meteorology-第15章

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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。







　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　2







　　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　was　due　to　the



rarity　of　the　occurrence：　it　was　not　observed；　for　though　it　does



happen　it　does　so　rarely。　The　reason　is　that　the　colours　are　not　so



easy　to　see　in　the　dark　and　that　many　other　conditions　must



coincide；　and　all　that　in　a　single　day　in　the　month。　For　if　there　is



to　be　one　it　must　be　at　full　moon；　and　then　as　the　moon　is　either



rising　or　setting。　So　we　have　only　met　with　two　instances　of　a　moon



rainbow　in　more　than　fifty　years。



　　We　must　accept　from　the　theory　of　optics　the　fact　that　sight　is



reflected　from　air　and　any　object　with　a　smooth　surface　just　as　it



is　from　water；　also　that　in　some　mirrors　the　forms　of　things　are



reflected；　in　others　only　their　colours。　Of　the　latter　kind　are



those　mirrors　which　are　so　small　as　to　be　indivisible　for　sense。　It　is



impossible　that　the　figure　of　a　thing　should　be　reflected　in　them；　for



if　it　is　the　mirror　will　be　sensibly　divisible　since　divisibility　is



involved　in　the　notion　of　figure。　But　since　something　must　be



reflected　in　them　and　figure　cannot　be；　it　remains　that　colour　alone



should　be　reflected。　The　colour　of　a　bright　object　sometimes　appears



bright　in　the　reflection；　but　it　sometimes；　either　owing　to　the



admixture　of　the　colour　of　the　mirror　or　to　weakness　of　sight；　gives



rise　to　the　appearance　of　another　colour。



　　However；　we　must　accept　the　account　we　have　given　of　these　things　in



the　theory　of　sensation；　and　take　some　things　for　granted　while　we



explain　others。







　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　3







　　Let　us　begin　by　explaining　the　shape　of　the　halo；　why　it　is　a　circle



and　why　it　appears　round　the　sun　or　the　moon　or　one　of　the　other



stars：　the　explanation　being　in　all　these　cases　the　same。



　　Sight　is　reflected　in　this　way　when　air　and　vapour　are　condensed



into　a　cloud　and　the　condensed　matter　is　uniform　and　consists　of　small



parts。　Hence　in　itself　it　is　a　sign　of　rain；　but　if　it　fades　away；



of　fine　weather；　if　it　is　broken　up；　of　wind。　For　if　it　does　not



fade　away　and　is　not　broken　up　but　is　allowed　to　attain　its　normal



state；　it　is　naturally　a　sign　of　rain　since　it　shows　that　a　process　of



condensation　is　proceeding　which　must；　when　it　is　carried　to　an　end；



result　in　rain。　For　the　same　reason　these　haloes　are　the　darkest。　It



is　a　sign　of　wind　when　it　is　broken　up　because　its　breaking　up　is



due　to　a　wind　which　exists　there　but　has　not　reached　us。　This　view



finds　support　in　the　fact　that　the　wind　blows　from　the　quarter　in



which　the　main　division　appears　in　the　halo。　Its　fading　away　is　a　sign



of　fine　weather　because　if　the　air　is　not　yet　in　a　state　to　get　the



better　of　the　heat　it　contains　and　proceed　to　condense　into　water；



this　shows　that　the　moist　vapour　has　not　yet　separated　from　the　dry



and　firelike　exhalation：　and　this　is　the　cause　of　fine　weather。



　　So　much　for　the　atmospheric　conditions　under　which　the　reflection



takes　place。　The　reflection　is　from　the　mist　that　forms　round　the



sun　or　the　moon；　and　that　is　why　the　halo　is　not　seen　opposite　the　sun



like　the　rainbow。



　　Since　the　reflection　takes　place　in　the　same　way　from　every　point



the　result　is　necessarily　a　circle　or　a　segment　of　a　circle：　for　if



the　lines　start　from　the　same　point　and　end　at　the　same　point　and



are　equal；　the　points　where　they　form　an　angle　will　always　lie　on　a



circle。



　　Let　AGB　and　AZB　and　ADB　be　lines　each　of　which　goes　from　the　point　A



to　the　point　B　and　forms　an　angle。　Let　the　lines　AG；　AZ；　AD　be　equal



and　those　at　B；　GB；　ZB；　DB　equal　too。　（See　diagram。）



　　Draw　the　line　AEB。　Then　the　triangles　are　equal；　for　their　base



AEB　is　equal。　Draw　perpendiculars　to　AEB　from　the　angles；　GE　from　G；



ZE　from　Z；　DE　from　D。　Then　these　perpendiculars　are　equal；　being　in



equal　triangles。　And　they　are　all　in　one　plane；　being　all　at　right



angles　to　AEB　and　meeting　at　a　single　point　E。　So　if　you　draw　the　line



it　will　be　a　circle　and　E　its　centre。　Now　B　is　the　sun；　A　the　eye；　and



the　circumference　passing　through　the　points　GZD　the　cloud　from



which　the　line　of　sight　is　reflected　to　the　sun。



　　The　mirrors　must　be　thought　of　as　contiguous：　each　of　them　is　too



small　to　be　visible；　but　their　contiguity　makes　the　whole　made　up　of



them　all　to　seem　one。　The　bright　band　is　the　sun；　which　is　seen　as　a



circle；　appearing　successively　in　each　of　the　mirrors　as　a　point



indivisible　to　sense。　The　band　of　cloud　next　to　it　is　black；　its



colour　being　intensified　by　contrast　with　the　brightness　of　the



halo。　The　halo　is　formed　rather　near　the　earth　because　that　is　calmer：



for　where　there　is　wind　it　is　clear　that　no　halo　can　maintain　its



position。



　　Haloes　are　commoner　round　the　moon　because　the　greater　heat　of　the



sun　dissolves　the　condensations　of　the　air　more　rapidly。



　　Haloes　are　formed　round　stars　for　the　same　reasons；　but　they　are　not



prognostic　in　the　same　way　because　the　cond