meteorology-第15章

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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　relation　to　the



southern　pole　as　the　place　we　live　in　bears　to　our　pole；　it　will



clearly　correspond　in　the　ordering　of　its　winds　as　well　as　in　other



things。　So　just　as　we　have　a　north　wind　here；　they　must　have　a



corresponding　wind　from　the　antarctic。　This　wind　cannot　reach　us　since



our　own　north　wind　is　like　a　land　breeze　and　does　not　even　reach　the



limits　of　the　region　we　live　in。　The　prevalence　of　north　winds　here　is



due　to　our　lying　near　the　north。　Yet　even　here　they　give　out　and



fail　to　penetrate　far：　in　the　southern　sea　beyond　Libya　east　and



west　winds　are　always　blowing　alternately；　like　north　and　south



winds　with　us。　So　it　is　clear　that　the　south　wind　is　not　the　wind　that



blows　from　the　south　pole。　It　is　neither　that　nor　the　wind　from　the



winter　tropic。　For　symmetry　would　require　another　wind　blowing　from



the　summer　tropic；　which　there　is　not；　since　we　know　that　only　one



wind　blows　from　that　quarter。　So　the　south　wind　clearly　blows　from　the



torrid　region。　Now　the　sun　is　so　near　to　that　region　that　it　has　no



water；　or　snow　which　might　melt　and　cause　Etesiae。　But　because　that



place　is　far　more　extensive　and　open　the　south　wind　is　greater　and



stronger　and　warmer　than　the　north　and　penetrates　farther　to　the　north



than　the　north　wind　does　to　the　south。



　　The　origin　of　these　winds　and　their　relation　to　one　another　has



now　been　explained。







　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　6







　　Let　us　now　explain　the　position　of　the　winds；　their　oppositions；



which　can　blow　simultaneously　with　which；　and　which　cannot；　their



names　and　number；　and　any　other　of　their　affections　that　have　not　been



treated　in　the　'particular　questions'。　What　we　say　about　their



position　must　be　followed　with　the　help　of　the　figure。　For



clearness'　sake　we　have　drawn　the　circle　of　the　horizon；　which　is



round；　but　it　represents　the　zone　in　which　we　live；　for　that　can　be



divided　in　the　same　way。　Let　us　also　begin　by　laying　down　that　those



things　are　locally　contrary　which　are　locally　most　distant　from　one



another；　just　as　things　specifically　most　remote　from　one　another



are　specific　contraries。　Now　things　that　face　one　another　from



opposite　ends　of　a　diameter　are　locally　most　distant　from　one　another。



（See　diagram。）



　　Let　A　be　the　point　where　the　sun　sets　at　the　equinox　and　B；　the



point　opposite；　the　place　where　it　rises　at　the　equinox。　Let　there



be　another　diameter　cutting　this　at　right　angles；　and　let　the　point



H　on　it　be　the　north　and　its　diametrical　opposite　O　the　south。　Let　Z



be　the　rising　of　the　sun　at　the　summer　solstice　and　E　its　setting　at



the　summer　solstice；　D　its　rising　at　the　winter　solstice；　and　G　its



setting　at　the　winter　solstice。　Draw　a　diameter　from　Z　to　G　from　D



to　E。　Then　since　those　things　are　locally　contrary　which　are　most



distant　from　one　another　in　space；　and　points　diametrically　opposite



are　most　distant　from　one　another；　those　win