meteorology-第13节

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　　The　cause　of　the　predominance　of　winds　from　the　north　and　from　the



south　is　the　same。　（Most　winds；　as　a　matter　of　fact；　are　north　winds



or　south　winds。）　These　are　the　only　regions　which　the　sun　does　not



visit：　it　approaches　them　and　recedes　from　them；　but　its　course　is



always　over　the…west　and　the　east。　Hence　clouds　collect　on　either



side；　and　when　the　sun　approaches　it　provokes　the　moist　evaporation；



and　when　it　recedes　to　the　opposite　side　there　are　storms　and　rain。　So



summer　and　winter　are　due　to　the　sun's　motion　to　and　from　the



solstices；　and　water　ascends　and　falls　again　for　the　same　reason。



Now　since　most　rain　falls　in　those　regions　towards　which　and　from



which　the　sun　turns　and　these　are　the　north　and　the　south；　and　since



most　evaporation　must　take　place　where　there　is　the　greatest　rainfall；



just　as　green　wood　gives　most　smoke；　and　since　this　evaporation　is



wind；　it　is　natural　that　the　most　and　most　important　winds　should　come



from　these　quarters。　（The　winds　from　the　north　are　called　Boreae；



those　from　the　south　Noti。）



　　The　course　of　winds　is　oblique：　for　though　the　evaporation　rises



straight　up　from　the　earth；　they　blow　round　it　because　all　the



surrounding　air　follows　the　motion　of　the　heavens。　Hence　the



question　might　be　asked　whether　winds　originate　from　above　or　from



below。　The　motion　comes　from　above：　before　we　feel　the　wind　blowing



the　air　betrays　its　presence　if　there　are　clouds　or　a　mist；　for



their　motion　shows　that　the　wind　has　begun　to　blow　before　it　has



actually　reached　us；　and　this　implies　that　the　source　of　winds　is



above。　But　since　wind　is　defined　as　'a　quantity　of　dry　evaporation



from　the　earth　moving　round　the　earth'；　it　is　clear　that　while　the



origin　of　the　motion　is　from　above；　the　matter　and　the　generation　of



wind　come　from　below。　The　oblique　movement　of　the　rising　evaporation



is　caused　from　above：　for　the　motion　of　the　heavens　determines　the



processes　that　are　at　a　distance　from　the　earth；　and　the　motion　from



below　is　vertical　and　every　cause　is　more　active　where　it　is　nearest



to　the　effect；　but　in　its　generation　and　origin　wind　plainly　derives



from　the　earth。



　　The　facts　bear　out　the　view　that　winds　are　formed　by　the　gradual



union　of　many　evaporations　just　as　rivers　derive　their　sources　from



the　water　that　oozes　from　the　earth。　Every　wind　is　weakest　in　the　spot



from　which　it　blows；　as　they　proceed　and　leave　their　source　at　a



distance　they　gather　strength。　Thus　the　winter　in　the　north　is



windless　and　calm：　that　is；　in　the　north　itself；　but；　the　breeze



that　blows　from　there　so　gently　as　to　escape　observation　becomes　a



great　wind　as　it　passes　on。



　　We　have　explained　the　nature　and　origin　of　wind；　the　occurrence　of



drought　and　rains；　the　reason　why　rain　stops　wind　and　wind　rises　after



rain；　the　prevalence　of　north　and　south　winds　and　also　why　wind



moves　in　the　way　it　does。







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　　The　sun　both　checks　the　formation　of　winds　and　stimulates　it。　When



the　evaporation　is　small　in　amount　and　faint　the　sun　wastes　it　and



dissipates　by　its　greater　heat　the　lesser　heat　contained　in　the



evaporation。　It　also　dries　up　the　earth；　the　source　of　the



evaporation；　before　the　latter　has　appeared　in　bulk：　just　as；　when　you



throw　a　little　fuel　into　a　great　fire；　it　is　often　burnt　up　before



giving　off　any　smoke。　In　these　ways　the　sun　checks　winds　and



prevents　them　from　rising　at　all：　it　checks　them　by　wasting　the



evaporation；　and　prevents　their　rising　by　drying　up　the　earth　quickly。



Hence　calm　is　very　apt　to　prevail　about　the　rising　of　Orion　and



lasts　until　the　coming　of　the　Etesiae　and　their　'forerunners'。



　　Calm　is　due　to　two　causes。　Either　cold　quenches　the　evaporation；　for



instance　a　sharp　frost：　or　excessive　heat　wastes　it。　In　the



intermediate　periods；　too；　the　causes　are　generally　either　that　the



evaporation　has　not　had　time　to　develop　or　that　it　has　passed　away　and



there　is　none　as　yet　to　replace　it。



　　Both　the　setting　and　the　rising　of　Orion　are　considered　to　be



treacherous　and　stormy；　because　they　place　at　a　change　of　season



（namely　of　summer　or　winter；　and　because　the　size　of　the　constellation



makes　its　rise　last　over　many　days）　and　a　state　of　change　is　always



indefinite　and　therefore　liable　to　disturbance。



　　The　Etesiae　blow　after　the　summer　solstice　and　the　rising　of　the



dog…star：　not　at　the　time　when　the　sun　is　closest　nor　when　it　is



distant；　and　they　blow　by　day　and　cease　at　night。　The　reason　is　that



when　the　sun　is　near　it　dries　up　the　earth　before　evaporation　has



taken　place；　but　when　it　has　receded　a　little　its　heat　and　the



evaporation　are　present　in　the　right　proportion；　so　the　ice　melts



and　the　earth；　dried　by　its　own　heat　and　that　of　the　sun；　smokes　and



vapours。　They　abate　at　night　because　the　cold　pf　the　nights　checks　the



melting　of　the　ice。　What　is　frozen　gives　off　no　evaporation；　nor



does　that　which　contains　no　dryness　at　all：　it　is　only　where　something



dry　contains　moisture　that　it　gives　off　evaporation　under　the



influence　of　heat。



　　The　question　is　sometimes　asked：　why　do　the　north　winds　which　we



call　the　Etesiae　blow　continuously　after　the　summer　solstice；　when



there　are　no　corresponding　south　winds　after　the　winter　solstice？



The　facts　are　reasonable　enough：　for　the　so…called　'white　south　winds'



do　blow　at　the　corresponding　season；　though　they　are　not　equally



continuous　and　so　escape　observation　and　give　rise　to　this　inquiry。



The　reason　for　this　is　that　the　north　wind　I　from　the　arctic　regions



which　are　full　of　water　and　snow。　The　sun　thaws　them　and　so　the



Etesiae　blow：　after　rather　than　at　the　summer　solstice。　（For　the



greatest　heat　is　developed　not　when　the　sun　is　nearest　to　the　north；



but　when　its　heat　has　been　felt　for　a　considerable　period　and　it　has



not　yet　receded　far。　The　'bird　winds'　blow　in　the　same　way　after　the



winter　solstice。　They；　too；　are　weak　Etesiae；　but　they　blow　less　and



later　than　the　Etesiae。　They　begin　to　blow　only　on　the　seventieth



day　because　the　sun　is　distant　and　therefore　weaker。　They　do　not



blow　so　continuously　because　only　things　on　the　surface　of　the　earth



and　offering　little　resistance　evaporate　then；　the　thoroughly　frozen



parts　requiring　greater　heat　to　melt　them。　So　they　blow　intermittently



till　the　true　Etesiae　come　on　again　at　the　summer　solstice：　for　from



that　time　onwards　the　wind　tends　to　blow　continuously。）　But　the



south　wind　blows　from　the　tropic　of　Cancer　and　not　from　the



antarctic　region。



　　There　are　two　inhabitable　sections　of　the　earth：　one　near　our　upper；



or　nothern　pole；　the　other　near　the　other　or　southern　pole；　and



their　shape　is　like　that　of　a　tambourine。　If　you　draw　lines　from　the



centre　of　the　earth　they　cut　out　a　drum…shaped　figure。　The　lines



form　two　cones；　the　base　of　the　one　is　the　tropic；　of　the　other　the



ever　visible　circle；　their　vertex　is　at　the　centre　of　the　earth。　Two



other　cones　towards　the　south　pole　give　corresponding　segments　of



the　earth。　These　sections　alone　are　habitable。　Beyond　the　tropics　no



one　can　live：　for　there　the　shade　would　not　fall　to　the　north；　whereas



the　earth　is　known　to　be　uninhabitable　before　the　sun　is　in　the　zenith



or　the　shade　is　thrown　to　the　south：　and　the　regions　below　the　Bear



are　uninhabitable　because　of　the　cold。



　　（The　Crown；　too；　moves　over　this　region：　for　it　is　in　the　zenith



when　it　is　on　our　meridian。）



　　So　we　see　that　the　way　in　which　they　now　describe　the　geography　of



the　earth　is　ridiculous。　They　depict　the　inhabited　earth　as　round；　but



both　ascertained　facts　and　general　considerations　show　this　to　be



impossible。　If　we　reflect　we　see　that　the　inhabited　region　is



limited　in　breadth；　while　the　climate　admits　of　its　extending　all



round　the　earth。　For　we　meet　with　no　excessive　heat　or　cold　in　the



direction　of　its　length　but　only　in　that　of　its　breadth；　so　that　there



is　nothing　to　prevent　our　travelling　round　the　earth　unless　the　extent



of　the　sea　presents　an　obstacle　anywhere。　The　records　of　journeys　by



sea　and　land　bear　this　out。　They　make　the　length　far　greater　than



the　breadth。　If　we　compute　these　voyages　and　journeys　the　distance



from　the　Pillars　of　Heracles　to　India　exceeds　that　from　Aethiopia　to



Maeotis　and　the　northernmost　Scythians　by　a　ratio　of　more　than　5　to　3；



as　far　as　such　matters　admit　of　accurate　statement。　Yet　we　know　the



whole　breadth　of　the　region　we　dwell　in　up　to　the　uninhabited　parts：



in　one　direction　no　one　lives　because　of　the　cold；　in　the　other



because　of　the　heat。



　　But　it　is　the　sea　which　divides　as　it　seems　the　parts　beyond　India



from　those　beyond　the　Pillars　of　Heracles　and　prevents　the　earth



from　being　inhabited　all　round。



　　Now　since　there　must　be　a　region　bearing　the　same　relat