meteorology-第19节

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！




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　condensation　they　imply　is　so



insignificant　as　to　be　barren。







　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　4







　　We　have　already　stated　that　the　rainbow　is　a　reflection：　we　have　now



to　explain　what　sort　of　reflection　it　is；　to　describe　its　various



concomitants；　and　to　assign　their　causes。



　　Sight　is　reflected　from　all　smooth　surfaces；　such　as　are　air　and



water　among　others。　Air　must　be　condensed　if　it　is　to　act　as　a　mirror；



though　it　often　gives　a　reflection　even　uncondensed　when　the　sight



is　weak。　Such　was　the　case　of　a　man　whose　sight　was　faint　and



indistinct。　He　always　saw　an　image　in　front　of　him　and　facing　him　as



he　walked。　This　was　because　his　sight　was　reflected　back　to　him。　Its



morbid　condition　made　it　so　weak　and　delicate　that　the　air　close　by



acted　as　a　mirror；　just　as　distant　and　condensed　air　normally　does；



and　his　sight　could　not　push　it　back。　So　promontories　in　the　sea



'loom'　when　there　is　a　south…east　wind；　and　everything　seems　bigger；



and　in　a　mist；　too；　things　seem　bigger：　so；　too；　the　sun　and　the　stars



seem　bigger　when　rising　and　setting　than　on　the　meridian。　But　things



are　best　reflected　from　water；　and　even　in　process　of　formation　it



is　a　better　mirror　than　air；　for　each　of　the　particles；　the　union　of



which　constitutes　a　raindrop；　is　necessarily　a　better　mirror　than



mist。　Now　it　is　obvious　and　has　already　been　stated　that　a　mirror　of



this　kind　renders　the　colour　of　an　object　only；　but　not　its　shape。



Hence　it　follows　that　when　it　is　on　the　point　of　raining　and　the　air



in　the　clouds　is　in　process　of　forming　into　raindrops　but　the　rain



is　not　yet　actually　there；　if　the　sun　is　opposite；　or　any　other　object



bright　enough　to　make　the　cloud　a　mirror　and　cause　the　sight　to　be



reflected　to　the　object　then　the　reflection　must　render　the　colour



of　the　object　without　its　shape。　Since　each　of　the　mirrors　is　so　small



as　to　be　invisible　and　what　we　see　is　the　continuous　magnitude　made　up



of　them　all；　the　reflection　necessarily　gives　us　a　continuous



magnitude　made　up　of　one　colour；　each　of　the　mirrors　contributing



the　same　colour　to　the　whole。　We　may　deduce　that　since　these



conditions　are　realizable　there　will　be　an　appearance　due　to



reflection　whenever　the　sun　and　the　cloud　are　related　in　the　way



described　and　we　are　between　them。　But　these　are　just　the　conditions



under　which　the　rainbow　appears。　So　it　is　clear　that　the　rainbow　is



a　reflection　of　sight　to　the　sun。



　　So　the　rainbow　always　appears　opposite　the　sun　whereas　the　halo　is



round　it。　They　are　both　reflections；　but　the　rainbow　is



distinguished　by　the　variety　of　its　colours。　The　reflection　in　the　one



case　is　from　water　which　is　dark　and　from　a　distance；　in　the　other



from　air　which　is　nearer　and　lighter　in　colour。　White　light　through



a　dark　medium　or　on　a　dark　surface　（it　makes　no　difference）　looks　red。



We　know　how　red　the　flame　of　green　wood　is：　this　is　because　so　much



smoke　is　mixed　with　the　bright　white　firelight：　so；　too；　the　sun



appears　red　through　smoke　and　mist。　That　is　why　in　the　rainbow



reflection　the　outer　circumference　is　red　（the　reflection　being　from



small　particles　of　water）；　but　not　in　the　case　of　the　halo。　The



other　colours　shall　be　explained　later。　Again；　a　condensation　of



this　kind　cannot　persist　in　the　neighbourhood　of　the　sun：　it　must



either　turn　to　rain　or　be　dissolved；　but　opposite　to　the　sun　there



is　an　interval　during　which　the　water　is　formed。　If　there　were　not



this　distinction　haloes　would　be　coloured　like　the　rainbow。　Actually



no　complete　or　circular　halo　presents　this　colour；　only　small　and



fragmentary　appearances　called　'rods'。　But　if　a　haze　due　to　water　or



any　other　dark　substance　formed　there　we　should　have　had；　as　we



maintain；　a　complete　rainbow　like　that　which　we　do　find　lamps。　A



rainbow　appears　round　these　in　winter；　generally　with　southerly　winds。



Persons　whose　eyes　are　moist　see　it　most　clearly　because　their　sight



is　weak　and　easily　reflected。　It　is　due　to　the　moistness　of　the　air



and　the　soot　which　the　flame　gives　off　and　which　mixes　with　the　air



and　makes　it　a　mirror；　and　to　the　blackness　which　that　mirror



derives　from　the　smoky　nature　of　the　soot。　The　light　of　the　lamp



appears　as　a　circle　which　is　not　white　but　purple。　It　shows　the



colours　of　the　rainbow；　but　because　the　sight　that　is　reflected　is　too



weak　and　the　mirror　too　dark；　red　is　absent。　The　rainbow　that　is



seen　when　oars　are　raised　out　of　the　sea　involves　the　same　relative



positions　as　that　in　the　sky；　but　its　colour　is　more　like　that　round



the　lamps；　being　purple　rather　than　red。　The　reflection　is　from　very



small　particles　continuous　with　one　another；　and　in　this　case　the



particles　are　fully　formed　water。　We　get　a　rainbow；　too；　if　a　man



sprinkles　fine　drops　in　a　room　turned　to　the　sun　so　that　the　sun　is



shining　in　part　of　the　room　and　throwing　a　shadow　in　the　rest。　Then　if



one　man　sprinkles　in　the　room；　another；　standing　outside；　sees　a



rainbow　where　the　sun's　rays　cease　and　make　the　shadow。　Its　nature　and



colour　is　like　that　from　the　oars　and　its　cause　is　the　same；　for　the



sprinkling　hand　corresponds　to　the　oar。



　　That　the　colours　of　the　rainbow　are　those　we　described　and　how　the



other　colours　come　to　appear　in　it　will　be　clear　from　the　following



considerations。　We　must　recognize；　as　we　have　said；　and　lay　down：



first；　that　white　colour　on　a　black　surface　or　seen　through　a　black



medium　gives　red；　second；　that　sight　when　strained　to　a　distance



becomes　weaker　and　less；　third；　that　black　is　in　a　sort　the　negation



of　sight：　an　object　is　black　because　sight　fails；　so　everything　at　a



distance　looks　blacker；　because　sight　does　not　reach　it。　The　theory　of



these　matters　belongs　to　the　account　of　the　senses；　which　are　the



proper　subjects　of　such　an