meteorology-第20章

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and　rain　and　a　quantity　of　water　are　produced。　As　far　as　the　matter



from　which　they　are　developed　goes　both　sets　of　phenomena　are　the



same。　As　soon　as　a　stimulus　to　the　development　of　either



potentiality　appears；　that　of　which　there　is　the　greater　quantity



present　in　the　cloud　is　at　once　secreted　from　it；　and　there　results



either　rain；　or；　if　the　other　exhalation　prevails；　a　hurricane。



　　Sometimes　the　exhalation　in　the　cloud；　when　it　is　being　secreted；



collides　with　another　under　circumstances　like　those　found　when　a　wind



is　forced　from　an　open　into　a　narrow　space　in　a　gateway　or　a　road。



It　often　happens　in　such　cases　that　the　first　part　of　the　moving



body　is　deflected　because　of　the　resistance　due　either　to　the



narrowness　or　to　a　contrary　current；　and　so　the　wind　forms　a　circle



and　eddy。　It　is　prevented　from　advancing　in　a　straight　line：　at　the



same　time　it　is　pushed　on　from　behind；　so　it　is　compelled　to　move



sideways　in　the　direction　of　least　resistance。　The　same　thing



happens　to　the　next　part；　and　the　next；　and　so　on；　till　the　series



becomes　one；　that　is；　till　a　circle　is　formed：　for　if　a　figure　is



described　by　a　single　motion　that　figure　must　itself　be　one。　This　is



how　eddies　are　generated　on　the　earth；　and　the　case　is　the　same　in　the



clouds　as　far　as　the　beginning　of　them　goes。　Only　here　（as　in　the　case



of　the　hurricane　which　shakes　off　the　cloud　without　cessation　and



becomes　a　continuous　wind）　the　cloud　follows　the　exhalation



unbroken；　and　the　exhalation；　failing　to　break　away　from　the　cloud



because　of　its　density；　first　moves　in　a　circle　for　the　reason　given



and　then　descends；　because　clouds　are　always　densest　on　the　side　where



the　heat　escapes。　This　phenomenon　is　called　a　whirlwind　when　it　is



colourless；　and　it　is　a　sort　of　undigested　hurricane。　There　is　never　a



whirlwind　when　the　weather　is　northerly；　nor　a　hurricane　when　there　is



snow。　The　reason　is　that　all　these　phenomena　are　'wind'；　and　wind　is　a



dry　and　warm　evaporation。　Now　frost　and　cold　prevail　over　this



principle　and　quench　it　at　its　birth：　that　they　do　prevail　is　clear　or



there　could　be　no　snow　or　northerly　rain；　since　these　occur　when　the



cold　does　prevail。



　　So　the　whirlwind　originates　in　the　failure　of　an　incipient　hurricane



to　escape　from　its　cloud：　it　is　due　to　the　resistance　which



generates　the　eddy；　and　it　consists　in　the　spiral　which　descends　to



the　earth　and　drags　with　it　the　cloud　which　it　cannot　shake　off。　It



moves　things　by　its　wind　in　the　direction　in　which　it　is　blowing　in



a　straight　line；　and　whirls　round　by　its　circular　motion　and



forcibly　snatches　up　whatever　it　meets。



　　When　the　cloud　burns　as　it　is　drawn　downwards；　that　is；　when　the



exhalation　becomes　rarer；　it　is　called　a　fire…wind；　for　its　fire



colours　the　neighbouring　air　and　inflames　it。



　　When　there　is　a　great　quantity　of　exhalation　and　it　is　rare　and　is



squeezed　out　in　the　cloud　itself　we　get　a　thunderbolt。　If　the



exhalation　is　exceedingly　rare　this　rareness　prevents　the



thunderbolt　from　scorching　and　the　poets　call　it　'bright'：　if　the



rareness　is　less　it　does　scorch　and　they　call　it　'smoky'。　The　former



moves　rapidly　because　of　its　rareness；　and　because　of　its　rapidity



passes　through　an　object　before　setting　fire　to　it　or　dwelling　on　it



so　as　to　blacken　it：　the　slower　one　does　blacken　the　object；　but



passes　through　it　before　it　can　actually　burn　it。　Further；　resisting



substances　are　affected；　unresisting　ones　are　not。　For　instance；　it



has　happened　that　the　bronze　of　a　shield　has　been　melted　while　the



woodwork　remained　intact　because　its　texture　was　so　loose　that　the



exhalation　filtered　through　without　affecting　it。　So　it　has　passed



through　clothes；　too；　without　burning　them；　and　has　merely　reduced



them　to　shreds。



　　Such　evidence　is　enough　by　itself　to　show　that　the　exhalation　is



at　work　in　all　these　cases；　but　we　sometimes　get　direct　evidence　as



well；　as　in　the　case　of　the　conflagration　of　the　temple　at　Ephesus



which　we　lately　witnessed。　There　independent　sheets　of　flame　left



the　main　fire　and　were　carried　bodily　in　many　directions。　Now　that



smoke　is　exhalation　and　that　smoke　burns　is　certain；　and　has　been



stated　in　another　place　before；　but　when　the　flame　moves　bodily；



then　we　have　ocular　proof　that　smoke　is　exhalation。　On　this　occasion



what　is　seen　in　small　fires　appeared　on　a　much　larger　scale　because　of



the　quantity　of　matter　that　was　burning。　The　beams　which　were　the



source　of　the　exhalation　split；　and　a　quantity　of　it　rushed　in　a



body　from　the　place　from　which　it　issued　forth　and　went　up　in　a　blaze：



so　that　the　flame　was　actually　seen　moving　through　the　air　away　and



falling　on　the　houses。　For　we　must　recognize　that　exhalation



accompanies　and　precedes　thunderbolts　though　it　is　colourless　and　so



invisible。　Hence；　where　the　thunderbolt　is　going　to　strike；　the　object



moves　before　it　is　struck；　showing　that　the　exhalation　leads　the　way



and　falls　on　the　object　first。　Thunder；　too；　splits　things　not　by



its　noise　but　because　the　exhalation　that　strikes　the　object　and



that　which　makes　the　noise　are　ejected　simultaneously。　This　exhalation



splits　the　thing　it　strikes　but　does　not　scorch　it　at　all。



　　We　have　now　explained　thunder　and　lightning　and　hurricane；　and



further　firewinds；　whirlwinds；　and　thunderbolts；　and　shown　that　they



are　all　of　them　forms　of　the　same　thing　and　wherein　they　all　differ。







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　　Let　us　now　explain　the　nature　and　cause　of　halo；　rainbow；　mock　suns；



and　rods；　since　the　same　account　applies　to　them　all。



　　We　must　first　describe　the　phenomena　and　the　circumstances　in



which　each　of　them　occurs。　The　halo　often　appears　as　a　complete



circle：　it　is　seen　round　the　sun　and　the　moon　and　bright　stars；　by



night　as　well　as　by　day；　and　at　midday　or　in　the　afternoon；　more



rarely　about　sunrise　or　sunset。



　　The　rainbow　never　forms　a　full　circle；　nor　any　segment　greater



than　a　semicircle。　At　sunset　and　sunrise　the　circle　is　smallest　and



the　segment　largest：　as　the　sun　rises　higher　the　circle　is　larger



and　the　segment　smaller。　After　the　autumn　equinox　in　the　shorter



days　it　is　seen　at　every　hour　of　the　day；　in　the　summer　not　about



midday。　There　are　never　more　than　two　rainbows　at　one　time。　Each　of



them　is　three…coloured；　the　colours　are　the　same　in　both　and　their



number　is　the　same；　but　in　the　outer　rainbow　they　are　fainter　and



their　position　is　reversed。　In　the　inner　rainbow　the　first　and　largest



band　is　red；　in　the　outer　rainbow　the　band　that　is　nearest　to　this　one



and　smallest　is　of　the　same　colour：　the　other　bands　correspond　on



the　same　principle。　These　are　almost　the　only　colours　which　painters



cannot　manufacture：　for　there　are　colours　which　they　create　by　mixing；



but　no　mixing　will　give　red；　green；　or　purple。　These　are　the　colours



of　the　rainbow；　though　between　the　red　and　the　green　an　orange



colour　is　often　seen。



　　Mock　suns　and　rods　are　always　seen　by　the　side　of　the　sun；　not　above



or　below　it　nor　in　the　opposite　quarter　of　the　sky。　They　are　not



seen　at　night　but　always　in　the　neighbourhood　of　the　sun；　either　as　it



is　rising　or　setting　but　more　commonly　towards　sunset。　They　have



scarcely　ever　appeared　when　the　sun　was　on　the　meridian；　though　this



once　happened　in　Bosporus　where　two　mock　suns　rose　with　the　sun　and



followed　it　all　through　the　day　till　sunset。



　　These　are　the　facts　about　each　of　these　phenomena：　the　cause　of　them



all　is　the　same；　for　they　are　all　reflections。　But　they　are



different　varieties；　and　are　distinguished　by　the　surface　from　which



and　the　way　in　which　the　reflection　to　the　sun　or　some　other　bright



object　takes　place。



　　The　rainbow　is　seen　by　day；　and　it　was　formerly　thought　that　it



never　appeared　by　night　as　a　moon　rainbow。　This　opinion　w