meteorology-第14章

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the　dry　evaporation　is　the　source　and　substance　of　all　winds。　That



things　must　necessarily　take　this　course　is　clear　from　the　resulting



phenomena　themselves；　for　the　evaporation　that　is　to　produce　them　must



necessarily　differ；　and　the　sun　and　the　warmth　in　the　earth　not　only



can　but　must　produce　these　evaporations。



　　Since　the　two　evaporations　are　specifically　distinct；　wind　and



rain　obviously　differ　and　their　substance　is　not　the　same；　as　those



say　who　maintain　that　one　and　the　same　air　when　in　motion　is　wind；　but



when　it　condenses　again　is　water。　Air；　as　we　have　explained　in　an



earlier　book；　is　made　up　of　these　as　constituents。　Vapour　is　moist



and　cold　（for　its　fluidity　is　due　to　its　moistness；　and　because　it



derives　from　water　it　is　naturally　cold；　like　water　that　has　not



been　warmed）：　whereas　the　smoky　evaporation　is　hot　and　dry。　Hence　each



contributes　a　part；　and　air　is　moist　and　hot。　It　is　absurd　that　this



air　that　surrounds　us　should　become　wind　when　in　motion；　whatever　be



the　source　of　its　motion　on　the　contrary　the　case　of　winds　is　like



that　of　rivers。　We　do　not　call　water　that　flows　anyhow　a　river；　even



if　there　is　a　great　quantity　of　it；　but　only　if　the　flow　comes　from



a　spring。　So　too　with　the　winds；　a　great　quantity　of　air　might　be



moved　by　the　fall　of　some　large　object　without　flowing　from　any　source



or　spring。



　　The　facts　bear　out　our　theory。　It　is　because　the　evaporation　takes



place　uninterruptedly　but　differs　in　degree　and　quantity　that　clouds



and　winds　appear　in　their　natural　proportion　according　to　the



season；　and　it　is　because　there　is　now　a　great　excess　of　the　vaporous；



now　of　the　dry　and　smoky　exhalation；　that　some　years　are　rainy　and



wet；　others　windy　and　dry。　Sometimes　there　is　much　drought　or　rain；



and　it　prevails　over　a　great　and　continuous　stretch　of　country。　At



other　times　it　is　local；　the　surrounding　country　often　getting



seasonable　or　even　excessive　rains　while　there　is　drought　in　a　certain



part；　or；　contrariwise；　all　the　surrounding　country　gets　little　or



even　no　rain　while　a　certain　part　gets　rain　in　abundance。　The　reason



for　all　this　is　that　while　the　same　affection　is　generally　apt　to



prevail　over　a　considerable　district　because　adjacent　places　（unless



there　is　something　special　to　differentiate　them）　stand　in　the　same



relation　to　the　sun；　yet　on　occasion　the　dry　evaporation　will



prevail　in　one　part　and　the　moist　in　another；　or　conversely。　Again　the



reason　for　this　latter　is　that　each　evaporation　goes　over　to　that　of



the　neighbouring　district：　for　instance；　the　dry　evaporation



circulates　in　its　own　place　while　the　moist　migrates　to　the　next



district　or　is　even　driven　by　winds　to　some　distant　place：　or　else　the



moist　evaporation　remains　and　the　dry　moves　away。　Just　as　in　the



case　of　the　body　when　the　stomach　is　dry　the　lower　belly　is　often　in



the　contrary　state；　and　when　it　is　dry　the　stomach　is　moist　and



cold；　so　it　often　happens　that　the　evaporations　reciprocally　take



one　another's　place　and　interchange。



　　Further；　after　rain　wind　generally　rises　in　those　places　where　the



rain　fell；　and　when　rain　has　come　on　the　wind　ceases。　These　are



necessary　effects　of　the　principles　we　have　explained。　After　rain



the　earth　is　being　dried　by　its　own　heat　and　that　from　above　and　gives



off　the　evaporation　which　we　saw　to　be　the　material　cause　of。　wind。



Again；　suppose　this　secretion　is　present　and　wind　prevails；　the　heat



is　continually　being　thrown　off；　rising　to　the　upper　region；　and　so



the　wind　ceases；　then　the　fall　in　temperature　makes　vapour　form　and



condense　into　water。　Water　also　forms　and　cools　the　dry　evaporation



when　the　clouds　are　driven　together　and　the　cold　concentrated　in　them。



These　are　the　causes　that　make　wind　cease　on　the　advent　of　rain；　and



rain　fall　on　the　cessation　of　wind。



　　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。







　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　5







　　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　re