meteorology-第15部分

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the　agency　which　created　the　evaporation　and　gave　it　an　impulse　to



motion　clearly　does　not　at　once　exhaust　the　whole　of　the　material　from



which　it　forms　the　wind　which　we　call　an　earthquake。　So　until　the　rest



of　this　is　exhausted　the　shocks　must　continue；　though　more　gently；　and



they　must　go　on　until　there　is　too　little　of　the　evaporation　left　to



have　any　perceptible　effect　on　the　earth　at　all。



　　Subterranean　noises；　too；　are　due　to　the　wind；　sometimes　they



portend　earthquakes　but　sometimes　they　have　been　heard　without　any



earthquake　following。　Just　as　the　air　gives　off　various　sounds　when　it



is　struck；　so　it　does　when　it　strikes　other　things；　for　striking



involves　being　struck　and　so　the　two　cases　are　the　same。　The　sound



precedes　the　shock　because　sound　is　thinner　and　passes　through



things　more　readily　than　wind。　But　when　the　wind　is　too　weak　by　reason



of　thinness　to　cause　an　earthquake　the　absence　of　a　shock　is　due　to



its　filtering　through　readily；　though　by　striking　hard　and　hollow



masses　of　different　shapes　it　makes　various　noises；　so　that　the



earth　sometimes　seems　to　'bellow'　as　the　portentmongers　say。



　　Water　has　been　known　to　burst　out　during　an　earthquake。　But　that



does　not　make　water　the　cause　of　the　earthquake。　The　wind　is　the



efficient　cause　whether　it　drives　the　water　along　the　surface　or　up



from　below：　just　as　winds　are　the　causes　of　waves　and　not　waves　of



winds。　Else　we　might　as　well　say　that　earth　was　the　cause；　for　it　is



upset　in　an　earthquake；　just　like　water　（for　effusion　is　a　form　of



upsetting）。　No；　earth　and　water　are　material　causes　（being　patients；



not　agents）：　the　true　cause　is　the　wind。



　　The　combination　of　a　tidal　wave　with　an　earthquake　is　due　to　the



presence　of　contrary　winds。　It　occurs　when　the　wind　which　is　shaking



the　earth　does　not　entirely　succeed　in　driving　off　the　sea　which



another　wind　is　bringing　on；　but　pushes　it　back　and　heaps　it　up　in　a



great　mass　in　one　place。　Given　this　situation　it　follows　that　when



this　wind　gives　way　the　whole　body　of　the　sea；　driven　on　by　the



other　wind；　will　burst　out　and　overwhelm　the　land。　This　is　what



happened　in　Achaea。　There　a　south　wind　was　blowing；　but　outside　a



north　wind；　then　there　was　a　calm　and　the　wind　entered　the　earth；



and　then　the　tidal　wave　came　on　and　simultaneously　there　was　an



earthquake。　This　was　the　more　violent　as　the　sea　allowed　no　exit　to



the　wind　that　had　entered　the　earth；　but　shut　it　in。　So　in　their



struggle　with　one　another　the　wind　caused　the　earthquake；　and　the　wave



by　its　settling　down　the　inundation。



　　Earthquakes　are　local　and　often　affect　a　small　district　only；



whereas　winds　are　not　local。　Such　phenomena　are　local　when　the



evaporations　at　a　given　place　are　joined　by　those　from　the　next　and



unite；　this；　as　we　explained；　is　what　happens　when　there　is　drought　or



excessive　rain　locally。　Now　earthquakes　do　come　about　in　this　way



but　winds　do　not。　For　earthquakes；　rains；　and　droughts　have　their



source　and　origin　inside　the　earth；　so　that　the　sun　is　not　equally



able　to　direct　all　the　evaporations　in　one　direction。　But　on　the



evaporations　in　the　air　the　sun　has　more　influence　so　that；　when



once　they　have　been　given　an　impulse　by　its　motion；　which　is



determined　by　its　various　positions；　they　flow　in　one　direction。



　　When　the　wind　is　present　in　sufficient　quantity　there　is　an



earthquake。　The　shocks　are　horizontal　like　a　tremor；　except



occasionally；　in　a　few　places；　where　they　act　vertically；　upwards　from



below；　like　a　throbbing。　It　is　the　vertical　direction　which　makes　this



kind　of　earthquake　so　rare。　The　motive　force　does　not　easily



accumulate　in　great　quantity　in　the　position　required；　since　the



surface　of　the　earth　secretes　far　more　of　the　evaporation　than　its



depths。　Wherever　an　earthquake　of　this　kind　does　occur　a　quantity　of



stones　comes　to　the　surface　of　the　earth　（as　when　you　throw　up



things　in　a　winnowing　fan）；　as　we　see　from　Sipylus　and　the



Phlegraean　plain　and　the　district　in　Liguria；　which　were　devastated　by



this　kind　of　earthquake。



　　Islands　in　the　middle　of　the　sea　are　less　exposed　to　earthquakes



than　those　near　land。　First；　the　volume　of　the　sea　cools　the



evaporations　and　overpowers　them　by　its　weight　and　so　crushes　them。



Then；　currents　and　not　shocks　are　produced　in　the　sea　by　the　action　of



the　winds。　Again；　it　is　so　extensive　that　evaporations　do　not



collect　in　it　but　issue　from　it；　and　these　draw　the　evaporations



from　the　earth　after　them。　Islands　near　the　continent　really　form　part



of　it：　the　intervening　sea　is　not　enough　to　make　any　difference；　but



those　in　the　open　sea　can　only　be　shaken　if　the　whole　of　the　sea



that　surrounds　them　is　shaken　too。



　　We　have　now　explained　earthquakes；　their　nature　and　cause；　and　the



most　important　of　the　circumstances　attendant　on　their　appearance。







　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　9







　　Let　us　go　on　to　explain　lightning　and　thunder；　and　further



whirlwind；　fire…wind；　and　thunderbolts：　for　the　cause　of　them　all　is



the　same。



　　As　we　have　said；　there　are　two　kinds　of　exhalation；　moist　and　dry；



and　the　atmosphere　contains　them　both　potentially。　It；　as　we　have　said



before；　condenses　into　cloud；　and　the　density　of　the　clouds　is　highest



at　their　upper　limit。　（For　they　must　be　denser　and　colder　on　the



side　where　the　heat　escapes　to　the　upper　region　and　leaves　them。



This　explains　why　hurricanes　and　thunderbolts　and　all　analogous



phenomena　move　downwards　in　spite　of　the　fact　that　everything　hot



has　a　natural　tendency　upwards。　Just　as　the　pips　that　we　squeeze



between　our　fingers　are　heavy　but　often　jump　upwards：　so　these



things　are　necessarily　squeezed　out　away　from　the　densest　part　of



the　cloud。）　Now　the　heat　that　escapes　disperses　to　the　up　region。



But　if　any　of　the　dry　exhalation　is　caught　in　the　process　as　the　air



cools；　it　is　squeezed　out　as　the　clouds　contract；　and　collides　in



its　rapid　course　with　the　neighbouring　clouds；　and　the　sound　of　this



collision　is　what　we　call　thunder。　This　collision　is　analogous；　to



compare　small　with　great；　to　the　sound　we　hear　in　a　flame　which　men



call　the　laughter　or　the　threat　of　Hephaestus　or　of　Hestia。　This



occurs　when　the　wood　dries　and　cracks　and　the　exhalation　rushes　on　the



flame　in　a　body。　So　in　the　clouds；　the　exhalation　is　projected　and　its



impact　on　dense　clouds　causes　thunder：　the　variety　of　the　sound　is　due



to　the　irregularity　of　the　clouds　and　the　hollows　that　intervene　where



their　density　is　interrupted。　This　then；　is　thunder；　and　this　its



cause。



　　It　usually　happens　that　the　exhalation　that　is　ejected　is　inflamed



and　burns　with　a　thin　and　faint　fire：　this　is　what　we　call



lightning；　where　we　see　as　it　were　the　exhalation　coloured　in　the



act　of　its　ejection。　It　comes　into　existence　after　the　collision　and



the　thunder；　though　we　see　it　earlier　because　sight　is　quicker　than



hearing。　The　rowing　of　triremes　illustrates　this：　the　oars　are　going



back　again　before　the　sound　of　their　striking　the　water　reaches　us。



　　However；　there　are　some　who　maintain　that　there　is　actually　fire



in　the　clouds。　Empedocles　says　that　it　consists　of　some　of　the　sun's



rays　which　are　intercepted：　Anaxagoras　that　it　is　part　of　the　upper



ether　（which　he　calls　fire）　which　has　descended　from　above。　Lightning；



then；　is　the　gleam　of　this　fire；　and　thunder　the　hissing　noise　of



its　extinction　in　the　cloud。



　　But　this　involves　the　view　that　lightning　actually　is　prior　to



thunder　and　does　not　merely　appear　to　be　so。　Again；　this



intercepting　of　the　fire　is　impossible　on　either　theory；　but



especially　it　is　said　to　be　drawn　down　from　the　upper　ether。　Some



reason　ought　to　be　given　why　that　which　naturally　ascends　should



descend；　and　why　it　should　not　always　do　so；　but　only　when　it　is



cloudy。　When　the　sky　is　clear　there　is　no　lightning：　to　say　that　there



is；　is　altogether　wanton。



　　The　view　that　the　heat　of　the　sun's　rays　intercepted　in　the　clouds



is　the　cause　of　these　phenomena　is　equally　unattractive：　this；　too；　is



a　most　careless　explanation。　Thunder；　lightning；　and　the　rest　must



have　a　separate　and　determinate　cause　assigned　to　them　on　which　they



ensue。　But　this　theory　does　nothing　of　the　sort。　It　is　like



supposing　that　water；　snow；　and　hail　existed　all　along　and　were



produced　when　the　time　came　and　not　generated　at　all；　as　if　the



atmosphere　brought　each　to　hand　out　of　its　stock　from　time　to　time。



They　are　concretions　in　the　same　way　as　thunder　and　lightning　are



discretions；　so　that　if　it　is　true　of　either　that　they　are　not



generated　but　pre…exist；　the　same　must　be　true　of　the　other。　Again；



how　can　any　distinction　be　made　about　the　intercepting　between　this



case　and　that　of　interception　in　denser　substances　such　as　water？



Water；　too；　is　heated　by　the　sun　and　by　fire：　yet　when　it　contracts



again　and　grows　cold　and　freezes　no　such　ejection　as　they　describe



occurs；　though　it　ought　on　their　the。　to　take　place　on　a　proportionate



scale。　Boiling　is　due　to　the　exhalation　generated　by　fire：　but　it　is



impossible　for　it　to　exist　in　the　water　beforehand；　and　besides　they



call　the　noise　'hissing'；　not　'boiling'。　But　hissing　is　really　boiling



on　a　small　scale：　for　when　that　which　is　brought　into　contact　with



moisture　and　is　in　process　of　being　extinguished　gets　the　better　of



it；　then　it　boils　and　makes　the　noise　in　question。　Some…Cleidemus　is



one　of　them…say　that　lightning　is　nothing　objective　but　merely　an



appearance。　They　compare　it　to　what　happens　when　you　strike　the　sea



with　a　rod　by　night　and　the　water　is　seen　to　shine。　They　say　that



the　moisture　in　the　cloud　is　beaten　about　in　the　same　way；　and　that



lightning　is　the　appearance　of　brightness　that　ensues。



　　This　theory　is　du