meteorology-第18章

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going　to　blow　there　is　a　premonitory　indication：　a　sound　is　heard　in



the　places　from　which　the　eruptions　issue。　This　is　because　the　sea



is　being　pushed　on　from　a　distance　and　its　advance　thrusts　back　into



the　earth　the　wind　that　was　issuing　from　it。　The　reason　why　there　is　a



noise　and　no　earthquake　is　that　the　underground　spaces　are　so



extensive　in　proportion　to　the　quantity　of　the　air　that　is　being



driven　on　that　the　wind　slips　away　into　the　void　beyond。



　　Again；　our　theory　is　supported　by　the　facts　that　the　sun　appears



hazy　and　is　darkened　in　the　absence　of　clouds；　and　that　there　is



sometimes　calm　and　sharp　frost　before　earthquakes　at　sunrise。　The



sun　is　necessarily　obscured　and　darkened　when　the　evaporation　which



dissolves　and　rarefies　the　air　begins　to　withdraw　into　the　earth。



The　calm；　too；　and　the　cold　towards　sunrise　and　dawn　follow　from　the



theory。　The　calm　we　have　already　explained。　There　must　as　a　rule　be



calm　because　the　wind　flows　back　into　the　earth：　again；　it　must　be



most　marked　before　the　more　violent　earthquakes；　for　when　the　wind



is　not　part　outside　earth；　part　inside；　but　moves　in　a　single　body；



its　strength　must　be　greater。　The　cold　comes　because　the　evaporation



which　is　naturally　and　essentially　hot　enters　the　earth。　（Wind　is



not　recognized　to　be　hot；　because　it　sets　the　air　in　motion；　and



that　is　full　of　a　quantity　of　cold　vapour。　It　is　the　same　with　the



breath　we　blow　from　our　mouth：　close　by　it　is　warm；　as　it　is　when　we



breathe　out　through　the　mouth；　but　there　is　so　little　of　it　that　it　is



scarcely　noticed；　whereas　at　a　distance　it　is　cold　for　the　same　reason



as　wind。）　Well；　when　this　evaporation　disappears　into　the　earth　the



vaporous　exhalation　concentrates　and　causes　cold　in　any　place　in　which



this　disappearance　occurs。



　　A　sign　which　sometimes　precedes　earthquakes　can　be　explained　in



the　same　way。　Either　by　day　or　a　little　after　sunset；　in　fine　weather；



a　little；　light；　long…drawn　cloud　is　seen；　like　a　long　very　straight



line。　This　is　because　the　wind　is　leaving　the　air　and　dying　down。



Something　analogous　to　this　happens　on　the　sea…shore。　When　the　sea



breaks　in　great　waves　the　marks　left　on　the　sand　are　very　thick　and



crooked；　but　when　the　sea　is　calm　they　are　slight　and　straight



（because　the　secretion　is　small）。　As　the　sea　is　to　the　shore　so　the



wind　is　to　the　cloudy　air；　so；　when　the　wind　drops；　this　very　straight



and　thin　cloud　is　left；　a　sort　of　wave…mark　in　the　air。



　　An　earthquake　sometimes　coincides　with　an　eclipse　of　the　moon　for



the　same　reason。　When　the　earth　is　on　the　point　of　being　interposed；



but　the　light　and　heat　of　the　sun　has　not　quite　vanished　from　the



air　but　is　dying　away；　the　wind　which　causes　the　earthquake　before　the



eclipse；　turns　off　into　the　earth；　and　calm　ensues。　For　there　often



are　winds　before　eclipses：　at　nightfall　if　the　eclipse　is　at　midnight；



and　at　midnight　if　the　eclipse　is　at　dawn。　They　are　caused　by　the



lessening　of　the　warmth　from　the　moon　when　its　sphere　approaches　the



point　at　which　the　eclipse　is　going　to　take　place。　So　the　influence



which　restrained　and　quieted　the　air　weakens　and　the　air　moves　again



and　a　wind　rises；　and　does　so　later；　the　later　the　eclipse。



　　A　severe　earthquake　does　not　stop　at　once　or　after　a　single　shock；



but　first　the　shocks　go　on；　often　for　about　forty　days；　after　that；



for　one　or　even　two　years　it　gives　premonitory　indications　in　the　same



place。　The　severity　of　the　earthquake　is　determined　by　the　quantity　of



wind　and　the　shape　of　the　passages　through　which　it　flows。　Where　it　is



beaten　back　and　cannot　easily　find　its　way　out　the　shocks　are　most



violent；　and　there　it　must　remain　in　a　cramped　space　like　water　that



cannot　escape。　Any　throbbing　in　the　body　does　not　cease　suddenly　or



quickly；　but　by　degrees　according　as　the　affection　passes　off。　So　here



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　Sipylu