　other　half；　which　shows　that　the　cause　of



the　light　is　the　motion　of　the　constellations　and　nothing　else。　For　if



it　is　found　in　the　circle　in　which　there　are　most　constellations　and



at　that　point　in　the　circle　at　which　they　are　densest　and　contain



the　biggest　and　the　most　stars；　it　is　natural　to　suppose　that　they　are



the　true　cause　of　the　affection　in　question。　The　circle　and　the



constellations　in　it　may　be　seen　in　the　diagram。　The　so…called



'scattered'　stars　it　is　not　possible　to　set　down　in　the　same　way　on



the　sphere　because　none　of　them　have　an　evident　permanent　position；



but　if　you　look　up　to　the　sky　the　point　is　clear。　For　in　this　circle



alone　are　the　intervals　full　of　these　stars：　in　the　other　circles



there　are　obvious　gaps。　Hence　if　we　accept　the　cause　assigned　for



the　appearance　of　comets　as　plausible　we　must　assume　that　the　same



kind　of　thing　holds　good　of　the　milky　way。　For　the　fringe　which　in　the



former　case　is　an　affection　of　a　single　star　here　forms　in　the　same



way　in　relation　to　a　whole　circle。　So　if　we　are　to　define　the　milky



way　we　may　call　it　'a　fringe　attaching　to　the　greatest　circle；　and　due



to　the　matter　secreted'。　This；　as　we　said　before；　explains　why　there



are　few　comets　and　why　they　appear　rarely；　it　is　because　at　each



revolution　of　the　heavens　this　matter　has　always　been　and　is　always



being　separated　off　and　gathered　into　this　region。



　　We　have　now　explained　the　phenomena　that　occur　in　that　part　of　the



terrestrial　world　which　is　continuous　with　the　motions　of　the　heavens；



namely；　shooting…stars　and　the　burning　flame；　comets　and　the　milky



way；　these　being　the　chief　affections　that　appear　in　that　region。







　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　9







　　Let　us　go　on　to　treat　of　the　region　which　follows　next　in　order



after　this　and　which　immediately　surrounds　the　earth。　It　is　the　region



common　to　water　and　air；　and　the　processes　attending　the　formation



of　water　above　take　place　in　it。　We　must　consider　the　principles　and



causes　of　all　these　phenomena　too　as　before。　The　efficient　and　chief



and　first　cause　is　the　circle　in　which　the　sun　moves。　For　the　sun　as



it　approaches　or　recedes；　obviously　causes　dissipation　and



condensation　and　so　gives　rise　to　generation　and　destruction。　Now



the　earth　remains　but　the　moisture　surrounding　it　is　made　to　evaporate



by　the　sun's　rays　and　the　other　heat　from　above；　and　rises。　But　when



the　heat　which　was　raising　it　leaves　it；　in　part　dispersing　to　the



higher　region；　in　part　quenched　through　rising　so　far　into　the　upper



air；　then　the　vapour　cools　because　its　heat　is　gone　and　because　the



place　is　cold；　and　condenses　again　and　turns　from　air　into　water。



And　after　the　water　has　formed　it　falls　down　again　to　the　earth。



　　The　exhalation　of　water　is　vapour：　air　condensing　into　water　is



cloud。　Mist　is　what　is　left　over　when　a　cloud　condenses　into　water；



and　is　therefore　rather　a　sign　of　fine　weather　than　of　rain；　for



mist　might　be　called　a　barren　cloud。　So　we　get　a　circular　process　that



follows　the　course　of　the　sun。　For　according　as　the　sun　moves　to



this　side　or　that；　the　moisture　in　this　process　rises　or　falls。　We



must　think　of　it　as　a　river　flowing　up　and　down　in　a　circle　and　made



up　partly　of　air；　partly　of　water。　When　the　sun　is　near；　the　stream　of



vapour　flows　upwards；　when　it　recedes；　the　stream　of　water　flows　down：



and　the　order　of　sequence；　at　all　events；　in　this　process　always



remains　the　same。　So　if　'Oceanus'　had　some　secret　meaning　in　early



writers；　perhaps　they　may　have　meant　this　river　that　flows　in　a　circle



about　the　earth。



　　So　the　moisture　is　always　raised　by　the　heat　and　descends　to　the



earth　again　when　it　gets　cold。　These　processes　and；　in　some　cases；



their　varieties　are　distinguished　by　special　names。　When　the　water



falls　in　small　drops　it　is　called　a　drizzle；　when　the　drops　are　larger



it　is　rain。







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　　Some　of　the　vapour　that　is　formed　by　day　does　not　rise　high



because　the　ratio　of　the　fire　that　is　raising　it　to　the　water　that



is　being　raised　is　small。　When　this　cools　and　descends　at　night　it



is　called　dew　and　hoar…frost。　When　the　vapour　is　frozen　before　it



has　condensed　to　water　again　it　is　hoar…frost；　and　this　appears　in



winter　and　is　commoner　in　cold　places。　It　is　dew　when　the　vapour　has



condensed　into　water　and　the　heat　is　not　so　great　as　to　dry　up　the



moisture　that　has　been　raised　nor　the　cold　sufficient　（owing　to　the



warmth　of　the　climate　or　season）　for　the　vapour　itself　to　freeze。



For　dew　is　more　commonly　found　when　the　season　or　the　place　is　warm；



whereas　the　opposite；　as　has　been　said；　is　the　case　with　hoar…frost。



For　obviously　vapour　is　warmer　than　water；　having　still　the　fire



that　raised　it：　consequently　more　cold　is　needed　to　freeze　it。



　　Both　dew　and　hoar…frost　are　found　when　the　sky　is　clear　and　there　is



no　wind。　For　the　vapour　could　not　be　raised　unless　the　sky　were　clear；



and　if　a　wind　were　blowing　it　could　not　condense。



　　The　fact　that　hoar…frost　is　not　found　on　mountains　contributes　to



prove　that　these　phenomena　occur　because　the　vapour　does　not　rise



high。　One　reason　for　this　is　that　it　rises　from　hollow　and　watery



places；　so　that　the　heat　that　is　raising　it；　bearing　as　it　were　too



heavy　a　burden　cannot　lift　it　to　a　great　height　but　soon　lets　it



fall　again。　A　second　reason　is　that　the　motion　of　the　air　is　more



pronounced　at　a　height；　and　this　dissolves　a　gathering　of　this　kind。



　　Everywhere；　except　in　Pontus；　dew　is　found　with　south　winds　and



not　with　north　winds。　There　the　opposite　is　the　case　and　it　is　found



with　north　winds　and　not　with　south。　The　reason　is　the　same　as　that



which　explains　why　dew　is　found　in　warm　weather　and　not　in　cold。　For



the　south　wind　brings　warm；　and　the　north；　wintry　weather。　For　the



north　wind　is　cold　and　so　quenches　the　heat　of　the　evaporation。　But　in



Pontus　the　south　wind　does　not　bring　warmth　enough　to　cause



evaporation；　whereas　the　coldness　of　the　north　wind　concentrates　the



heat　by　a　sort　of　recoil；　so　that　there　is　more　evaporation　and　not



less。　This　is　a　thing　which　we　can　often　observe　in　other　places



too。　Wells；　for　inst